glasgow_exts.xml 434 KB
Newer Older
1
<?xml version="1.0" encoding="iso-8859-1"?>
2 3 4
<para>
<indexterm><primary>language, GHC</primary></indexterm>
<indexterm><primary>extensions, GHC</primary></indexterm>
rrt's avatar
rrt committed
5
As with all known Haskell systems, GHC implements some extensions to
Ian Lynagh's avatar
Ian Lynagh committed
6 7 8
the language.  They can all be enabled or disabled by commandline flags
or language pragmas. By default GHC understands the most recent Haskell
version it supports, plus a handful of extensions.
9
</para>
rrt's avatar
rrt committed
10

11
<para>
12 13 14 15 16 17 18 19
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!
20
</para>
rrt's avatar
rrt committed
21

22
<para>
rrt's avatar
rrt committed
23
Before you get too carried away working at the lowest level (e.g.,
24
sloshing <literal>MutableByteArray&num;</literal>s around your
25
program), you may wish to check if there are libraries that provide a
26
&ldquo;Haskellised veneer&rdquo; over the features you want.  The
27 28
separate <ulink url="../libraries/index.html">libraries
documentation</ulink> describes all the libraries that come with GHC.
29
</para>
rrt's avatar
rrt committed
30

31
<!-- LANGUAGE OPTIONS -->
32 33
  <sect1 id="options-language">
    <title>Language options</title>
34

35 36 37 38 39 40
    <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>
41

42
    <para>The language option flags control what variation of the language are
Ian Lynagh's avatar
Ian Lynagh committed
43
    permitted.</para>
44

45 46
    <para>Language options can be controlled in two ways:
    <itemizedlist>
47 48
      <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>";
49 50 51 52 53 54
        (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>
55

56
    <para>The flag <option>-fglasgow-exts</option>
57
          <indexterm><primary><option>-fglasgow-exts</option></primary></indexterm>
58
	  is equivalent to enabling the following extensions:
59
          &what_glasgow_exts_does;
60
	    Enabling these options is the <emphasis>only</emphasis>
Simon Marlow's avatar
Simon Marlow committed
61
	    effect of <option>-fglasgow-exts</option>.
62
          We are trying to move away from this portmanteau flag,
63
	  and towards enabling features individually.</para>
64

65
  </sect1>
66

67
<!-- UNBOXED TYPES AND PRIMITIVE OPERATIONS -->
68 69 70
<sect1 id="primitives">
  <title>Unboxed types and primitive operations</title>

71 72
<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.
73
While you really can use this stuff to write fast code,
74 75 76 77 78
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>
79

80 81
<para>All these primitive data types and operations are exported by the
library <literal>GHC.Prim</literal>, for which there is
82
<ulink url="&libraryGhcPrimLocation;/GHC-Prim.html">detailed online documentation</ulink>.
83 84
(This documentation is generated from the file <filename>compiler/prelude/primops.txt.pp</filename>.)
</para>
85

86 87 88 89 90 91 92
<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>

93
<para>The primops make extensive use of <link linkend="glasgow-unboxed">unboxed types</link>
94 95
and <link linkend="unboxed-tuples">unboxed tuples</link>, which
we briefly summarise here. </para>
96

97
<sect2 id="glasgow-unboxed">
98
<title>Unboxed types</title>
99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126

<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
127
bottom.  We use the convention (but it is only a convention)
128 129 130 131
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>.
132 133 134 135 136 137 138 139 140 141 142 143 144 145 146
</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.
147 148 149
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.
150 151 152
</para>

<para>
153 154 155 156
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
157 158 159 160 161 162 163 164 165 166 167
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>
168
</listitem>
169 170 171 172 173 174 175
<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>
176 177 178 179 180 181 182
<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,
183 184
non-top-level) pattern binding, but you must make any such pattern-match
strict.  For example, rather than:
185 186
<programlisting>
  data Foo = Foo Int Int#
187

188 189
  f x = let (Foo a b, w) = ..rhs.. in ..body..
</programlisting>
190
you must write:
191 192 193
<programlisting>
  data Foo = Foo Int Int#

194
  f x = let !(Foo a b, w) = ..rhs.. in ..body..
195
</programlisting>
196
since <literal>b</literal> has type <literal>Int#</literal>.
197 198 199
</para>
</listitem>
</itemizedlist>
200 201 202 203 204
</para>

</sect2>

<sect2 id="unboxed-tuples">
205
<title>Unboxed tuples</title>
206 207

<para>
208 209
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
210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226
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>

227 228 229 230 231 232 233 234
<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>

235 236 237 238 239 240
<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
241
of the primitive operations listed in <literal>primops.txt.pp</literal> return unboxed
242
tuples.
243 244
In particular, the <literal>IO</literal> and <literal>ST</literal> monads use unboxed
tuples to avoid unnecessary allocation during sequences of operations.
245 246 247
</para>

<para>
248
There are some restrictions on the use of unboxed tuples:
249 250
<itemizedlist>

251
<listitem>
252
<para>
253
Values of unboxed tuple types are subject to the same restrictions as
254 255 256 257 258
other unboxed types; i.e. they may not be stored in polymorphic data
structures or passed to polymorphic functions.
</para>
</listitem>

259 260
<listitem>
<para>
261 262 263 264 265 266 267 268 269 270 271
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,
272
the resulting binding is lazy like any other Haskell pattern binding.  The
273 274
above example desugars like this:
<programlisting>
275
  f x = let t = case h x of { (# p,q #) -> (p,q) }
276 277 278 279 280
            p = fst t
            q = snd t
        in ..body..
</programlisting>
Indeed, the bindings can even be recursive.
281 282 283 284
</para>
</listitem>
</itemizedlist>

285 286 287 288 289
</para>

</sect2>
</sect1>

rrt's avatar
rrt committed
290

291 292 293 294
<!-- ====================== SYNTACTIC EXTENSIONS =======================  -->

<sect1 id="syntax-extns">
<title>Syntactic extensions</title>
295

Simon Marlow's avatar
Simon Marlow committed
296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312
    <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>
313 314 315 316 317 318 319 320 321

<!--
               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.
            -->

Simon Marlow's avatar
Simon Marlow committed
322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361
	  <tbody>
	    <row>
	      <entry><literal>::</literal></entry>
	      <entry>::</entry> <!-- no special char, apparently -->
              <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>
362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407

	  <tbody>
	    <row>
	      <entry>-&lt;</entry>
	      <entry>&larrtl;</entry>
	      <entry>0x2919</entry>
	      <entry>LEFTWARDS ARROW-TAIL</entry>
	    </row>
          </tbody>

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

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

	  <tbody>
	    <row>
	      <entry>&gt;&gt;-</entry>
	      <entry></entry>
	      <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>

Simon Marlow's avatar
Simon Marlow committed
408 409 410 411
        </tgroup>
      </informaltable>
    </sect2>

412 413 414 415 416 417
    <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>

418 419
      <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>),
420
        but there is no requirement to do so; they are just plain ordinary variables.
421
	Nor does the <option>-XMagicHash</option> extension bring anything into scope.
422 423
	For example, to bring <literal>Int&num;</literal> into scope you must
	import <literal>GHC.Prim</literal> (see <xref linkend="primitives"/>);
424 425
	the <option>-XMagicHash</option> extension
	then allows you to <emphasis>refer</emphasis> to the <literal>Int&num;</literal>
426 427 428 429 430
	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>
431
      <para> The <option>-XMagicHash</option> also enables some new forms of literals (see <xref linkend="glasgow-unboxed"/>):
432
	<itemizedlist>
433 434 435
	  <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,
Ian Lynagh's avatar
Ian Lynagh committed
436
	  any Haskell integer lexeme followed by a <literal>&num;</literal> is an <literal>Int&num;</literal> literal, e.g.
Krzysztof Gogolewski's avatar
Typos  
Krzysztof Gogolewski committed
437
            <literal>-0x3A&num;</literal> as well as <literal>32&num;</literal>.</para></listitem>
438
	  <listitem><para> <literal>3&num;&num;</literal> has type <literal>Word&num;</literal>. In general,
Ian Lynagh's avatar
Ian Lynagh committed
439
	  any non-negative Haskell integer lexeme followed by <literal>&num;&num;</literal>
440 441 442 443 444 445 446
	      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>

447
    <sect2 id="negative-literals">
Krzysztof Gogolewski's avatar
Typos  
Krzysztof Gogolewski committed
448
      <title>Negative literals</title>
449 450 451 452
      <para>
          The literal <literal>-123</literal> is, according to
          Haskell98 and Haskell 2010, desugared as
          <literal>negate (fromInteger 123)</literal>.
453 454
         The language extension <option>-XNegativeLiterals</option>
         means that it is instead desugared as
Krzysztof Gogolewski's avatar
Typos  
Krzysztof Gogolewski committed
455
         <literal>fromInteger (-123)</literal>.
456 457 458
      </para>

      <para>
459 460 461 462 463
      This can make a difference when the positive and negative range of 
      a numeric data type don't match up.  For example, 
      in 8-bit arithmetic -128 is representable, but +128 is not.
      So <literal>negate (fromInteger 128)</literal> will elicit an 
      unexpected integer-literal-overflow message.
464 465 466
      </para>
   </sect2>

thoughtpolice's avatar
thoughtpolice committed
467 468 469 470 471
    <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
Krzysztof Gogolewski's avatar
Typos  
Krzysztof Gogolewski committed
472
          type <literal>Fractional a => a</literal>.
thoughtpolice's avatar
thoughtpolice committed
473 474 475
      </para>

      <para>
Krzysztof Gogolewski's avatar
Typos  
Krzysztof Gogolewski committed
476
         The language extension <option>-XNumDecimals</option> allows
thoughtpolice's avatar
thoughtpolice committed
477 478
         you to also use the floating literal syntax for instances of
         <literal>Integral</literal>, and have values like
479
         <literal>(1.2e6 :: Num a => a)</literal>
thoughtpolice's avatar
thoughtpolice committed
480 481 482
      </para>
   </sect2>

483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502
    <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>
thoughtpolice's avatar
thoughtpolice committed
503

504 505
    <!-- ====================== HIERARCHICAL MODULES =======================  -->

506

507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536
    <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>

537 538
      <para>For details on how GHC searches for source and interface
      files in the presence of hierarchical modules, see <xref
539
      linkend="search-path"/>.</para>
540 541

      <para>GHC comes with a large collection of libraries arranged
542 543 544 545 546
      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>
547 548 549 550 551 552 553 554 555
    </sect2>

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

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

<para>
<indexterm><primary>Pattern guards (Glasgow extension)</primary></indexterm>
556
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.)
557 558 559 560 561 562 563 564 565 566 567
</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,
568
where <varname>v</varname> is the value that the key maps to.  Now consider the following definition:
569 570 571
</para>

<programlisting>
572
clunky env var1 var2 | ok1 &amp;&amp; ok2 = val1 + val2
573 574 575 576 577 578 579 580 581 582 583
| 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>
584
The auxiliary functions are
585 586 587 588 589 590 591 592 593 594 595 596 597
</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>
598
What is <function>clunky</function> doing? The guard <literal>ok1 &amp;&amp;
599 600 601 602
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
603
returned values to <varname>val1</varname> and <varname>val2</varname>
604 605 606 607 608 609 610 611 612 613 614
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>
615
clunky env var1 var2 = case lookup env var1 of
616 617 618 619 620
  Nothing -&gt; fail
  Just val1 -&gt; case lookup env var2 of
    Nothing -&gt; fail
    Just val2 -&gt; val1 + val2
where
Simon Marlow's avatar
Simon Marlow committed
621
  fail = var1 + var2
622 623 624 625 626 627 628
</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
629
the cases we want to consider, one at a time, independently of each other.
630 631
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
632
tends to become more and more indented.
633 634 635 636 637 638 639
</para>

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

<programlisting>
640
clunky env var1 var2
641 642 643 644 645 646 647
  | Just val1 &lt;- lookup env var1
  , Just val2 &lt;- lookup env var2
  = val1 + val2
...other equations for clunky...
</programlisting>

<para>
648
The semantics should be clear enough.  The qualifiers are matched in order.
649
For a <literal>&lt;-</literal> qualifier, which I call a pattern guard, the
650
right hand side is evaluated and matched against the pattern on the left.
651 652 653 654 655 656 657 658 659 660 661 662 663 664 665
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>
666
f x | [y] &lt;- x
667
    , y > 3
668
    , Just z &lt;- h y
669 670 671 672 673 674 675
    = ...
</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>
676 677 678 679 680 681 682 683 684 685 686
</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
687
<ulink url="http://ghc.haskell.org/trac/ghc/wiki/ViewPatterns">Wiki
688 689 690 691 692 693 694 695 696 697 698 699
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
700

701 702 703
data TypView = Unit
             | Arrow Typ Typ

Gabor Greif's avatar
Gabor Greif committed
704
view :: Typ -> TypView
705 706 707 708 709

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

The representation of Typ is held abstract, permitting implementations
SamB's avatar
SamB committed
710
to use a fancy representation (e.g., hash-consing to manage sharing).
711

712
Without view patterns, using this signature a little inconvenient:
713 714 715 716 717 718 719 720 721 722 723 724 725 726
<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
727
matching against the result:
728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769
<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>
770
More precisely, the scoping rules are:
771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787
<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
788
were collected into a tuple.
789 790 791 792 793 794 795 796 797 798 799 800 801 802 803
</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>

804
(For some amplification on this design choice see
805
<ulink url="http://ghc.haskell.org/trac/ghc/ticket/4061">Trac #4061</ulink>.)
806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824

</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>
825 826
case v of { (e -> p) -> e1 ; _ -> e2 }
 =
827 828 829 830 831 832 833 834
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
835
<replaceable>pat</replaceable>.
836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869
</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>

870 871 872 873 874 875 876 877 878
</sect2>

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

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

<para>
879 880 881 882 883
Pattern synonyms are enabled by the flag
<literal>-XPatternSynonyms</literal>, which is required for both
defining them <emphasis>and</emphasis> using them.  More information
and examples of view patterns can be found on the <ulink
url="http://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms">Wiki
884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899
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>
Gabor Greif's avatar
Gabor Greif committed
900 901 902
Here are some examples of using said representation.
Consider a few types of the <literal>Type</literal> universe encoded
like this:
903 904 905 906 907 908 909 910 911 912 913
</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
Gabor Greif's avatar
Gabor Greif committed
914
types specially, for example the following two functions collect all
915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989
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
990 991
right-hand side. We can give an explicit inversion of a pattern
synonym using the following syntax:
992 993
</para>

994 995 996 997 998
<programlisting>
  pattern Head x &lt;- x:xs where
    Head x = [x]
</programlisting>

999
<para>
1000 1001 1002 1003 1004 1005
The syntax and semantics of pattern synonyms are elaborated in the
following subsections.  
See the <ulink
url="http://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms">Wiki
page</ulink> for more details.
</para>
1006

1007
<sect3> <title>Syntax and scoping of pattern synonyms</title>
1008 1009 1010 1011 1012 1013 1014 1015 1016
<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
1017 1018 1019 1020
</programlisting> or
<programlisting>
  pattern Name args &lt;- pat where
    Name args = expr
1021
</programlisting>
1022 1023 1024
  Either prefix or infix syntax can be
  used.
</para>
1025 1026 1027 1028 1029 1030 1031 1032
<para>
  Pattern synonym declarations can only occur in the top level of a
  module. In particular, they are not allowed as local
  definitions. Currently, they also don't work in GHCi, but that is a
  technical restriction that will be lifted in later versions.
</para>
<para>
  The variables in the left-hand side of the definition are bound by
1033 1034 1035 1036 1037 1038
  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.
1039 1040 1041 1042 1043
</para>

<para>
  Pattern synonyms cannot be defined recursively.
</para>
1044
</sect3>
1045

1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070
<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>
1071

1072
<sect3> <title>Typing of pattern synonyms</title>
1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146

<para>
  Given a pattern synonym definition of the form
</para>
<programlisting>
  pattern P var1 var2 ... varN &lt;- pat
</programlisting>
<para>
  it is assigned a <emphasis>pattern type</emphasis> of the form
</para>
<programlisting>
  pattern CProv => P t1 t2 ... tN :: CReq => t
</programlisting>
<para>
  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
  types.
</para>

<para>
A pattern synonym of this type can be used in a pattern if the
instatiated (monomorphic) type satisfies the constraints of
<replaceable>CReq</replaceable>. In this case, it extends the context
available in the right-hand side of the match with
<replaceable>CProv</replaceable>, just like how an existentially-typed
data constructor can extend the context.
</para>

<para>
For example, in the following program:
</para>
<programlisting>
{-# LANGUAGE PatternSynonyms, GADTs #-}
module ShouldCompile where

data T a where
	MkT :: (Show b) => a -> b -> T a

pattern ExNumPat x = MkT 42 x
</programlisting>

<para>
the pattern type of <literal>ExNumPat</literal> is
</para>

<programlisting>
pattern (Show b) => ExNumPat b :: (Num a, Eq a) => T a
</programlisting>

<para>
  and so can be used in a function definition like the following:
</para>

<programlisting>
  f :: (Num t, Eq t) => T t -> String
  f (ExNumPat x) = show x
</programlisting>

<para>
  For bidirectional pattern synonyms, uses as expressions have the type
</para>
<programlisting>
  (CProv, CReq) => t1 -> t2 -> ... -> tN -> t
</programlisting>

<para>
  So in the previous example, <literal>ExNumPat</literal>,
  when used in an expression, has type
</para>
<programlisting>
  ExNumPat :: (Show b, Num a, Eq a) => b -> T t
</programlisting>
1147
</sect3>
1148

1149
<sect3><title>Matching of pattern synonyms</title>
1150 1151

<para>
Gabor Greif's avatar
Gabor Greif committed
1152
A pattern synonym occurrence in a pattern is evaluated by first
1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180
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
f' _                                   = False
</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>
1181
</sect3>
1182

1183 1184 1185 1186 1187 1188
</sect2>

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

<sect2 id="n-k-patterns">
<title>n+k patterns</title>
1189
<indexterm><primary><option>-XNPlusKPatterns</option></primary></indexterm>
1190 1191

<para>
1192 1193
<literal>n+k</literal> pattern support is disabled by default. To enable
it, you can use the <option>-XNPlusKPatterns</option> flag.
1194 1195
</para>

Ian Lynagh's avatar
Ian Lynagh committed
1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208
</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>

1209 1210 1211 1212
</sect2>

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

1213
<sect2 id="recursive-do-notation">
1214 1215 1216 1217
<title>The recursive do-notation
</title>

<para>
1218 1219 1220
    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
thoughtpolice's avatar
thoughtpolice committed
1221 1222
    group.
</para>
1223

thoughtpolice's avatar
thoughtpolice committed
1224
<para>
1225 1226
    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
Ross Paterson's avatar
Ross Paterson committed
1227 1228 1229 1230 1231 1232
    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
1233 1234 1235
    <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.
1236
</para>
1237 1238 1239 1240 1241

<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>)
Ross Paterson's avatar
Ross Paterson committed
1242 1243 1244
    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>
1245 1246 1247 1248
    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>

1249
<para>
1250
    Here is a simple (albeit contrived) example:
1251
<programlisting>
1252 1253 1254
{-# LANGUAGE RecursiveDo #-}
justOnes = mdo { xs &lt;- Just (1:xs)
               ; return (map negate xs) }
1255
</programlisting>
Ross Paterson's avatar
Ross Paterson committed
1256 1257 1258 1259 1260 1261
or equivalently
<programlisting>
{-# LANGUAGE RecursiveDo #-}
justOnes = do { rec { xs &lt;- Just (1:xs) }
              ; return (map negate xs) }
</programlisting>
1262 1263
As you can guess <literal>justOnes</literal> will evaluate to <literal>Just [-1,-1,-1,...</literal>.
</para>
1264

thoughtpolice's avatar
thoughtpolice committed
1265
<para>
1266 1267 1268
   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
Ross Paterson's avatar
Ross Paterson committed
1269 1270
   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.
1271 1272
</para>

1273
<sect3>
1274 1275
<title>Recursive binding groups</title>

1276
<para>
1277 1278 1279 1280
    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
1281
<programlisting>
1282 1283 1284 1285
    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) }
1286
</programlisting>
1287 1288 1289 1290
    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.)
1291
</para>
1292

1293
<para>
1294 1295 1296 1297
    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:
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
1298
<programlisting>
1299 1300 1301
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) })
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
1302
</programlisting>
1303 1304
   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
1305
<programlisting>
1306
<replaceable>vs</replaceable> &lt;- mfix (\ ~<replaceable>vs</replaceable> -&gt; do { <replaceable>ss</replaceable>; return <replaceable>vs</replaceable> })
1307
</programlisting>
1308 1309 1310 1311
  where <replaceable>vs</replaceable> is a tuple of the variables bound by <replaceable>ss</replaceable>.
</para>

<para>
Ross Paterson's avatar
Ross Paterson committed
1312 1313 1314
    Note in particular that the translation for a <literal>rec</literal> block only involves wrapping a call
    to <literal>mfix</literal>: it performs no other analysis on the bindings. The latter is the task
    for the <literal>mdo</literal> notation, which is described next.
1315 1316 1317 1318 1319 1320 1321 1322
</para>
</sect3>

<sect3>
<title>The <literal>mdo</literal> notation</title>

<para>
    A <literal>rec</literal>-block tells the compiler where precisely the recursive knot should be tied. It turns out that
Ross Paterson's avatar
Ross Paterson committed
1323 1324
    the placement of the recursive knots can be rather delicate: in particular, we would like the knots to be wrapped
    around as minimal groups as possible. This process is known as <emphasis>segmentation</emphasis>, and is described
1325 1326
    in detail in Secton 3.2 of <ulink url="https://sites.google.com/site/leventerkok/recdo.pdf">A recursive do for
    Haskell</ulink>. Segmentation improves polymorphism and reduces the size of the recursive knot. Most importantly, it avoids
Ross Paterson's avatar
Ross Paterson committed
1327
    unnecessary interference caused by a fundamental issue with the so-called <emphasis>right-shrinking</emphasis>
1328 1329 1330 1331 1332
    axiom for monadic recursion. In brief, most monads of interest (IO, strict state, etc.) do <emphasis>not</emphasis>
    have recursion operators that satisfy this axiom, and thus not performing segmentation can cause unnecessary
    interference, changing the termination behavior of the resulting translation.
    (Details can be found in Sections 3.1 and 7.2.2 of
    <ulink url="http://sites.google.com/site/leventerkok/erkok-thesis.pdf">Value Recursion in Monadic Computations</ulink>.)
1333
</para>
1334

1335
<para>
1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388
    The <literal>mdo</literal> notation removes the burden of placing
    explicit <literal>rec</literal> blocks in the code.  Unlike an
    ordinary <literal>do</literal> expression, in which variables bound by
    statements are only in scope for later statements, variables bound in
    an <literal>mdo</literal> expression are in scope for all statements
    of the expression.  The compiler then automatically identifies minimal
    mutually recursively dependent segments of statements, treating them as
    if the user had wrapped a <literal>rec</literal> qualifier around them.
</para>

<para>
   The definition is syntactic:
</para>
<itemizedlist>
   <listitem>
       <para>
         A generator <replaceable>g</replaceable>
         <emphasis>depends</emphasis> on a textually following generator
         <replaceable>g'</replaceable>, if
       </para>
       <itemizedlist>
         <listitem>
           <para>
             <replaceable>g'</replaceable> defines a variable that
             is used by <replaceable>g</replaceable>, or
           </para>
         </listitem>
         <listitem>
           <para>
           <replaceable>g'</replaceable> textually appears between
           <replaceable>g</replaceable> and
           <replaceable>g''</replaceable>, where <replaceable>g</replaceable>
           depends on <replaceable>g''</replaceable>.
           </para>
         </listitem>
       </itemizedlist>
   </listitem>
   <listitem>
       <para>
         A <emphasis>segment</emphasis> of a given
         <literal>mdo</literal>-expression is a minimal sequence of generators
         such that no generator of the sequence depends on an outside
         generator. As a special case, although it is not a generator,
         the final expression in an <literal>mdo</literal>-expression is
         considered to form a segment by itself.
       </para>
   </listitem>
</itemizedlist>
<para>
   Segments in this sense are
   related to <emphasis>strongly-connected components</emphasis> analysis,
   with the exception that bindings in a segment cannot be reordered and
   must be contiguous.
1389 1390 1391
</para>

<para>
Ross Paterson's avatar
Ross Paterson committed
1392
    Here is an example <literal>mdo</literal>-expression, and its translation to <literal>rec</literal> blocks:
1393 1394 1395 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411 1412
<programlisting>
mdo { a &lt;- getChar      ===> do { a &lt;- getChar
    ; b &lt;- f a c                ; rec { b &lt;- f a c
    ; c &lt;- f b a                ;     ; c &lt;- f b a }
    ; z &lt;- h a b                ; z &lt;- h a b
    ; d &lt;- g d e                ; rec { d &lt;- g d e
    ; e &lt;- g a z                ;     ; e &lt;- g a z }
    ; putChar c }               ; putChar c }
</programlisting>
Note that a given <literal>mdo</literal> expression can cause the creation of multiple <literal>rec</literal> blocks.
If there are no recursive dependencies, <literal>mdo</literal> will introduce no <literal>rec</literal> blocks. In this
latter case an <literal>mdo</literal> expression is precisely the same as a <literal>do</literal> expression, as one
would expect.
</para>

<para>
    In summary, given an <literal>mdo</literal> expression, GHC first performs segmentation, introducing
    <literal>rec</literal> blocks to wrap over minimal recursive groups. Then, each resulting
    <literal>rec</literal> is desugared, using a call to <literal>Control.Monad.Fix.mfix</literal> as described
    in the previous section. The original <literal>mdo</literal>-expression typechecks exactly when the desugared
Ross Paterson's avatar
Ross Paterson committed
1413
    version would do so.
1414
</para>
1415

1416
<para>
1417
Here are some other important points in using the recursive-do notation:
1418

1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454
<itemizedlist>
    <listitem>
        <para>
            It is enabled with the flag <literal>-XRecursiveDo</literal>, or the <literal>LANGUAGE RecursiveDo</literal>
            pragma. (The same flag enables both <literal>mdo</literal>-notation, and the use of <literal>rec</literal>
            blocks inside <literal>do</literal> expressions.)
        </para>
    </listitem>
    <listitem>
        <para>
            <literal>rec</literal> blocks can also be used inside <literal>mdo</literal>-expressions, which will be
            treated as a single statement. However, it is good style to either use <literal>mdo</literal> or
            <literal>rec</literal> blocks in a single expression.
        </para>
    </listitem>
    <listitem>
        <para>
            If recursive bindings are required for a monad, then that monad must be declared an instance of
            the <literal>MonadFix</literal> class.
        </para>
    </listitem>
    <listitem>
        <para>
            The following instances of <literal>MonadFix</literal> are automatically provided: List, Maybe, IO.
            Furthermore, the <literal>Control.Monad.ST</literal> and <literal>Control.Monad.ST.Lazy</literal>
            modules provide the instances of the <literal>MonadFix</literal> class for Haskell's internal
            state monad (strict and lazy, respectively).
        </para>
    </listitem>
    <listitem>
        <para>
            Like <literal>let</literal> and <literal>where</literal> bindings, name shadowing is not allowed within
            an <literal>mdo</literal>-expression or a <literal>rec</literal>-block; that is, all the names bound in
            a single <literal>rec</literal> must be distinct. (GHC will complain if this is not the case.)
        </para>
    </listitem>
1455 1456
</itemizedlist>
</para>
1457 1458
</sect3>

1459

1460 1461 1462
</sect2>


1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481
   <!-- ===================== 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>
1482