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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 are all enabled by options; by default GHC
understands only plain Haskell 98.
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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>These flags control what variation of the language are
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    permitted.  Leaving out all of them gives you standard Haskell
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    98.</para>
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    <para>NB. turning on an option that enables special syntax
    <emphasis>might</emphasis> cause working Haskell 98 code to fail
    to compile, perhaps because it uses a variable name which has
    become a reserved word.  So, together with each option below, we
    list the special syntax which is enabled by this option.  We use
    notation and nonterminal names from the Haskell 98 lexical syntax
    (see the Haskell 98 Report).  There are two classes of special
    syntax:</para>

    <itemizedlist>
      <listitem>
	<para>New reserved words and symbols: character sequences
        which are no longer available for use as identifiers in the
        program.</para>
      </listitem>
      <listitem>
	<para>Other special syntax: sequences of characters that have
	a different meaning when this particular option is turned
	on.</para>
      </listitem>
    </itemizedlist>

    <para>We are only listing syntax changes here that might affect
    existing working programs (i.e. "stolen" syntax).  Many of these
    extensions will also enable new context-free syntax, but in all
    cases programs written to use the new syntax would not be
    compilable without the option enabled.</para>

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    <variablelist>
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      <varlistentry>
	<term><option>-fglasgow-exts</option>:</term>
	<indexterm><primary><option>-fglasgow-exts</option></primary></indexterm>
	<listitem>
	  <para>This simultaneously enables all of the extensions to
          Haskell 98 described in <xref
          linkend="ghc-language-features">, except where otherwise
          noted. </para>
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	  <para>New reserved words: <literal>forall</literal> (only in
	  types), <literal>mdo</literal>.</para>

	  <para>Other syntax stolen:
	      <replaceable>varid</replaceable>{<literal>&num;</literal>},
	      <replaceable>char</replaceable><literal>&num;</literal>,	    
	      <replaceable>string</replaceable><literal>&num;</literal>,    
	      <replaceable>integer</replaceable><literal>&num;</literal>,    
	      <replaceable>float</replaceable><literal>&num;</literal>,    
	      <replaceable>float</replaceable><literal>&num;&num;</literal>,    
	      <literal>(&num;</literal>, <literal>&num;)</literal>,	    
	      <literal>|)</literal>, <literal>{|</literal>.</para>
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	</listitem>
      </varlistentry>
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      <varlistentry>
	<term><option>-ffi</option> and <option>-fffi</option>:</term>
	<indexterm><primary><option>-ffi</option></primary></indexterm>
	<indexterm><primary><option>-fffi</option></primary></indexterm>
	<listitem>
	  <para>This option enables the language extension defined in the
	  Haskell 98 Foreign Function Interface Addendum plus deprecated
	  syntax of previous versions of the FFI for backwards
	  compatibility.</para> 
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	  <para>New reserved words: <literal>foreign</literal>.</para>
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	</listitem>
      </varlistentry>

      <varlistentry>
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	<term><option>-fno-monomorphism-restriction</option>:</term>
	<indexterm><primary><option>-fno-monomorphism-restriction</option></primary></indexterm>
	<listitem>
	  <para> Switch off the Haskell 98 monomorphism restriction.
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          Independent of the <option>-fglasgow-exts</option>
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          flag. </para>
	</listitem>
      </varlistentry>
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      <varlistentry>
	<term><option>-fallow-overlapping-instances</option></term>
	<term><option>-fallow-undecidable-instances</option></term>
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	<term><option>-fallow-incoherent-instances</option></term>
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	<term><option>-fcontext-stack</option></term>
	<indexterm><primary><option>-fallow-overlapping-instances</option></primary></indexterm>
	<indexterm><primary><option>-fallow-undecidable-instances</option></primary></indexterm>
	<indexterm><primary><option>-fcontext-stack</option></primary></indexterm>
	<listitem>
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	  <para> See <xref LinkEnd="instance-decls">.  Only relevant
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          if you also use <option>-fglasgow-exts</option>.</para>
	</listitem>
      </varlistentry>
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      <varlistentry>
	<term><option>-finline-phase</option></term>
	<indexterm><primary><option>-finline-phase</option></primary></indexterm>
	<listitem>
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	  <para>See <xref LinkEnd="rewrite-rules">.  Only relevant if
          you also use <option>-fglasgow-exts</option>.</para>
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	</listitem>
      </varlistentry>
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      <varlistentry>
	<term><option>-farrows</option></term>
	<indexterm><primary><option>-farrows</option></primary></indexterm>
	<listitem>
	  <para>See <xref LinkEnd="arrow-notation">.  Independent of
          <option>-fglasgow-exts</option>.</para>
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	  <para>New reserved words/symbols: <literal>rec</literal>,
	  <literal>proc</literal>, <literal>-&lt;</literal>,
	  <literal>&gt;-</literal>, <literal>-&lt;&lt;</literal>,
	  <literal>&gt;&gt;-</literal>.</para>

	  <para>Other syntax stolen: <literal>(|</literal>,
	  <literal>|)</literal>.</para>
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	</listitem>
      </varlistentry>

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      <varlistentry>
	<term><option>-fgenerics</option></term>
	<indexterm><primary><option>-fgenerics</option></primary></indexterm>
	<listitem>
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	  <para>See <xref LinkEnd="generic-classes">.  Independent of
          <option>-fglasgow-exts</option>.</para>
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	</listitem>
      </varlistentry>

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      <varlistentry>
	<term><option>-fno-implicit-prelude</option></term>
	<listitem>
	  <para><indexterm><primary>-fno-implicit-prelude
          option</primary></indexterm> GHC normally imports
          <filename>Prelude.hi</filename> files for you.  If you'd
          rather it didn't, then give it a
          <option>-fno-implicit-prelude</option> option.  The idea is
          that you can then import a Prelude of your own.  (But don't
          call it <literal>Prelude</literal>; the Haskell module
          namespace is flat, and you must not conflict with any
          Prelude module.)</para>

	  <para>Even though you have not imported the Prelude, most of
          the built-in syntax still refers to the built-in Haskell
          Prelude types and values, as specified by the Haskell
          Report.  For example, the type <literal>[Int]</literal>
          still means <literal>Prelude.[] Int</literal>; tuples
          continue to refer to the standard Prelude tuples; the
          translation for list comprehensions continues to use
          <literal>Prelude.map</literal> etc.</para>

	  <para>However, <option>-fno-implicit-prelude</option> does
	  change the handling of certain built-in syntax: see <xref
	  LinkEnd="rebindable-syntax">.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fth</option></term>
	<listitem>
	  <para>Enables Template Haskell (see <xref
	  linkend="template-haskell">).  Currently also implied by
	  <option>-fglasgow-exts</option>.</para>
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	  <para>Syntax stolen: <literal>[|</literal>,
	  <literal>[e|</literal>, <literal>[p|</literal>,
	  <literal>[d|</literal>, <literal>[t|</literal>,
	  <literal>$(</literal>,
	  <literal>$<replaceable>varid</replaceable></literal>.</para>
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	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-fimplicit-params</option></term>
	<listitem>
	  <para>Enables implicit parameters (see <xref
	  linkend="implicit-parameters">).  Currently also implied by 
	  <option>-fglasgow-exts</option>.</para>
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	  <para>Syntax stolen:
	  <literal>?<replaceable>varid</replaceable></literal>,
	  <literal>%<replaceable>varid</replaceable></literal>.</para>
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	</listitem>
      </varlistentry>
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    </variablelist>
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  </sect1>
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<!-- UNBOXED TYPES AND PRIMITIVE OPERATIONS -->
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<!--    included from primitives.sgml  -->
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<!-- &primitives; -->
<sect1 id="primitives">
  <title>Unboxed types and primitive operations</title>

<para>GHC is built on a raft of primitive data types and operations.
While you really can use this stuff to write fast code,
  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>

<para>We do not currently have good, up-to-date documentation about the
primitives, perhaps because they are mainly intended for internal use.
There used to be a long section about them here in the User Guide, but it
became out of date, and wrong information is worse than none.</para>

<para>The Real Truth about what primitive types there are, and what operations
work over those types, is held in the file
<filename>fptools/ghc/compiler/prelude/primops.txt</filename>.
This file is used directly to generate GHC's primitive-operation definitions, so
it is always correct!  It is also intended for processing into text.</para>

<para> Indeed,
the result of such processing is part of the description of the 
 <ulink
      url="http://haskell.cs.yale.edu/ghc/docs/papers/core.ps.gz">External
	 Core language</ulink>.
So that document is a good place to look for a type-set version.
We would be very happy if someone wanted to volunteer to produce an SGML
back end to the program that processes <filename>primops.txt</filename> so that
we could include the results here in the User Guide.</para>

<para>What follows here is a brief summary of some main points.</para>
  
<sect2 id="glasgow-unboxed">
<title>Unboxed types
</title>

<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
bottom.  We use the convention that primitive types, values, and
operations have a <literal>&num;</literal> suffix.
</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.
</para>

<para>
There are some restrictions on the use of primitive types, the main
one being that you can't pass a primitive value to a polymorphic
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>

<para>
Nevertheless, 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.
</para>

</sect2>

<sect2 id="unboxed-tuples">
<title>Unboxed Tuples
</title>

<para>
Unboxed tuples aren't really exported by <literal>GHC.Exts</literal>,
they're available by default with <option>-fglasgow-exts</option>.  An
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>

<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
of the primitive operations listed in this section return unboxed
tuples.
</para>

<para>
There are some pretty stringent restrictions on the use of unboxed tuples:
</para>

<para>

<itemizedlist>
<listitem>

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

</para>
</listitem>
<listitem>

<para>
 Unboxed tuples may only be constructed as the direct result of
a function, and may only be deconstructed with a <literal>case</literal> expression.
eg. the following are valid:


<programlisting>
f x y = (# x+1, y-1 #)
g x = case f x x of { (# a, b #) -&#62; a + b }
</programlisting>


but the following are invalid:


<programlisting>
f x y = g (# x, y #)
g (# x, y #) = x + y
</programlisting>


</para>
</listitem>
<listitem>

<para>
 No variable can have an unboxed tuple type.  This is illegal:


<programlisting>
f :: (# Int, Int #) -&#62; (# Int, Int #)
f x = x
</programlisting>


because <literal>x</literal> has an unboxed tuple type.

</para>
</listitem>

</itemizedlist>

</para>

<para>
Note: we may relax some of these restrictions in the future.
</para>

<para>
The <literal>IO</literal> and <literal>ST</literal> monads use unboxed
tuples to avoid unnecessary allocation during sequences of operations.
</para>

</sect2>
</sect1>

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

<sect1 id="syntax-extns">
<title>Syntactic extensions</title>
 
    <!-- ====================== HIERARCHICAL MODULES =======================  -->

    <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
      linkend="search-path">.</para>
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      <para>GHC comes with a large collection of libraries arranged
      hierarchically; see the accompanying library documentation.
      There is an ongoing project to create and maintain a stable set
      of <quote>core</quote> libraries used by several Haskell
      compilers, and the libraries that GHC comes with represent the
      current status of that project.  For more details, see <ulink
      url="http://www.haskell.org/~simonmar/libraries/libraries.html">Haskell
      Libraries</ulink>.</para>

    </sect2>

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

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

<para>
<indexterm><primary>Pattern guards (Glasgow extension)</primary></indexterm>
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.)
</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,
where <VarName>v</VarName> is the value that the key maps to.  Now consider the following definition:
</para>

<programlisting>
clunky env var1 var2 | ok1 && ok2 = val1 + val2
| 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>
The auxiliary functions are 
</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>
What is <function>clunky</function> doing? The guard <literal>ok1 &&
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
returned values to <VarName>val1</VarName> and <VarName>val2</VarName>
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>
clunky env var1 var1 = case lookup env var1 of
  Nothing -&gt; fail
  Just val1 -&gt; case lookup env var2 of
    Nothing -&gt; fail
    Just val2 -&gt; val1 + val2
where
  fail = val1 + val2
</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
the cases we want to consider, one at a time, independently of each other. 
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
tends to become more and more indented. 
</para>

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

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

<para>
The semantics should be clear enough.  The qualifers are matched in order. 
For a <literal>&lt;-</literal> qualifier, which I call a pattern guard, the
right hand side is evaluated and matched against the pattern on the left. 
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>
f x | [y] <- x
    , y > 3
    , Just z <- h y
    = ...
</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>
</sect2>

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

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

<para> The recursive do-notation (also known as mdo-notation) is implemented as described in
"A recursive do for Haskell",
Levent Erkok, John Launchbury",
Haskell Workshop 2002, pages: 29-37. Pittsburgh, Pennsylvania. 
</para>
<para>
The do-notation of Haskell 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
group. It turns out that several applications can benefit from recursive bindings in
the do-notation, and this extension provides the necessary syntactic support.
</para>
<para>
Here is a simple (yet contrived) example:
</para>
<programlisting>
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import Control.Monad.Fix

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justOnes = mdo xs <- Just (1:xs)
               return xs
</programlisting>
<para>
As you can guess <literal>justOnes</literal> will evaluate to <literal>Just [1,1,1,...</literal>.
</para>

<para>
The Control.Monad.Fix library introduces the <literal>MonadFix</literal> class. It's definition is:
</para>
<programlisting>
class Monad m => MonadFix m where
   mfix :: (a -> m a) -> m a
</programlisting>
<para>
The function <literal>mfix</literal>
dictates how the required recursion operation should be performed. If recursive bindings are required for a monad,
then that monad must be declared an instance of the <literal>MonadFix</literal> class.
For details, see the above mentioned reference.
</para>
<para>
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The following instances of <literal>MonadFix</literal> are automatically provided: List, Maybe, IO. 
Furthermore, the Control.Monad.ST and Control.Monad.ST.Lazy modules provide the instances of the MonadFix class 
for Haskell's internal state monad (strict and lazy, respectively).
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</para>
<para>
There are three important points in using the recursive-do notation:
<itemizedlist>
<listitem><para>
The recursive version of the do-notation uses the keyword <literal>mdo</literal> (rather
than <literal>do</literal>).
</para></listitem>

<listitem><para>
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You should <literal>import Control.Monad.Fix</literal>.
(Note: Strictly speaking, this import is required only when you need to refer to the name
<literal>MonadFix</literal> in your program, but the import is always safe, and the programmers
are encouraged to always import this module when using the mdo-notation.)
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</para></listitem>

<listitem><para>
As with other extensions, ghc should be given the flag <literal>-fglasgow-exts</literal>
</para></listitem>
</itemizedlist>
</para>

<para>
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The web page: <ulink url="http://www.cse.ogi.edu/PacSoft/projects/rmb">http://www.cse.ogi.edu/PacSoft/projects/rmb</ulink>
contains up to date information on recursive monadic bindings.
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</para>

<para>
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Historical note: The old implementation of the mdo-notation (and most
of the existing documents) used the name
<literal>MonadRec</literal> for the class and the corresponding library.
This name is not supported by GHC.
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</para>

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</sect2>


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   <!-- ===================== PARALLEL LIST COMPREHENSIONS ===================  -->

  <sect2 id="parallel-list-comprehensions">
    <title>Parallel List Comprehensions</title>
    <indexterm><primary>list comprehensions</primary><secondary>parallel</secondary>
    </indexterm>
    <indexterm><primary>parallel list comprehensions</primary>
    </indexterm>

    <para>Parallel list comprehensions are a natural extension to list
    comprehensions.  List comprehensions can be thought of as a nice
    syntax for writing maps and filters.  Parallel comprehensions
    extend this to include the zipWith family.</para>

    <para>A parallel list comprehension has multiple independent
    branches of qualifier lists, each separated by a `|' symbol.  For
    example, the following zips together two lists:</para>

<programlisting>
   [ (x, y) | x <- xs | y <- ys ] 
</programlisting>

    <para>The behavior of parallel list comprehensions follows that of
    zip, in that the resulting list will have the same length as the
    shortest branch.</para>

    <para>We can define parallel list comprehensions by translation to
    regular comprehensions.  Here's the basic idea:</para>

    <para>Given a parallel comprehension of the form: </para>

<programlisting>
   [ e | p1 <- e11, p2 <- e12, ... 
       | q1 <- e21, q2 <- e22, ... 
       ... 
   ] 
</programlisting>

    <para>This will be translated to: </para>

<programlisting>
   [ e | ((p1,p2), (q1,q2), ...) <- zipN [(p1,p2) | p1 <- e11, p2 <- e12, ...] 
                                         [(q1,q2) | q1 <- e21, q2 <- e22, ...] 
                                         ... 
   ] 
</programlisting>

    <para>where `zipN' is the appropriate zip for the given number of
    branches.</para>

  </sect2>

<sect2 id="rebindable-syntax">
<title>Rebindable syntax</title>


      <para>GHC allows most kinds of built-in syntax to be rebound by
      the user, to facilitate replacing the <literal>Prelude</literal>
      with a home-grown version, for example.</para>

            <para>You may want to define your own numeric class
            hierarchy.  It completely defeats that purpose if the
            literal "1" means "<literal>Prelude.fromInteger
            1</literal>", which is what the Haskell Report specifies.
            So the <option>-fno-implicit-prelude</option> flag causes
            the following pieces of built-in syntax to refer to
            <emphasis>whatever is in scope</emphasis>, not the Prelude
            versions:</para>

	    <itemizedlist>
	      <listitem>
		<para>Integer and fractional literals mean
                "<literal>fromInteger 1</literal>" and
                "<literal>fromRational 3.2</literal>", not the
                Prelude-qualified versions; both in expressions and in
                patterns. </para>
		<para>However, the standard Prelude <literal>Eq</literal> class
		is still used for the equality test necessary for literal patterns.</para>
	      </listitem>

	      <listitem>
		<para>Negation (e.g. "<literal>- (f x)</literal>")
		means "<literal>negate (f x)</literal>" (not
		<literal>Prelude.negate</literal>).</para>
	      </listitem>

	      <listitem>
		<para>In an n+k pattern, the standard Prelude
                <literal>Ord</literal> class is still used for comparison,
                but the necessary subtraction uses whatever
                "<literal>(-)</literal>" is in scope (not
                "<literal>Prelude.(-)</literal>").</para>
	      </listitem>

	      <listitem>
	  <para>"Do" notation is translated using whatever
	      functions <literal>(>>=)</literal>,
	      <literal>(>>)</literal>, <literal>fail</literal>, and
	      <literal>return</literal>, are in scope (not the Prelude
	      versions).  List comprehensions, and parallel array
	      comprehensions, are unaffected.  </para></listitem>
	    </itemizedlist>

	     <para>Be warned: this is an experimental facility, with fewer checks than
	     usual.  In particular, it is essential that the functions GHC finds in scope
	     must have the appropriate types, namely:
	     <screen>
	        fromInteger  :: forall a. (...) => Integer  -> a
		fromRational :: forall a. (...) => Rational -> a
		negate       :: forall a. (...) => a -> a
		(-)          :: forall a. (...) => a -> a -> a
		(>>=)	     :: forall m a. (...) => m a -> (a -> m b) -> m b
		(>>)	     :: forall m a. (...) => m a -> m b -> m b
		return	     :: forall m a. (...) => a      -> m a
		fail	     :: forall m a. (...) => String -> m a
	     </screen>
	     (The (...) part can be any context including the empty context; that part 
	     is up to you.)
	     If the functions don't have the right type, very peculiar things may 
	     happen.  Use <literal>-dcore-lint</literal> to
	     typecheck the desugared program.  If Core Lint is happy you should be all right.</para>

</sect2>
</sect1>

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<!-- TYPE SYSTEM EXTENSIONS -->
<sect1 id="type-extensions">
<title>Type system extensions</title>
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<sect2>
<title>Data types and type synonyms</title>

<sect3 id="nullary-types">
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<title>Data types with no constructors</title>

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<para>With the <option>-fglasgow-exts</option> flag, GHC lets you declare
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a data type with no constructors.  For example:</para>
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<programlisting>
  data S      -- S :: *
  data T a    -- T :: * -> *
</programlisting>
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<para>Syntactically, the declaration lacks the "= constrs" part.  The 
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type can be parameterised over types of any kind, but if the kind is
not <literal>*</literal> then an explicit kind annotation must be used
(see <xref linkend="sec-kinding">).</para>
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<para>Such data types have only one value, namely bottom.
Nevertheless, they can be useful when defining "phantom types".</para>
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</sect3>
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<sect3 id="infix-tycons">
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<title>Infix type constructors</title>

<para>
GHC allows type constructors to be operators, and to be written infix, very much 
like expressions.  More specifically:
<itemizedlist>
<listitem><para>
  A type constructor can be an operator, beginning with a colon; e.g. <literal>:*:</literal>.
  The lexical syntax is the same as that for data constructors.
  </para></listitem>
<listitem><para>
  Types can be written infix.  For example <literal>Int :*: Bool</literal>.  
  </para></listitem>
<listitem><para>
  Back-quotes work
  as for expressions, both for type constructors and type variables;  e.g. <literal>Int `Either` Bool</literal>, or
  <literal>Int `a` Bool</literal>.  Similarly, parentheses work the same; e.g.  <literal>(:*:) Int Bool</literal>.
  </para></listitem>
<listitem><para>
  Fixities may be declared for type constructors just as for data constructors.  However,
  one cannot distinguish between the two in a fixity declaration; a fixity declaration
  sets the fixity for a data constructor and the corresponding type constructor.  For example:
<screen>
  infixl 7 T, :*:
</screen>
  sets the fixity for both type constructor <literal>T</literal> and data constructor <literal>T</literal>,
  and similarly for <literal>:*:</literal>.
  <literal>Int `a` Bool</literal>.
  </para></listitem>
<listitem><para>
  Function arrow is <literal>infixr</literal> with fixity 0.  (This might change; I'm not sure what it should be.)
  </para></listitem>
<listitem><para>
  Data type and type-synonym declarations can be written infix.  E.g.
<screen>
  data a :*: b = Foo a b
  type a :+: b = Either a b
</screen>
  </para></listitem>
<listitem><para>
  The only thing that differs between operators in types and operators in expressions is that
  ordinary non-constructor operators, such as <literal>+</literal> and <literal>*</literal>
  are not allowed in types. Reason: the uniform thing to do would be to make them type
  variables, but that's not very useful.  A less uniform but more useful thing would be to
  allow them to be type <emphasis>constructors</emphasis>.  But that gives trouble in export
  lists.  So for now we just exclude them.
  </para></listitem>

</itemizedlist>
</para>
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</sect3>
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<sect3 id="type-synonyms">
<title>Liberalised type synonyms</title>
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<para>
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Type synonmys are like macros at the type level, and
GHC does validity checking on types <emphasis>only after expanding type synonyms</emphasis>.
That means that GHC can be very much more liberal about type synonyms than Haskell 98:
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<itemizedlist>
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<listitem> <para>You can write a <literal>forall</literal> (including overloading)
in a type synonym, thus:
<programlisting>
  type Discard a = forall b. Show b => a -> b -> (a, String)
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  f :: Discard a
  f x y = (x, show y)
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  g :: Discard Int -> (Int,Bool)    -- A rank-2 type
  g f = f Int True
</programlisting>
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</para>
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</listitem>
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<listitem><para>
You can write an unboxed tuple in a type synonym:
<programlisting>
  type Pr = (# Int, Int #)
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  h :: Int -> Pr
  h x = (# x, x #)
</programlisting>
</para></listitem>

<listitem><para>
You can apply a type synonym to a forall type:
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<programlisting>
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  type Foo a = a -> a -> Bool
 
  f :: Foo (forall b. b->b)
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</programlisting>
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After expanding the synonym, <literal>f</literal> has the legal (in GHC) type:
<programlisting>
  f :: (forall b. b->b) -> (forall b. b->b) -> Bool
</programlisting>
</para></listitem>
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<listitem><para>
You can apply a type synonym to a partially applied type synonym:
<programlisting>
  type Generic i o = forall x. i x -> o x
  type Id x = x
  
  foo :: Generic Id []
</programlisting>
After epxanding the synonym, <literal>foo</literal> has the legal (in GHC) type:
<programlisting>
  foo :: forall x. x -> [x]
</programlisting>
</para></listitem>
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</itemizedlist>
</para>
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<para>
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GHC currently does kind checking before expanding synonyms (though even that
could be changed.)
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</para>
<para>
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After expanding type synonyms, GHC does validity checking on types, looking for
the following mal-formedness which isn't detected simply by kind checking:
<itemizedlist>
<listitem><para>
Type constructor applied to a type involving for-alls.
</para></listitem>
<listitem><para>
Unboxed tuple on left of an arrow.
</para></listitem>
<listitem><para>
Partially-applied type synonym.
</para></listitem>
</itemizedlist>
So, for example,
this will be rejected:
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<programlisting>
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  type Pr = (# Int, Int #)
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  h :: Pr -> Int
  h x = ...
</programlisting>
because GHC does not allow  unboxed tuples on the left of a function arrow.
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</sect3>
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<sect3 id="existential-quantification">
<title>Existentially quantified data constructors
</title>
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<para>
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The idea of using existential quantification in data type declarations
was suggested by Laufer (I believe, thought doubtless someone will
correct me), and implemented in Hope+. It's been in Lennart
Augustsson's <Command>hbc</Command> Haskell compiler for several years, and
proved very useful.  Here's the idea.  Consider the declaration:
</para>
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<para>
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<programlisting>
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  data Foo = forall a. MkFoo a (a -> Bool)
           | Nil
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</programlisting>
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</para>
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<para>
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The data type <literal>Foo</literal> has two constructors with types:
</para>
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<para>
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<programlisting>
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  MkFoo :: forall a. a -> (a -> Bool) -> Foo
  Nil   :: Foo
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</programlisting>
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</para>
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<para>
Notice that the type variable <literal>a</literal> in the type of <function>MkFoo</function>
does not appear in the data type itself, which is plain <literal>Foo</literal>.
For example, the following expression is fine:
</para>
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<para>
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<programlisting>
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  [MkFoo 3 even, MkFoo 'c' isUpper] :: [Foo]
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</programlisting>
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</para>
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<para>
Here, <literal>(MkFoo 3 even)</literal> packages an integer with a function
<function>even</function> that maps an integer to <literal>Bool</literal>; and <function>MkFoo 'c'
isUpper</function> packages a character with a compatible function.  These
two things are each of type <literal>Foo</literal> and can be put in a list.
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</para>
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<para>
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What can we do with a value of type <literal>Foo</literal>?.  In particular,
what happens when we pattern-match on <function>MkFoo</function>?
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</para>
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<para>
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<programlisting>
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  f (MkFoo val fn) = ???
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</programlisting>
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</para>
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<para>
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Since all we know about <literal>val</literal> and <function>fn</function> is that they
are compatible, the only (useful) thing we can do with them is to
apply <function>fn</function> to <literal>val</literal> to get a boolean.  For example:
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</para>
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<para>
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<programlisting>
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  f :: Foo -> Bool
  f (MkFoo val fn) = fn val
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</programlisting>
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</para>
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<para>
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What this allows us to do is to package heterogenous values
together with a bunch of functions that manipulate them, and then treat
that collection of packages in a uniform manner.  You can express
quite a bit of object-oriented-like programming this way.
</para>
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<sect4 id="existential">
<title>Why existential?
</title>
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<para>
What has this to do with <emphasis>existential</emphasis> quantification?
Simply that <function>MkFoo</function> has the (nearly) isomorphic type
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</para>
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<para>
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<programlisting>
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  MkFoo :: (exists a . (a, a -> Bool)) -> Foo
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</programlisting>
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</para>
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<para>
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But Haskell programmers can safely think of the ordinary
<emphasis>universally</emphasis> quantified type given above, thereby avoiding
adding a new existential quantification construct.
</para>
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</sect4>
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<sect4>
<title>Type classes</title>
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<para>
An easy extension (implemented in <Command>hbc</Command>) is to allow
arbitrary contexts before the constructor.  For example:
</para>
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<para>
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<programlisting>
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data Baz = forall a. Eq a => Baz1 a a
         | forall b. Show b => Baz2 b (b -> b)
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</programlisting>
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</para>
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<para>
The two constructors have the types you'd expect:
</para>
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<para>
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<programlisting>
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Baz1 :: forall a. Eq a => a -> a -> Baz
Baz2 :: forall b. Show b => b -> (b -> b) -> Baz
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</programlisting>
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</para>
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<para>
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But when pattern matching on <function>Baz1</function> the matched values can be compared
for equality, and when pattern matching on <function>Baz2</function> the first matched
value can be converted to a string (as well as applying the function to it).
So this program is legal:
</para>
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<para>
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<programlisting>
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  f :: Baz -> String
  f (Baz1 p q) | p == q    = "Yes"
               | otherwise = "No"
  f (Baz2 v fn)            = show (fn v)
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</programlisting>
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</para>
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<para>
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Operationally, in a dictionary-passing implementation, the
constructors <function>Baz1</function> and <function>Baz2</function> must store the
dictionaries for <literal>Eq</literal> and <literal>Show</literal> respectively, and
extract it on pattern matching.
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</para>
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<para>
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Notice the way that the syntax fits smoothly with that used for
universal quantification earlier.
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</para>
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</sect4>
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<sect4>
<title>Restrictions</title>
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<para>
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There are several restrictions on the ways in which existentially-quantified
constructors can be use.
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</para>
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<para>
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<itemizedlist>
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<listitem>
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<para>
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 When pattern matching, each pattern match introduces a new,
distinct, type for each existential type variable.  These types cannot
be unified with any other type, nor can they escape from the scope of
the pattern match.  For example, these fragments are incorrect:
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<programlisting>
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f1 (MkFoo a f) = a
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</programlisting>
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Here, the type bound by <function>MkFoo</function> "escapes", because <literal>a</literal>
is the result of <function>f1</function>.  One way to see why this is wrong is to
ask what type <function>f1</function> has:
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<programlisting>
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  f1 :: Foo -> a             -- Weird!
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</programlisting>
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What is this "<literal>a</literal>" in the result type? Clearly we don't mean
this:
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<programlisting>
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  f1 :: forall a. Foo -> a   -- Wrong!
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</programlisting>
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The original program is just plain wrong.  Here's another sort of error
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<programlisting>
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  f2 (Baz1 a b) (Baz1 p q) = a==q
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</programlisting>
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It's ok to say <literal>a==b</literal> or <literal>p==q</literal>, but
<literal>a==q</literal> is wrong because it equates the two distinct types arising
from the two <function>Baz1</function> constructors.
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</para>
</listitem>
<listitem>
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<para>
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You can't pattern-match on an existentially quantified
constructor in a <literal>let</literal> or <literal>where</literal> group of
bindings. So this is illegal:
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<programlisting>
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  f3 x = a==b where { Baz1 a b = x }
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</programlisting>
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Instead, use a <literal>case</literal> expression:
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<programlisting>
  f3 x = case x of Baz1 a b -> a==b
</programlisting>

In general, you can only pattern-match
on an existentially-quantified constructor in a <literal>case</literal> expression or
in the patterns of a function definition.

The reason for this restriction is really an implementation one.
Type-checking binding groups is already a nightmare without
existentials complicating the picture.  Also an existential pattern
binding at the top level of a module doesn't make sense, because it's
not clear how to prevent the existentially-quantified type "escaping".
So for now, there's a simple-to-state restriction.  We'll see how
annoying it is.

</para>
</listitem>
<listitem>

<para>
You can't use existential quantification for <literal>newtype</literal>
declarations.  So this is illegal:
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<programlisting>
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  newtype T = forall a. Ord a => MkT a
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</programlisting>
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Reason: a value of type <literal>T</literal> must be represented as a
pair of a dictionary for <literal>Ord t</literal> and a value of type
<literal>t</literal>.  That contradicts the idea that
<literal>newtype</literal> should have no concrete representation.
You can get just the same efficiency and effect by using
<literal>data</literal> instead of <literal>newtype</literal>.  If
there is no overloading involved, then there is more of a case for
allowing an existentially-quantified <literal>newtype</literal>,
because the <literal>data</literal> version does carry an
implementation cost, but single-field existentially quantified
constructors aren't much use.  So the simple restriction (no
existential stuff on <literal>newtype</literal>) stands, unless there
are convincing reasons to change it.
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</para>
</listitem>
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<para>
 You can't use <literal>deriving</literal> to define instances of a
data type with existentially quantified data constructors.

Reason: in most cases it would not make sense. For example:&num;

<programlisting>
data T = forall a. MkT [a] deriving( Eq )
</programlisting>

To derive <literal>Eq</literal> in the standard way we would need to have equality
between the single component of two <function>MkT</function> constructors:
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<programlisting>
instance Eq T where
  (MkT a) == (MkT b) = ???
</programlisting>
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But <VarName>a</VarName> and <VarName>b</VarName> have distinct types, and so can't be compared.
It's just about possible to imagine examples in which the derived instance
would make sense, but it seems altogether simpler simply to prohibit such
declarations.  Define your own instances!
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</para>
</listitem>
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</itemizedlist>
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</para>
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</sect4>
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</sect3>
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</sect2>
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<sect2 id="multi-param-type-classes">
<title>Class declarations</title>

<para>
This section documents GHC's implementation of multi-parameter type
classes.  There's lots of background in the paper <ULink
URL="http://research.microsoft.com/~simonpj/multi.ps.gz" >Type
classes: exploring the design space</ULink > (Simon Peyton Jones, Mark
Jones, Erik Meijer).
</para>
<para>
There are the following constraints on class declarations:
<OrderedList>
<listitem>

<para>
 <emphasis>Multi-parameter type classes are permitted</emphasis>. For example:


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<programlisting>
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  class Collection c a where
    union :: c a -> c a -> c a
    ...etc.
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</programlisting>
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</para>
</listitem>
<listitem>

<para>
 <emphasis>The class hierarchy must be acyclic</emphasis>.  However, the definition
of "acyclic" involves only the superclass relationships.  For example,
this is OK:
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<programlisting>
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  class C a where {
    op :: D b => a -> b -> b
  }

  class C a => D a where { ... }
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</programlisting>


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Here, <literal>C</literal> is a superclass of <literal>D</literal>, but it's OK for a
class operation <literal>op</literal> of <literal>C</literal> to mention <literal>D</literal>.  (It
would not be OK for <literal>D</literal> to be a superclass of <literal>C</literal>.)

</para>
</listitem>
<listitem>

<para>
 <emphasis>There are no restrictions on the context in a class declaration
(which introduces superclasses), except that the class hierarchy must
be acyclic</emphasis>.  So these class declarations are OK:
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<programlisting>
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  class Functor (m k) => FiniteMap m k where
    ...
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  class (Monad m, Monad (t m)) => Transform t m where
    lift :: m a -> (t m) a
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</programlisting>


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</para>
</listitem>

<listitem>

<para>
 <emphasis>All of the class type variables must be reachable (in the sense 
mentioned in <xref linkend="type-restrictions">)
from the free varibles of each method type
</emphasis>.  For example:
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<programlisting>
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  class Coll s a where
    empty  :: s
    insert :: s -> a -> s
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</programlisting>


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is not OK, because the type of <literal>empty</literal> doesn't mention
<literal>a</literal>.  This rule is a consequence of Rule 1(a), above, for
types, and has the same motivation.

Sometimes, offending class declarations exhibit misunderstandings.  For
example, <literal>Coll</literal> might be rewritten
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<programlisting>
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  class Coll s a where
    empty  :: s a
    insert :: s a -> a -> s a
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</programlisting>


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which makes the connection between the type of a collection of
<literal>a</literal>'s (namely <literal>(s a)</literal>) and the element type <literal>a</literal>.
Occasionally this really doesn't work, in which case you can split the
class like this:


<programlisting>
  class CollE s where
    empty  :: s

  class CollE s => Coll s a where
    insert :: s -> a -> s
</programlisting>
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</para>
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</listitem>

</OrderedList>
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</para>
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<sect3 id="class-method-types">
<title>Class method types</title>
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Haskell 98 prohibits class method types to mention constraints on the
class type variable, thus:
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<programlisting>
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  class Seq s a where
    fromList :: [a] -> s a
    elem     :: Eq a => a -> s a -> Bool
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</programlisting>
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The type of <literal>elem</literal> is illegal in Haskell 98, because it
contains the constraint <literal>Eq a</literal>, constrains only the 
class type variable (in this case <literal>a</literal>).
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</para>
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With the <option>-fglasgow-exts</option> GHC lifts this restriction.
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</para>

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</sect3>

</sect2>

<sect2 id="type-restrictions">
<title>Type signatures</title>

<sect3><title>The context of a type signature</title>
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Unlike Haskell 98, constraints in types do <emphasis>not</emphasis> have to be of
the form <emphasis>(class type-variable)</emphasis> or
<emphasis>(class (type-variable type-variable ...))</emphasis>.  Thus,
these type signatures are perfectly OK
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  g :: Eq [a] => ...
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  g :: Ord (T a ()) => ...
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</programlisting>
</para>
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GHC imposes the following restrictions on the constraints in a type signature.
Consider the type:
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<programlisting>
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(Here, we write the "foralls" explicitly, although the Haskell source
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language omits them; in Haskell 98, all the free type variables of an
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explicit source-language type signature are universally quantified,
except for the class type variables in a class declaration.  However,
in GHC, you can give the foralls if you want.  See <xref LinkEnd="universal-quantification">).
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<para>
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<OrderedList>
<listitem>

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<para>
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 <emphasis>Each universally quantified type variable
<literal>tvi</literal> must be reachable from <literal>type</literal></emphasis>.

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A type variable <literal>a</literal> is "reachable" if it it appears
in the same constraint as either a type variable free in in
<literal>type</literal>, or another reachable type variable.  
A value with a type that does not obey 
this reachability restriction cannot be used without introducing
ambiguity; that is why the type is rejected.
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Here, for example, is an illegal type:

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<programlisting>
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  forall a. Eq a => Int
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</programlisting>
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When a value with this type was used, the constraint <literal>Eq tv</literal>
would be introduced where <literal>tv</literal> is a fresh type variable, and
(in the dictionary-translation implementation) the value would be
applied to a dictionary for <literal>Eq tv</literal>.  The difficulty is that we
can never know which instance of <literal>Eq</literal> to use because we never
get any more information about <literal>tv</literal>.
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</para>
<para>
Note
that the reachability condition is weaker than saying that <literal>a</literal> is
functionally dependendent on a type variable free in
<literal>type</literal> (see <xref
linkend="functional-dependencies">).  The reason for this is there
might be a "hidden" dependency, in a superclass perhaps.  So
"reachable" is a conservative approximation to "functionally dependent".
For example, consider:
<programlisting>
  class C a b | a -> b where ...
  class C a b => D a b where ...
  f :: forall a b. D a b => a -> a
</programlisting>
This is fine, because in fact <literal>a</literal> does functionally determine <literal>b</literal>
but that is not immediately apparent from <literal>f</literal>'s type.
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</listitem>
<listitem>
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<para>
 <emphasis>Every constraint <literal>ci</literal> must mention at least one of the
universally quantified type variables <literal>tvi</literal></emphasis>.
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For example, this type is OK because <literal>C a b</literal> mentions the
universally quantified type variable <literal>b</literal>:
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<programlisting>
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</programlisting>
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The next type is illegal because the constraint <literal>Eq b</literal> does not
mention <literal>a</literal>: