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<?xml version="1.0" encoding="iso-8859-1"?>
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<para>
<indexterm><primary>language, GHC</primary></indexterm>
<indexterm><primary>extensions, GHC</primary></indexterm>
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As with all known Haskell systems, GHC implements some extensions to
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the language.  They can all be enabled or disabled by command line flags
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or language pragmas. By default GHC understands the most recent Haskell
version it supports, plus a handful of extensions.
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</para>
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<para>
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Some of the Glasgow extensions serve to give you access to the
underlying facilities with which we implement Haskell.  Thus, you can
get at the Raw Iron, if you are willing to write some non-portable
code at a more primitive level.  You need not be &ldquo;stuck&rdquo;
on performance because of the implementation costs of Haskell's
&ldquo;high-level&rdquo; features&mdash;you can always code
&ldquo;under&rdquo; them.  In an extreme case, you can write all your
time-critical code in C, and then just glue it together with Haskell!
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</para>
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<para>
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Before you get too carried away working at the lowest level (e.g.,
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sloshing <literal>MutableByteArray&num;</literal>s around your
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program), you may wish to check if there are libraries that provide a
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&ldquo;Haskellised veneer&rdquo; over the features you want.  The
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separate <ulink url="../libraries/index.html">libraries
documentation</ulink> describes all the libraries that come with GHC.
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</para>
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<!-- LANGUAGE OPTIONS -->
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  <sect1 id="options-language">
    <title>Language options</title>
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    <indexterm><primary>language</primary><secondary>option</secondary>
    </indexterm>
    <indexterm><primary>options</primary><secondary>language</secondary>
    </indexterm>
    <indexterm><primary>extensions</primary><secondary>options controlling</secondary>
    </indexterm>
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    <para>The language option flags control what variation of the language are
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    permitted.</para>
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    <para>Language options can be controlled in two ways:
    <itemizedlist>
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      <listitem><para>Every language option can switched on by a command-line flag "<option>-X...</option>"
        (e.g. <option>-XTemplateHaskell</option>), and switched off by the flag "<option>-XNo...</option>";
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        (e.g. <option>-XNoTemplateHaskell</option>).</para></listitem>
      <listitem><para>
          Language options recognised by Cabal can also be enabled using the <literal>LANGUAGE</literal> pragma,
          thus <literal>{-# LANGUAGE TemplateHaskell #-}</literal> (see <xref linkend="language-pragma"/>). </para>
          </listitem>
      </itemizedlist></para>
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    <para>The flag <option>-fglasgow-exts</option>
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          <indexterm><primary><option>-fglasgow-exts</option></primary></indexterm>
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	  is equivalent to enabling the following extensions:
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          &what_glasgow_exts_does;
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	    Enabling these options is the <emphasis>only</emphasis>
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	    effect of <option>-fglasgow-exts</option>.
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          We are trying to move away from this portmanteau flag,
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	  and towards enabling features individually.</para>
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  </sect1>
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<!-- UNBOXED TYPES AND PRIMITIVE OPERATIONS -->
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<sect1 id="primitives">
  <title>Unboxed types and primitive operations</title>

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<para>GHC is built on a raft of primitive data types and operations;
"primitive" in the sense that they cannot be defined in Haskell itself.
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While you really can use this stuff to write fast code,
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we generally find it a lot less painful, and more satisfying in the
long run, to use higher-level language features and libraries.  With
any luck, the code you write will be optimised to the efficient
unboxed version in any case.  And if it isn't, we'd like to know
about it.</para>
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<para>All these primitive data types and operations are exported by the
library <literal>GHC.Prim</literal>, for which there is
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<ulink url="&libraryGhcPrimLocation;/GHC-Prim.html">detailed online documentation</ulink>.
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(This documentation is generated from the file <filename>compiler/prelude/primops.txt.pp</filename>.)
</para>
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<para>
If you want to mention any of the primitive data types or operations in your
program, you must first import <literal>GHC.Prim</literal> to bring them
into scope.  Many of them have names ending in "&num;", and to mention such
names you need the <option>-XMagicHash</option> extension (<xref linkend="magic-hash"/>).
</para>

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<para>The primops make extensive use of <link linkend="glasgow-unboxed">unboxed types</link>
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and <link linkend="unboxed-tuples">unboxed tuples</link>, which
we briefly summarise here. </para>
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<sect2 id="glasgow-unboxed">
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<title>Unboxed types</title>
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<para>
<indexterm><primary>Unboxed types (Glasgow extension)</primary></indexterm>
</para>

<para>Most types in GHC are <firstterm>boxed</firstterm>, which means
that values of that type are represented by a pointer to a heap
object.  The representation of a Haskell <literal>Int</literal>, for
example, is a two-word heap object.  An <firstterm>unboxed</firstterm>
type, however, is represented by the value itself, no pointers or heap
allocation are involved.
</para>

<para>
Unboxed types correspond to the &ldquo;raw machine&rdquo; types you
would use in C: <literal>Int&num;</literal> (long int),
<literal>Double&num;</literal> (double), <literal>Addr&num;</literal>
(void *), etc.  The <emphasis>primitive operations</emphasis>
(PrimOps) on these types are what you might expect; e.g.,
<literal>(+&num;)</literal> is addition on
<literal>Int&num;</literal>s, and is the machine-addition that we all
know and love&mdash;usually one instruction.
</para>

<para>
Primitive (unboxed) types cannot be defined in Haskell, and are
therefore built into the language and compiler.  Primitive types are
always unlifted; that is, a value of a primitive type cannot be
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bottom.  We use the convention (but it is only a convention)
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that primitive types, values, and
operations have a <literal>&num;</literal> suffix (see <xref linkend="magic-hash"/>).
For some primitive types we have special syntax for literals, also
described in the <link linkend="magic-hash">same section</link>.
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</para>

<para>
Primitive values are often represented by a simple bit-pattern, such
as <literal>Int&num;</literal>, <literal>Float&num;</literal>,
<literal>Double&num;</literal>.  But this is not necessarily the case:
a primitive value might be represented by a pointer to a
heap-allocated object.  Examples include
<literal>Array&num;</literal>, the type of primitive arrays.  A
primitive array is heap-allocated because it is too big a value to fit
in a register, and would be too expensive to copy around; in a sense,
it is accidental that it is represented by a pointer.  If a pointer
represents a primitive value, then it really does point to that value:
no unevaluated thunks, no indirections&hellip;nothing can be at the
other end of the pointer than the primitive value.
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A numerically-intensive program using unboxed types can
go a <emphasis>lot</emphasis> faster than its &ldquo;standard&rdquo;
counterpart&mdash;we saw a threefold speedup on one example.
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</para>

<para>
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There are some restrictions on the use of primitive types:
<itemizedlist>
<listitem><para>The main restriction
is that you can't pass a primitive value to a polymorphic
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function or store one in a polymorphic data type.  This rules out
things like <literal>[Int&num;]</literal> (i.e. lists of primitive
integers).  The reason for this restriction is that polymorphic
arguments and constructor fields are assumed to be pointers: if an
unboxed integer is stored in one of these, the garbage collector would
attempt to follow it, leading to unpredictable space leaks.  Or a
<function>seq</function> operation on the polymorphic component may
attempt to dereference the pointer, with disastrous results.  Even
worse, the unboxed value might be larger than a pointer
(<literal>Double&num;</literal> for instance).
</para>
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</listitem>
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<listitem><para> You cannot define a newtype whose representation type
(the argument type of the data constructor) is an unboxed type.  Thus,
this is illegal:
<programlisting>
  newtype A = MkA Int#
</programlisting>
</para></listitem>
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<listitem><para> You cannot bind a variable with an unboxed type
in a <emphasis>top-level</emphasis> binding.
</para></listitem>
<listitem><para> You cannot bind a variable with an unboxed type
in a <emphasis>recursive</emphasis> binding.
</para></listitem>
<listitem><para> You may bind unboxed variables in a (non-recursive,
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non-top-level) pattern binding, but you must make any such pattern-match
strict.  For example, rather than:
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<programlisting>
  data Foo = Foo Int Int#
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  f x = let (Foo a b, w) = ..rhs.. in ..body..
</programlisting>
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you must write:
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<programlisting>
  data Foo = Foo Int Int#

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  f x = let !(Foo a b, w) = ..rhs.. in ..body..
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</programlisting>
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since <literal>b</literal> has type <literal>Int#</literal>.
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</para>
</listitem>
</itemizedlist>
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</para>

</sect2>

<sect2 id="unboxed-tuples">
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<title>Unboxed tuples</title>
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<para>
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Unboxed tuples aren't really exported by <literal>GHC.Exts</literal>;
they are a syntactic extension enabled by the language flag <option>-XUnboxedTuples</option>.  An
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unboxed tuple looks like this:
</para>

<para>

<programlisting>
(# e_1, ..., e_n #)
</programlisting>

</para>

<para>
where <literal>e&lowbar;1..e&lowbar;n</literal> are expressions of any
type (primitive or non-primitive).  The type of an unboxed tuple looks
the same.
</para>

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<para>
Note that when unboxed tuples are enabled,
<literal>(#</literal> is a single lexeme, so for example when using
operators like <literal>#</literal> and <literal>#-</literal> you need
to write <literal>( # )</literal> and <literal>( #- )</literal> rather than
<literal>(#)</literal> and <literal>(#-)</literal>.
</para>

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<para>
Unboxed tuples are used for functions that need to return multiple
values, but they avoid the heap allocation normally associated with
using fully-fledged tuples.  When an unboxed tuple is returned, the
components are put directly into registers or on the stack; the
unboxed tuple itself does not have a composite representation.  Many
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of the primitive operations listed in <literal>primops.txt.pp</literal> return unboxed
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tuples.
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In particular, the <literal>IO</literal> and <literal>ST</literal> monads use unboxed
tuples to avoid unnecessary allocation during sequences of operations.
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</para>

<para>
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There are some restrictions on the use of unboxed tuples:
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<itemizedlist>

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<listitem>
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<para>
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Values of unboxed tuple types are subject to the same restrictions as
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other unboxed types; i.e. they may not be stored in polymorphic data
structures or passed to polymorphic functions.
</para>
</listitem>

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<listitem>
<para>
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The typical use of unboxed tuples is simply to return multiple values,
binding those multiple results with a <literal>case</literal> expression, thus:
<programlisting>
  f x y = (# x+1, y-1 #)
  g x = case f x x of { (# a, b #) -&#62; a + b }
</programlisting>
You can have an unboxed tuple in a pattern binding, thus
<programlisting>
  f x = let (# p,q #) = h x in ..body..
</programlisting>
If the types of <literal>p</literal> and <literal>q</literal> are not unboxed,
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the resulting binding is lazy like any other Haskell pattern binding.  The
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above example desugars like this:
<programlisting>
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  f x = let t = case h x of { (# p,q #) -> (p,q) }
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            p = fst t
            q = snd t
        in ..body..
</programlisting>
Indeed, the bindings can even be recursive.
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</para>
</listitem>
</itemizedlist>

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</para>

</sect2>
</sect1>

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<!-- ====================== SYNTACTIC EXTENSIONS =======================  -->

<sect1 id="syntax-extns">
<title>Syntactic extensions</title>
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    <sect2 id="unicode-syntax">
      <title>Unicode syntax</title>
      <para>The language
      extension <option>-XUnicodeSyntax</option><indexterm><primary><option>-XUnicodeSyntax</option></primary></indexterm>
      enables Unicode characters to be used to stand for certain ASCII
      character sequences.  The following alternatives are provided:</para>

      <informaltable>
	<tgroup cols="2" align="left" colsep="1" rowsep="1">
	  <thead>
	    <row>
	      <entry>ASCII</entry>
              <entry>Unicode alternative</entry>
	      <entry>Code point</entry>
	      <entry>Name</entry>
	    </row>
	  </thead>
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<!--
               to find the DocBook entities for these characters, find
               the Unicode code point (e.g. 0x2237), and grep for it in
               /usr/share/sgml/docbook/xml-dtd-*/ent/* (or equivalent on
               your system.  Some of these Unicode code points don't have
               equivalent DocBook entities.
            -->

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	  <tbody>
	    <row>
	      <entry><literal>::</literal></entry>
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	      <entry>&#x2237;</entry>
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              <entry>0x2237</entry>
	      <entry>PROPORTION</entry>
	    </row>
          </tbody>
	  <tbody>
	    <row>
	      <entry><literal>=&gt;</literal></entry>
	      <entry>&rArr;</entry>
	      <entry>0x21D2</entry>
              <entry>RIGHTWARDS DOUBLE ARROW</entry>
	    </row>
          </tbody>
	  <tbody>
	    <row>
	      <entry><literal>forall</literal></entry>
	      <entry>&forall;</entry>
	      <entry>0x2200</entry>
              <entry>FOR ALL</entry>
	    </row>
          </tbody>
	  <tbody>
	    <row>
	      <entry><literal>-&gt;</literal></entry>
	      <entry>&rarr;</entry>
	      <entry>0x2192</entry>
              <entry>RIGHTWARDS ARROW</entry>
	    </row>
          </tbody>
	  <tbody>
	    <row>
	      <entry><literal>&lt;-</literal></entry>
	      <entry>&larr;</entry>
	      <entry>0x2190</entry>
              <entry>LEFTWARDS ARROW</entry>
	    </row>
          </tbody>
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	  <tbody>
	    <row>
	      <entry>-&lt;</entry>
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	      <entry>&#x2919;</entry>
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	      <entry>0x2919</entry>
	      <entry>LEFTWARDS ARROW-TAIL</entry>
	    </row>
          </tbody>

	  <tbody>
	    <row>
	      <entry>&gt;-</entry>
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	      <entry>&#x291A;</entry>
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	      <entry>0x291A</entry>
	      <entry>RIGHTWARDS ARROW-TAIL</entry>
	    </row>
          </tbody>

	  <tbody>
	    <row>
	      <entry>-&lt;&lt;</entry>
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	      <entry>&#x291B;</entry>
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	      <entry>0x291B</entry>
	      <entry>LEFTWARDS DOUBLE ARROW-TAIL</entry>
	    </row>
          </tbody>

	  <tbody>
	    <row>
	      <entry>&gt;&gt;-</entry>
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	      <entry>&#x291C;</entry>
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	      <entry>0x291C</entry>
	      <entry>RIGHTWARDS DOUBLE ARROW-TAIL</entry>
	    </row>
          </tbody>

	  <tbody>
	    <row>
	      <entry>*</entry>
	      <entry>&starf;</entry>
	      <entry>0x2605</entry>
	      <entry>BLACK STAR</entry>
	    </row>
          </tbody>

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        </tgroup>
      </informaltable>
    </sect2>

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    <sect2 id="magic-hash">
      <title>The magic hash</title>
      <para>The language extension <option>-XMagicHash</option> allows "&num;" as a
	postfix modifier to identifiers.  Thus, "x&num;" is a valid variable, and "T&num;" is
	a valid type constructor or data constructor.</para>

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      <para>The hash sign does not change semantics at all.  We tend to use variable
	names ending in "&num;" for unboxed values or types (e.g. <literal>Int&num;</literal>),
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        but there is no requirement to do so; they are just plain ordinary variables.
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	Nor does the <option>-XMagicHash</option> extension bring anything into scope.
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	For example, to bring <literal>Int&num;</literal> into scope you must
	import <literal>GHC.Prim</literal> (see <xref linkend="primitives"/>);
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	the <option>-XMagicHash</option> extension
	then allows you to <emphasis>refer</emphasis> to the <literal>Int&num;</literal>
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	that is now in scope. Note that with this option, the meaning of <literal>x&num;y = 0</literal>
	is changed: it defines a function <literal>x&num;</literal> taking a single argument <literal>y</literal>;
        to define the operator <literal>&num;</literal>, put a space: <literal>x &num; y = 0</literal>.

</para>
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      <para> The <option>-XMagicHash</option> also enables some new forms of literals (see <xref linkend="glasgow-unboxed"/>):
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	<itemizedlist>
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	  <listitem><para> <literal>'x'&num;</literal> has type <literal>Char&num;</literal></para> </listitem>
	  <listitem><para> <literal>&quot;foo&quot;&num;</literal> has type <literal>Addr&num;</literal></para> </listitem>
	  <listitem><para> <literal>3&num;</literal> has type <literal>Int&num;</literal>. In general,
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	  any Haskell integer lexeme followed by a <literal>&num;</literal> is an <literal>Int&num;</literal> literal, e.g.
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            <literal>-0x3A&num;</literal> as well as <literal>32&num;</literal>.</para></listitem>
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	  <listitem><para> <literal>3&num;&num;</literal> has type <literal>Word&num;</literal>. In general,
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	  any non-negative Haskell integer lexeme followed by <literal>&num;&num;</literal>
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	      is a <literal>Word&num;</literal>. </para> </listitem>
	  <listitem><para> <literal>3.2&num;</literal> has type <literal>Float&num;</literal>.</para> </listitem>
	  <listitem><para> <literal>3.2&num;&num;</literal> has type <literal>Double&num;</literal></para> </listitem>
	  </itemizedlist>
      </para>
   </sect2>

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    <sect2 id="negative-literals">
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      <title>Negative literals</title>
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      <para>
          The literal <literal>-123</literal> is, according to
          Haskell98 and Haskell 2010, desugared as
          <literal>negate (fromInteger 123)</literal>.
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         The language extension <option>-XNegativeLiterals</option>
         means that it is instead desugared as
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         <literal>fromInteger (-123)</literal>.
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      </para>

      <para>
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      This can make a difference when the positive and negative range of
      a numeric data type don't match up.  For example,
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      in 8-bit arithmetic -128 is representable, but +128 is not.
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      So <literal>negate (fromInteger 128)</literal> will elicit an
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      unexpected integer-literal-overflow message.
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      </para>
   </sect2>

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    <sect2 id="num-decimals">
      <title>Fractional looking integer literals</title>
      <para>
          Haskell 2010 and Haskell 98 define floating literals with
          the syntax <literal>1.2e6</literal>. These literals have the
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          type <literal>Fractional a => a</literal>.
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      </para>

      <para>
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         The language extension <option>-XNumDecimals</option> allows
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         you to also use the floating literal syntax for instances of
         <literal>Integral</literal>, and have values like
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         <literal>(1.2e6 :: Num a => a)</literal>
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      </para>
   </sect2>

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    <sect2 id="binary-literals">
      <title>Binary integer literals</title>
      <para>
          Haskell 2010 and Haskell 98 allows for integer literals to
          be given in decimal, octal (prefixed by
          <literal>0o</literal> or <literal>0O</literal>), or
          hexadecimal notation (prefixed by <literal>0x</literal> or
          <literal>0X</literal>).
      </para>

      <para>
          The language extension <option>-XBinaryLiterals</option>
          adds support for expressing integer literals in binary
          notation with the prefix <literal>0b</literal> or
          <literal>0B</literal>. For instance, the binary integer
          literal <literal>0b11001001</literal> will be desugared into
          <literal>fromInteger 201</literal> when
          <option>-XBinaryLiterals</option> is enabled.
      </para>
   </sect2>
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    <!-- ====================== HIERARCHICAL MODULES =======================  -->

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    <sect2 id="hierarchical-modules">
      <title>Hierarchical Modules</title>

      <para>GHC supports a small extension to the syntax of module
      names: a module name is allowed to contain a dot
      <literal>&lsquo;.&rsquo;</literal>.  This is also known as the
      &ldquo;hierarchical module namespace&rdquo; extension, because
      it extends the normally flat Haskell module namespace into a
      more flexible hierarchy of modules.</para>

      <para>This extension has very little impact on the language
      itself; modules names are <emphasis>always</emphasis> fully
      qualified, so you can just think of the fully qualified module
      name as <quote>the module name</quote>.  In particular, this
      means that the full module name must be given after the
      <literal>module</literal> keyword at the beginning of the
      module; for example, the module <literal>A.B.C</literal> must
      begin</para>

<programlisting>module A.B.C</programlisting>


      <para>It is a common strategy to use the <literal>as</literal>
      keyword to save some typing when using qualified names with
      hierarchical modules.  For example:</para>

<programlisting>
import qualified Control.Monad.ST.Strict as ST
</programlisting>

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      <para>For details on how GHC searches for source and interface
      files in the presence of hierarchical modules, see <xref
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      linkend="search-path"/>.</para>
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      <para>GHC comes with a large collection of libraries arranged
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      hierarchically; see the accompanying <ulink
      url="../libraries/index.html">library
      documentation</ulink>.  More libraries to install are available
      from <ulink
      url="http://hackage.haskell.org/packages/hackage.html">HackageDB</ulink>.</para>
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    </sect2>

    <!-- ====================== PATTERN GUARDS =======================  -->

<sect2 id="pattern-guards">
<title>Pattern guards</title>

<para>
<indexterm><primary>Pattern guards (Glasgow extension)</primary></indexterm>
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The discussion that follows is an abbreviated version of Simon Peyton Jones's original <ulink url="http://research.microsoft.com/~simonpj/Haskell/guards.html">proposal</ulink>. (Note that the proposal was written before pattern guards were implemented, so refers to them as unimplemented.)
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</para>

<para>
Suppose we have an abstract data type of finite maps, with a
lookup operation:

<programlisting>
lookup :: FiniteMap -> Int -> Maybe Int
</programlisting>

The lookup returns <function>Nothing</function> if the supplied key is not in the domain of the mapping, and <function>(Just v)</function> otherwise,
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where <varname>v</varname> is the value that the key maps to.  Now consider the following definition:
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</para>

<programlisting>
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clunky env var1 var2 | ok1 &amp;&amp; ok2 = val1 + val2
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| otherwise  = var1 + var2
where
  m1 = lookup env var1
  m2 = lookup env var2
  ok1 = maybeToBool m1
  ok2 = maybeToBool m2
  val1 = expectJust m1
  val2 = expectJust m2
</programlisting>

<para>
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The auxiliary functions are
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</para>

<programlisting>
maybeToBool :: Maybe a -&gt; Bool
maybeToBool (Just x) = True
maybeToBool Nothing  = False

expectJust :: Maybe a -&gt; a
expectJust (Just x) = x
expectJust Nothing  = error "Unexpected Nothing"
</programlisting>

<para>
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What is <function>clunky</function> doing? The guard <literal>ok1 &amp;&amp;
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ok2</literal> checks that both lookups succeed, using
<function>maybeToBool</function> to convert the <function>Maybe</function>
types to booleans. The (lazily evaluated) <function>expectJust</function>
calls extract the values from the results of the lookups, and binds the
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returned values to <varname>val1</varname> and <varname>val2</varname>
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respectively.  If either lookup fails, then clunky takes the
<literal>otherwise</literal> case and returns the sum of its arguments.
</para>

<para>
This is certainly legal Haskell, but it is a tremendously verbose and
un-obvious way to achieve the desired effect.  Arguably, a more direct way
to write clunky would be to use case expressions:
</para>

<programlisting>
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clunky env var1 var2 = case lookup env var1 of
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  Nothing -&gt; fail
  Just val1 -&gt; case lookup env var2 of
    Nothing -&gt; fail
    Just val2 -&gt; val1 + val2
where
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  fail = var1 + var2
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</programlisting>

<para>
This is a bit shorter, but hardly better.  Of course, we can rewrite any set
of pattern-matching, guarded equations as case expressions; that is
precisely what the compiler does when compiling equations! The reason that
Haskell provides guarded equations is because they allow us to write down
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the cases we want to consider, one at a time, independently of each other.
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This structure is hidden in the case version.  Two of the right-hand sides
are really the same (<function>fail</function>), and the whole expression
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tends to become more and more indented.
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</para>

<para>
Here is how I would write clunky:
</para>

<programlisting>
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clunky env var1 var2
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  | Just val1 &lt;- lookup env var1
  , Just val2 &lt;- lookup env var2
  = val1 + val2
...other equations for clunky...
</programlisting>

<para>
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The semantics should be clear enough.  The qualifiers are matched in order.
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For a <literal>&lt;-</literal> qualifier, which I call a pattern guard, the
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right hand side is evaluated and matched against the pattern on the left.
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If the match fails then the whole guard fails and the next equation is
tried.  If it succeeds, then the appropriate binding takes place, and the
next qualifier is matched, in the augmented environment.  Unlike list
comprehensions, however, the type of the expression to the right of the
<literal>&lt;-</literal> is the same as the type of the pattern to its
left.  The bindings introduced by pattern guards scope over all the
remaining guard qualifiers, and over the right hand side of the equation.
</para>

<para>
Just as with list comprehensions, boolean expressions can be freely mixed
with among the pattern guards.  For example:
</para>

<programlisting>
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f x | [y] &lt;- x
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    , y > 3
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    , Just z &lt;- h y
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    = ...
</programlisting>

<para>
Haskell's current guards therefore emerge as a special case, in which the
qualifier list has just one element, a boolean expression.
</para>
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</sect2>

    <!-- ===================== View patterns ===================  -->

<sect2 id="view-patterns">
<title>View patterns
</title>

<para>
View patterns are enabled by the flag <literal>-XViewPatterns</literal>.
More information and examples of view patterns can be found on the
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<ulink url="http://ghc.haskell.org/trac/ghc/wiki/ViewPatterns">Wiki
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page</ulink>.
</para>

<para>
View patterns are somewhat like pattern guards that can be nested inside
of other patterns.  They are a convenient way of pattern-matching
against values of abstract types. For example, in a programming language
implementation, we might represent the syntax of the types of the
language as follows:

<programlisting>
type Typ
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data TypView = Unit
             | Arrow Typ Typ

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view :: Typ -> TypView
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-- additional operations for constructing Typ's ...
</programlisting>

The representation of Typ is held abstract, permitting implementations
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to use a fancy representation (e.g., hash-consing to manage sharing).
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Without view patterns, using this signature a little inconvenient:
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<programlisting>
size :: Typ -> Integer
size t = case view t of
  Unit -> 1
  Arrow t1 t2 -> size t1 + size t2
</programlisting>

It is necessary to iterate the case, rather than using an equational
function definition. And the situation is even worse when the matching
against <literal>t</literal> is buried deep inside another pattern.
</para>

<para>
View patterns permit calling the view function inside the pattern and
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matching against the result:
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<programlisting>
size (view -> Unit) = 1
size (view -> Arrow t1 t2) = size t1 + size t2
</programlisting>

That is, we add a new form of pattern, written
<replaceable>expression</replaceable> <literal>-></literal>
<replaceable>pattern</replaceable> that means "apply the expression to
whatever we're trying to match against, and then match the result of
that application against the pattern". The expression can be any Haskell
expression of function type, and view patterns can be used wherever
patterns are used.
</para>

<para>
The semantics of a pattern <literal>(</literal>
<replaceable>exp</replaceable> <literal>-></literal>
<replaceable>pat</replaceable> <literal>)</literal> are as follows:

<itemizedlist>

<listitem> Scoping:

<para>The variables bound by the view pattern are the variables bound by
<replaceable>pat</replaceable>.
</para>

<para>
Any variables in <replaceable>exp</replaceable> are bound occurrences,
but variables bound "to the left" in a pattern are in scope.  This
feature permits, for example, one argument to a function to be used in
the view of another argument.  For example, the function
<literal>clunky</literal> from <xref linkend="pattern-guards" /> can be
written using view patterns as follows:

<programlisting>
clunky env (lookup env -> Just val1) (lookup env -> Just val2) = val1 + val2
...other equations for clunky...
</programlisting>
</para>

<para>
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More precisely, the scoping rules are:
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<itemizedlist>
<listitem>
<para>
In a single pattern, variables bound by patterns to the left of a view
pattern expression are in scope. For example:
<programlisting>
example :: Maybe ((String -> Integer,Integer), String) -> Bool
example Just ((f,_), f -> 4) = True
</programlisting>

Additionally, in function definitions, variables bound by matching earlier curried
arguments may be used in view pattern expressions in later arguments:
<programlisting>
example :: (String -> Integer) -> String -> Bool
example f (f -> 4) = True
</programlisting>
That is, the scoping is the same as it would be if the curried arguments
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were collected into a tuple.
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</para>
</listitem>

<listitem>
<para>
In mutually recursive bindings, such as <literal>let</literal>,
<literal>where</literal>, or the top level, view patterns in one
declaration may not mention variables bound by other declarations.  That
is, each declaration must be self-contained.  For example, the following
program is not allowed:
<programlisting>
let {(x -> y) = e1 ;
     (y -> x) = e2 } in x
</programlisting>

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(For some amplification on this design choice see
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<ulink url="http://ghc.haskell.org/trac/ghc/ticket/4061">Trac #4061</ulink>.)
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</para>
</listitem>
</itemizedlist>

</para>
</listitem>

<listitem><para> Typing: If <replaceable>exp</replaceable> has type
<replaceable>T1</replaceable> <literal>-></literal>
<replaceable>T2</replaceable> and <replaceable>pat</replaceable> matches
a <replaceable>T2</replaceable>, then the whole view pattern matches a
<replaceable>T1</replaceable>.
</para></listitem>

<listitem><para> Matching: To the equations in Section 3.17.3 of the
<ulink url="http://www.haskell.org/onlinereport/">Haskell 98
Report</ulink>, add the following:
<programlisting>
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case v of { (e -> p) -> e1 ; _ -> e2 }
 =
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case (e v) of { p -> e1 ; _ -> e2 }
</programlisting>
That is, to match a variable <replaceable>v</replaceable> against a pattern
<literal>(</literal> <replaceable>exp</replaceable>
<literal>-></literal> <replaceable>pat</replaceable>
<literal>)</literal>, evaluate <literal>(</literal>
<replaceable>exp</replaceable> <replaceable> v</replaceable>
<literal>)</literal> and match the result against
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<replaceable>pat</replaceable>.
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</para></listitem>

<listitem><para> Efficiency: When the same view function is applied in
multiple branches of a function definition or a case expression (e.g.,
in <literal>size</literal> above), GHC makes an attempt to collect these
applications into a single nested case expression, so that the view
function is only applied once.  Pattern compilation in GHC follows the
matrix algorithm described in Chapter 4 of <ulink
url="http://research.microsoft.com/~simonpj/Papers/slpj-book-1987/">The
Implementation of Functional Programming Languages</ulink>.  When the
top rows of the first column of a matrix are all view patterns with the
"same" expression, these patterns are transformed into a single nested
case.  This includes, for example, adjacent view patterns that line up
in a tuple, as in
<programlisting>
f ((view -> A, p1), p2) = e1
f ((view -> B, p3), p4) = e2
</programlisting>
</para>

<para> The current notion of when two view pattern expressions are "the
same" is very restricted: it is not even full syntactic equality.
However, it does include variables, literals, applications, and tuples;
e.g., two instances of <literal>view ("hi", "there")</literal> will be
collected.  However, the current implementation does not compare up to
alpha-equivalence, so two instances of <literal>(x, view x ->
y)</literal> will not be coalesced.
</para>

</listitem>

</itemizedlist>
</para>

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</sect2>

    <!-- ===================== Pattern synonyms ===================  -->

<sect2 id="pattern-synonyms">
<title>Pattern synonyms
</title>

<para>
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Pattern synonyms are enabled by the flag
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<literal>-XPatternSynonyms</literal>, which is required for defining
them, but <emphasis>not</emphasis> for using them.  More information
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and examples of view patterns can be found on the <ulink
url="http://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms">Wiki
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page</ulink>.
</para>

<para>
Pattern synonyms enable giving names to parametrized pattern
schemes. They can also be thought of as abstract constructors that
don't have a bearing on data representation. For example, in a
programming language implementation, we might represent types of the
language as follows:
</para>

<programlisting>
data Type = App String [Type]
</programlisting>

<para>
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Here are some examples of using said representation.
Consider a few types of the <literal>Type</literal> universe encoded
like this:
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</para>

<programlisting>
  App "->" [t1, t2]          -- t1 -> t2
  App "Int" []               -- Int
  App "Maybe" [App "Int" []] -- Maybe Int
</programlisting>

<para>
This representation is very generic in that no types are given special
treatment. However, some functions might need to handle some known
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types specially, for example the following two functions collect all
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argument types of (nested) arrow types, and recognize the
<literal>Int</literal> type, respectively:
</para>

<programlisting>
  collectArgs :: Type -> [Type]
  collectArgs (App "->" [t1, t2]) = t1 : collectArgs t2
  collectArgs _                   = []

  isInt :: Type -> Bool
  isInt (App "Int" []) = True
  isInt _              = False
</programlisting>

<para>
Matching on <literal>App</literal> directly is both hard to read and
error prone to write. And the situation is even worse when the
matching is nested:
</para>

<programlisting>
  isIntEndo :: Type -> Bool
  isIntEndo (App "->" [App "Int" [], App "Int" []]) = True
  isIntEndo _                                       = False
</programlisting>

<para>
Pattern synonyms permit abstracting from the representation to expose
matchers that behave in a constructor-like manner with respect to
pattern matching. We can create pattern synonyms for the known types
we care about, without committing the representation to them (note
that these don't have to be defined in the same module as the
<literal>Type</literal> type):
</para>

<programlisting>
  pattern Arrow t1 t2 = App "->"    [t1, t2]
  pattern Int         = App "Int"   []
  pattern Maybe t     = App "Maybe" [t]
</programlisting>

<para>
Which enables us to rewrite our functions in a much cleaner style:
</para>

<programlisting>
  collectArgs :: Type -> [Type]
  collectArgs (Arrow t1 t2) = t1 : collectArgs t2
  collectArgs _             = []

  isInt :: Type -> Bool
  isInt Int = True
  isInt _   = False

  isIntEndo :: Type -> Bool
  isIntEndo (Arrow Int Int) = True
  isIntEndo _               = False
</programlisting>

<para>
  Note that in this example, the pattern synonyms
  <literal>Int</literal> and <literal>Arrow</literal> can also be used
  as expressions (they are <emphasis>bidirectional</emphasis>). This
  is not necessarily the case: <emphasis>unidirectional</emphasis>
  pattern synonyms can also be declared with the following syntax:
</para>

<programlisting>
  pattern Head x &lt;- x:xs
</programlisting>

<para>
In this case, <literal>Head</literal> <replaceable>x</replaceable>
cannot be used in expressions, only patterns, since it wouldn't
specify a value for the <replaceable>xs</replaceable> on the
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right-hand side. We can give an explicit inversion of a pattern
synonym using the following syntax:
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</para>

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<programlisting>
  pattern Head x &lt;- x:xs where
    Head x = [x]
</programlisting>

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<para>
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The syntax and semantics of pattern synonyms are elaborated in the
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following subsections.
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See the <ulink
url="http://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms">Wiki
page</ulink> for more details.
</para>
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<sect3> <title>Syntax and scoping of pattern synonyms</title>
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<para>
A pattern synonym declaration can be either unidirectional or
bidirectional. The syntax for unidirectional pattern synonyms is:
<programlisting>
  pattern Name args &lt;- pat
</programlisting>
  and the syntax for bidirectional pattern synonyms is:
<programlisting>
  pattern Name args = pat
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</programlisting> or
<programlisting>
  pattern Name args &lt;- pat where
    Name args = expr
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</programlisting>
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  Either prefix or infix syntax can be
  used.
</para>
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<para>
  Pattern synonym declarations can only occur in the top level of a
  module. In particular, they are not allowed as local
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  definitions.
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</para>
<para>
  The variables in the left-hand side of the definition are bound by
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  the pattern on the right-hand side. For implicitly bidirectional
  pattern synonyms, all the variables of the right-hand side must also
  occur on the left-hand side; also, wildcard patterns and view
  patterns are not allowed. For unidirectional and
  explicitly-bidirectional pattern synonyms, there is no restriction
  on the right-hand side pattern.
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</para>

<para>
  Pattern synonyms cannot be defined recursively.
</para>
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</sect3>
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<sect3 id="patsyn-impexp"> <title>Import and export of pattern synonyms</title>

<para>
  The name of the pattern synonym itself is in the same namespace as
  proper data constructors. In an export or import specification,
  you must prefix pattern
  names with the <literal>pattern</literal> keyword, e.g.:
<programlisting>
  module Example (pattern Single) where
  pattern Single x = [x]
</programlisting>
Without the <literal>pattern</literal> prefix, <literal>Single</literal> would
be interpreted as a type constructor in the export list.
</para>
<para>
You may also use the <literal>pattern</literal> keyword in an import/export
specification to import or export an ordinary data constructor.  For example:
<programlisting>
  import Data.Maybe( pattern Just )
</programlisting>
would bring into scope the data constructor <literal>Just</literal> from the
<literal>Maybe</literal> type, without also bringing the type constructor
<literal>Maybe</literal> into scope.
</para>
</sect3>
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<sect3> <title>Typing of pattern synonyms</title>
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<para>
  Given a pattern synonym definition of the form
<programlisting>
  pattern P var1 var2 ... varN &lt;- pat
</programlisting>
  it is assigned a <emphasis>pattern type</emphasis> of the form
<programlisting>
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  pattern P :: CProv => CReq => t1 -> t2 -> ... -> tN -> t
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</programlisting>
  where <replaceable>CProv</replaceable> and
  <replaceable>CReq</replaceable> are type contexts, and
  <replaceable>t1</replaceable>, <replaceable>t2</replaceable>, ...,
  <replaceable>tN</replaceable> and <replaceable>t</replaceable> are
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  types.
Notice the unusual form of the type, with two contexts <replaceable>CProv</replaceable> and <replaceable>CReq</replaceable>:
<itemizedlist>
<listitem><para><replaceable>CReq</replaceable> are the constraints <emphasis>required</emphasis> to match the pattern.</para></listitem>
<listitem><para><replaceable>CProv</replaceable> are the constraints <emphasis>made available (provided)</emphasis>
by a successful pattern match.</para></listitem>
</itemizedlist>
For example, consider
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<programlisting>
data T a where
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  MkT :: (Show b) => a -> b -> T a
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f1 :: (Eq a, Num a) => MkT a -> String
f1 (MkT 42 x) = show x
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pattern ExNumPat :: (Show b) => (Num a, Eq a) => b -> T a
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pattern ExNumPat x = MkT 42 x
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f2 :: (Eq a, Num a) => MkT a -> String
f2 (ExNumPat x) = show x
</programlisting>
Here <literal>f1</literal> does not use pattern synonyms.  To match against the
numeric pattern <literal>42</literal> <emphasis>requires</emphasis> the caller to
satisfy the constraints <literal>(Num a, Eq a)</literal>,
so they appear in <literal>f1</literal>'s type.  The call to <literal>show</literal> generates a <literal>(Show b)</literal>
constraint, where <literal>b</literal> is an existentially type variable bound by the pattern match
on <literal>MkT</literal>. But the same pattern match also <emphasis>provides</emphasis> the constraint
<literal>(Show b)</literal> (see <literal>MkT</literal>'s type), and so all is well.
</para>
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<para>
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Exactly the same reasoning applies to <literal>ExNumPat</literal>:
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matching against <literal>ExNumPat</literal> <emphasis>requires</emphasis>
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the constraints <literal>(Num a, Eq a)</literal>, and <emphasis>provides</emphasis>
the constraint <literal>(Show b)</literal>.
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</para>
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<para>
Note also the following points
<itemizedlist>
<listitem><para>
In the common case where <replaceable>CReq</replaceable> is empty,
  <literal>()</literal>, it can be omitted altogether.
</para> </listitem>

<listitem><para>
You may specify an explicit <emphasis>pattern signature</emphasis>, as
we did for <literal>ExNumPat</literal> above, to specify the type of a pattern,
just as you can for a function.  As usual, the type signature can be less polymorphic
than the inferred type.  For example
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<programlisting>
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  -- Inferred type would be 'a -> [a]'
  pattern SinglePair :: (a, a) -> [(a, a)]
  pattern SinglePair x = [x]
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</programlisting>
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</para> </listitem>
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<listitem><para>
The GHCi <literal>:info</literal> command shows pattern types in this format.
</para> </listitem>

<listitem><para>
For a bidirectional pattern synonym, a use of the pattern synonym as an expression has the type
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<programlisting>
  (CProv, CReq) => t1 -> t2 -> ... -> tN -> t
</programlisting>
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  So in the previous example, when used in an expression, <literal>ExNumPat</literal> has type
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<programlisting>
  ExNumPat :: (Show b, Num a, Eq a) => b -> T t
</programlisting>
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Notice that this is a tiny bit more restrictive than the expression <literal>MkT 42 x</literal>
which would not require <literal>(Eq a)</literal>.
</para> </listitem>
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<listitem><para>
Consider these two pattern synonyms:
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<programlisting>
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data S a where
   S1 :: Bool -> S Bool

pattern P1 b = Just b  -- P1 ::             Bool -> Maybe Bool
pattern P2 b = S1 b    -- P2 :: (b~Bool) => Bool -> S b

f :: Maybe a -> String
f (P1 x) = "no no no"     -- Type-incorrect

g :: S a -> String
g (P2 b) = "yes yes yes"  -- Fine
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</programlisting>
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Pattern <literal>P1</literal> can only match against a value of type <literal>Maybe Bool</literal>,
so function <literal>f</literal> is rejected because the type signature is <literal>Maybe a</literal>.
(To see this, imagine expanding the pattern synonym.)
</para>
<para>
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On the other hand, function <literal>g</literal> works fine, because matching against <literal>P2</literal>
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(which wraps the GADT <literal>S</literal>) provides the local equality <literal>(a~Bool)</literal>.
If you were to give an explicit pattern signature <literal>P2 :: Bool -> S Bool</literal>, then <literal>P2</literal>
would become less polymorphic, and would behave exactly like <literal>P1</literal> so that <literal>g</literal>
would then be rejected.
</para>
<para>
In short, if you want GADT-like behaviour for pattern synonyms,
then (unlike unlike concrete data constructors like <literal>S1</literal>)
you must write its type with explicit provided equalities.
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For a concrete data constructor like <literal>S1</literal> you can write
its type signature as either <literal>S1 :: Bool -> S Bool</literal> or
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<literal>S1 :: (b~Bool) => Bool -> S b</literal>; the two are equivalent.
Not so for pattern synonyms: the two forms are different, in order to
distinguish the two cases above. (See <ulink url="https://ghc.haskell.org/trac/ghc/ticket/9953">Trac #9953</ulink> for
discussion of this choice.)
</para></listitem>
</itemizedlist>
</para>
</sect3>
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<sect3><title>Matching of pattern synonyms</title>
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<para>
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A pattern synonym occurrence in a pattern is evaluated by first
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matching against the pattern synonym itself, and then on the argument
patterns. For example, in the following program, <literal>f</literal>
and <literal>f'</literal> are equivalent:
</para>

<programlisting>
pattern Pair x y &lt;- [x, y]

f (Pair True True) = True
f _                = False

f' [x, y] | True &lt;- x, True &lt;- y = True
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f' _                              = False
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</programlisting>

<para>
  Note that the strictness of <literal>f</literal> differs from that
  of <literal>g</literal> defined below:
<programlisting>
g [True, True] = True
g _            = False

*Main> f (False:undefined)
*** Exception: Prelude.undefined
*Main> g (False:undefined)
False
</programlisting>
</para>
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</sect3>
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</sect2>

    <!-- ===================== n+k patterns ===================  -->

<sect2 id="n-k-patterns">
<title>n+k patterns</title>
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<indexterm><primary><option>-XNPlusKPatterns</option></primary></indexterm>
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<para>
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<literal>n+k</literal> pattern support is disabled by default. To enable
it, you can use the <option>-XNPlusKPatterns</option> flag.
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</para>

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</sect2>

    <!-- ===================== Traditional record syntax ===================  -->

<sect2 id="traditional-record-syntax">
<title>Traditional record syntax</title>
<indexterm><primary><option>-XNoTraditionalRecordSyntax</option></primary></indexterm>

<para>
Traditional record syntax, such as <literal>C {f = x}</literal>, is enabled by default.
To disable it, you can use the <option>-XNoTraditionalRecordSyntax</option> flag.
</para>

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</sect2>

    <!-- ===================== Recursive do-notation ===================  -->

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<sect2 id="recursive-do-notation">
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<title>The recursive do-notation
</title>

<para>
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    The do-notation of Haskell 98 does not allow <emphasis>recursive bindings</emphasis>,
    that is, the variables bound in a do-expression are visible only in the textually following
    code block. Compare this to a let-expression, where bound variables are visible in the entire binding
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    group.
</para>
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<para>
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    It turns out that such recursive bindings do indeed make sense for a variety of monads, but
    not all. In particular, recursion in this sense requires a fixed-point operator for the underlying
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    monad, captured by the <literal>mfix</literal> method of the <literal>MonadFix</literal> class, defined in <literal>Control.Monad.Fix</literal> as follows:
<programlisting>
class Monad m => MonadFix m where
   mfix :: (a -> m a) -> m a
</programlisting>
    Haskell's
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    <literal>Maybe</literal>, <literal>[]</literal> (list), <literal>ST</literal> (both strict and lazy versions),
    <literal>IO</literal>, and many other monads have <literal>MonadFix</literal> instances. On the negative
    side, the continuation monad, with the signature <literal>(a -> r) -> r</literal>, does not.
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</para>
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<para>
    For monads that do belong to the <literal>MonadFix</literal> class, GHC provides
    an extended version of the do-notation that allows recursive bindings.
    The <option>-XRecursiveDo</option> (language pragma: <literal>RecursiveDo</literal>)
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    provides the necessary syntactic support, introducing the keywords <literal>mdo</literal> and
    <literal>rec</literal> for higher and lower levels of the notation respectively. Unlike
    bindings in a <literal>do</literal> expression, those introduced by <literal>mdo</literal> and <literal>rec</literal>
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    are recursively defined, much like in an ordinary let-expression. Due to the new
    keyword <literal>mdo</literal>, we also call this notation the <emphasis>mdo-notation</emphasis>.
</para>

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<para>
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    Here is a simple (albeit contrived) example:
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<programlisting>
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{-# LANGUAGE RecursiveDo #-}
justOnes = mdo { xs &lt;- Just (1:xs)
               ; return (map negate xs) }
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</programlisting>
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or equivalently
<programlisting>
{-# LANGUAGE RecursiveDo #-}
justOnes = do { rec { xs &lt;- Just (1:xs) }
              ; return (map negate xs) }
</programlisting>
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As you can guess <literal>justOnes</literal> will evaluate to <literal>Just [-1,-1,-1,...</literal>.
</para>
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<para>
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   GHC's implementation the mdo-notation closely follows the original translation as described in the paper
   <ulink url="https://sites.google.com/site/leventerkok/recdo.pdf">A recursive do for Haskell</ulink>, which
   in turn is based on the work <ulink url="http://sites.google.com/site/leventerkok/erkok-thesis.pdf">Value Recursion
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   in Monadic Computations</ulink>. Furthermore, GHC extends the syntax described in the former paper
   with a lower level syntax flagged by the <literal>rec</literal> keyword, as we describe next.
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</para>

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<sect3>
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<title>Recursive binding groups</title>

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<para>
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    The flag <option>-XRecursiveDo</option> also introduces a new keyword <literal>rec</literal>, which wraps a
    mutually-recursive group of monadic statements inside a <literal>do</literal> expression, producing a single statement.
    Similar to a <literal>let</literal> statement inside a <literal>do</literal>, variables bound in
    the <literal>rec</literal> are visible throughout the <literal>rec</literal> group, and below it.  For example, compare
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<programlisting>
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    do { a &lt;- getChar            do { a &lt;- getChar
       ; let { r1 = f a r2          ; rec { r1 &lt;- f a r2
       ;     ; r2 = g r1 }          ;     ; r2 &lt;- g r1 }
       ; return (r1 ++ r2) }        ; return (r1 ++ r2) }
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</programlisting>
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    In both cases, <literal>r1</literal> and <literal>r2</literal> are available both throughout
    the <literal>let</literal> or <literal>rec</literal> block, and in the statements that follow it.
    The difference is that <literal>let</literal> is non-monadic, while <literal>rec</literal> is monadic.
    (In Haskell <literal>let</literal> is really <literal>letrec</literal>, of course.)
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</para>
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<para>
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    The semantics of <literal>rec</literal> is fairly straightforward. Whenever GHC finds a <literal>rec</literal>
    group, it will compute its set of bound variables, and will introduce an appropriate call
    to the underlying monadic value-recursion operator <literal>mfix</literal>, belonging to the
    <literal>MonadFix</literal> class. Here is an example:
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<programlisting>
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rec { b &lt;- f a c     ===>    (b,c) &lt;- mfix (\ ~(b,c) -> do { b &lt;- f a c
    ; c &lt;- f b a }                                         ; c &lt;- f b a
                                                           ; return (b,c) })
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</programlisting>
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   As usual, the meta-variables <literal>b</literal>, <literal>c</literal> etc., can be arbitrary patterns.
   In general, the statement <literal>rec <replaceable>ss</replaceable></literal> is desugared to the statement
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<programli