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glasgow_exts.xml
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glasgow_exts.xml
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<?xml version="1.0" encoding="iso-8859-1"?>
<para>
<indexterm><primary>language, GHC</primary></indexterm>
<indexterm><primary>extensions, GHC</primary></indexterm>
As with all known Haskell systems, GHC implements some extensions to
the language. They are all enabled by options; by default GHC
understands only plain Haskell 98.
</para>
<para>
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 “stuck”
on performance because of the implementation costs of Haskell's
“high-level” features—you can always code
“under” them. In an extreme case, you can write all your
time-critical code in C, and then just glue it together with Haskell!
</para>
<para>
Before you get too carried away working at the lowest level (e.g.,
sloshing <literal>MutableByteArray#</literal>s around your
program), you may wish to check if there are libraries that provide a
“Haskellised veneer” over the features you want. The
separate <ulink url="../libraries/index.html">libraries
documentation</ulink> describes all the libraries that come with GHC.
</para>
<!-- LANGUAGE OPTIONS -->
<sect1 id="options-language">
<title>Language options</title>
<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>
<para>The language option flag control what variation of the language are
permitted. Leaving out all of them gives you standard Haskell
98.</para>
<para>Generally speaking, all the language options are introduced by "<option>-X</option>",
e.g. <option>-XTemplateHaskell</option>.
</para>
<para> All the language options can be turned off by using the prefix "<option>No</option>";
e.g. "<option>-XNoTemplateHaskell</option>".</para>
<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>
<para>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>
<variablelist>
<varlistentry>
<term>
<option>-fglasgow-exts</option>:
<indexterm><primary><option>-fglasgow-exts</option></primary></indexterm>
</term>
<listitem>
<para>This simultaneously enables all of the extensions to
Haskell 98 described in <xref
linkend="ghc-language-features"/>, except where otherwise
noted. We are trying to move away from this portmanteau flag,
and towards enabling features individaully.</para>
<para>New reserved words: <literal>forall</literal> (only in
types), <literal>mdo</literal>.</para>
<para>Other syntax stolen:
<replaceable>varid</replaceable>{<literal>#</literal>},
<replaceable>char</replaceable><literal>#</literal>,
<replaceable>string</replaceable><literal>#</literal>,
<replaceable>integer</replaceable><literal>#</literal>,
<replaceable>float</replaceable><literal>#</literal>,
<replaceable>float</replaceable><literal>##</literal>,
<literal>(#</literal>, <literal>#)</literal>,
<literal>|)</literal>, <literal>{|</literal>.</para>
<para>Implies these specific language options:
<option>-XForeignFunctionInterface</option>,
<option>-XImplicitParams</option>,
<option>-XScopedTypeVariables</option>,
<option>-XGADTs</option>,
<option>-XTypeFamilies</option>. </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-XForeignFunctionInterface</option>:
<indexterm><primary><option>-XForeignFunctionInterface</option></primary></indexterm>
</term>
<listitem>
<para>This option enables the language extension defined in the
Haskell 98 Foreign Function Interface Addendum.</para>
<para>New reserved words: <literal>foreign</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-XMonomorphismRestriction</option>,<option>-XMonoPatBinds</option>:
</term>
<listitem>
<para> These two flags control how generalisation is done.
See <xref linkend="monomorphism"/>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-XExtendedDefaultRules</option>:
<indexterm><primary><option>-XExtendedDefaultRules</option></primary></indexterm>
</term>
<listitem>
<para> Use GHCi's extended default rules in a regular module (<xref linkend="extended-default-rules"/>).
Independent of the <option>-fglasgow-exts</option>
flag. </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-XOverlappingInstances</option>
<indexterm><primary><option>-XOverlappingInstances</option></primary></indexterm>
</term>
<term>
<option>-XUndecidableInstances</option>
<indexterm><primary><option>-XUndecidableInstances</option></primary></indexterm>
</term>
<term>
<option>-XIncoherentInstances</option>
<indexterm><primary><option>-XIncoherentInstances</option></primary></indexterm>
</term>
<term>
<option>-fcontext-stack=N</option>
<indexterm><primary><option>-fcontext-stack</option></primary></indexterm>
</term>
<listitem>
<para> See <xref linkend="instance-decls"/>. Only relevant
if you also use <option>-fglasgow-exts</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-finline-phase</option>
<indexterm><primary><option>-finline-phase</option></primary></indexterm>
</term>
<listitem>
<para>See <xref linkend="rewrite-rules"/>. Only relevant if
you also use <option>-fglasgow-exts</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-XArrows</option>
<indexterm><primary><option>-XArrows</option></primary></indexterm>
</term>
<listitem>
<para>See <xref linkend="arrow-notation"/>. Independent of
<option>-fglasgow-exts</option>.</para>
<para>New reserved words/symbols: <literal>rec</literal>,
<literal>proc</literal>, <literal>-<</literal>,
<literal>>-</literal>, <literal>-<<</literal>,
<literal>>>-</literal>.</para>
<para>Other syntax stolen: <literal>(|</literal>,
<literal>|)</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-XGenerics</option>
<indexterm><primary><option>-XGenerics</option></primary></indexterm>
</term>
<listitem>
<para>See <xref linkend="generic-classes"/>. Independent of
<option>-fglasgow-exts</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-XNoImplicitPrelude</option></term>
<listitem>
<para><indexterm><primary>-XNoImplicitPrelude
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>-XNoImplicitPrelude</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>-XNoImplicitPrelude</option> does
change the handling of certain built-in syntax: see <xref
linkend="rebindable-syntax"/>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-XImplicitParams</option></term>
<listitem>
<para>Enables implicit parameters (see <xref
linkend="implicit-parameters"/>). Currently also implied by
<option>-fglasgow-exts</option>.</para>
<para>Syntax stolen:
<literal>?<replaceable>varid</replaceable></literal>,
<literal>%<replaceable>varid</replaceable></literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-XOverloadedStrings</option></term>
<listitem>
<para>Enables overloaded string literals (see <xref
linkend="overloaded-strings"/>).</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-XScopedTypeVariables</option></term>
<listitem>
<para>Enables lexically-scoped type variables (see <xref
linkend="scoped-type-variables"/>). Implied by
<option>-fglasgow-exts</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-XTemplateHaskell</option></term>
<listitem>
<para>Enables Template Haskell (see <xref
linkend="template-haskell"/>). This flag must
be given explicitly; it is no longer implied by
<option>-fglasgow-exts</option>.</para>
<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>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<!-- UNBOXED TYPES AND PRIMITIVE OPERATIONS -->
<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.pp</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 “raw machine” types you
would use in C: <literal>Int#</literal> (long int),
<literal>Double#</literal> (double), <literal>Addr#</literal>
(void *), etc. The <emphasis>primitive operations</emphasis>
(PrimOps) on these types are what you might expect; e.g.,
<literal>(+#)</literal> is addition on
<literal>Int#</literal>s, and is the machine-addition that we all
know and love—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>#</literal> suffix.
</para>
<para>
Primitive values are often represented by a simple bit-pattern, such
as <literal>Int#</literal>, <literal>Float#</literal>,
<literal>Double#</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#</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…nothing can be at the
other end of the pointer than the primitive value.
A numerically-intensive program using unboxed types can
go a <emphasis>lot</emphasis> faster than its “standard”
counterpart—we saw a threefold speedup on one example.
</para>
<para>
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
function or store one in a polymorphic data type. This rules out
things like <literal>[Int#]</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#</literal> for instance).
</para>
</listitem>
<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>
<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,
non-top-level) pattern binding, but any such variable causes the entire
pattern-match
to become strict. For example:
<programlisting>
data Foo = Foo Int Int#
f x = let (Foo a b, w) = ..rhs.. in ..body..
</programlisting>
Since <literal>b</literal> has type <literal>Int#</literal>, the entire pattern
match
is strict, and the program behaves as if you had written
<programlisting>
data Foo = Foo Int Int#
f x = case ..rhs.. of { (Foo a b, w) -> ..body.. }
</programlisting>
</para>
</listitem>
</itemizedlist>
</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_1..e_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 <literal>primops.txt.pp</literal> return unboxed
tuples.
In particular, the <literal>IO</literal> and <literal>ST</literal> monads use unboxed
tuples to avoid unnecessary allocation during sequences of operations.
</para>
<para>
There are some pretty stringent restrictions on the use of unboxed tuples:
<itemizedlist>
<listitem>
<para>
Values of 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>
No variable can have an unboxed tuple type, nor may a constructor or function
argument have an unboxed tuple type. The following are all illegal:
<programlisting>
data Foo = Foo (# Int, Int #)
f :: (# Int, Int #) -> (# Int, Int #)
f x = x
g :: (# Int, Int #) -> Int
g (# a,b #) = a
h x = let y = (# x,x #) in ...
</programlisting>
</para>
</listitem>
</itemizedlist>
</para>
<para>
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 #) -> 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,
the resulting binding is lazy like any other Haskell pattern binding. The
above example desugars like this:
<programlisting>
f x = let t = case h x o f{ (# p,q #) -> (p,q)
p = fst t
q = snd t
in ..body..
</programlisting>
Indeed, the bindings can even be recursive.
</para>
</sect2>
</sect1>
<!-- ====================== 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>‘.’</literal>. This is also known as the
“hierarchical module namespace” 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>
<para>For details on how GHC searches for source and interface
files in the presence of hierarchical modules, see <xref
linkend="search-path"/>.</para>
<para>GHC comes with a large collection of libraries arranged
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>
</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 -> Bool
maybeToBool (Just x) = True
maybeToBool Nothing = False
expectJust :: Maybe a -> 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 var2 = case lookup env var1 of
Nothing -> fail
Just val1 -> case lookup env var2 of
Nothing -> fail
Just val2 -> val1 + val2
where
fail = var1 + var2
</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 var2
| Just val1 <- lookup env var1
, Just val2 <- lookup env var2
= val1 + val2
...other equations for clunky...
</programlisting>
<para>
The semantics should be clear enough. The qualifiers are matched in order.
For a <literal><-</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><-</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
<ulink url="http://citeseer.ist.psu.edu/erk02recursive.html">A recursive do for Haskell</ulink>,
by Levent Erkok, John Launchbury,
Haskell Workshop 2002, pages: 29-37. Pittsburgh, Pennsylvania.
This paper is essential reading for anyone making non-trivial use of mdo-notation,
and we do not repeat it here.
</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>
import Control.Monad.Fix
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. For example,
<literal>justOnes</literal> desugars as follows:
<programlisting>
justOnes = mfix (\xs' -> do { xs <- Just (1:xs'); return xs }
</programlisting>
For full details of the way in which mdo is typechecked and desugared, see
the paper <ulink url="http://citeseer.ist.psu.edu/erk02recursive.html">A recursive do for Haskell</ulink>.
In particular, GHC implements the segmentation technique described in Section 3.2 of the paper.
</para>
<para>
If recursive bindings are required for a monad,
then that monad must be declared an instance of the <literal>MonadFix</literal> class.
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).
</para>
<para>
Here are some 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>
It is enabled with the flag <literal>-XRecursiveDo</literal>, which is in turn implied by
<literal>-fglasgow-exts</literal>.
</para></listitem>
<listitem><para>
Unlike ordinary do-notation, but like <literal>let</literal> and <literal>where</literal> bindings,
name shadowing is not allowed; that is, all the names bound in a single <literal>mdo</literal> must
be distinct (Section 3.3 of the paper).
</para></listitem>
<listitem><para>
Variables bound by a <literal>let</literal> statement in an <literal>mdo</literal>
are monomorphic in the <literal>mdo</literal> (Section 3.1 of the paper). However
GHC breaks the <literal>mdo</literal> into segments to enhance polymorphism,
and improve termination (Section 3.2 of the paper).
</para></listitem>
</itemizedlist>
</para>
<para>
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.
</para>
<para>
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.
</para>
</sect2>
<!-- ===================== 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>-XNoImplicitPrelude</option> flag causes
the following pieces of built-in syntax to refer to
<emphasis>whatever is in scope</emphasis>, not the Prelude
versions:
<itemizedlist>
<listitem>
<para>An integer literal <literal>368</literal> means
"<literal>fromInteger (368::Integer)</literal>", rather than
"<literal>Prelude.fromInteger (368::Integer)</literal>".
</para> </listitem>
<listitem><para>Fractional literals are handed in just the same way,
except that the translation is
<literal>fromRational (3.68::Rational)</literal>.
</para> </listitem>
<listitem><para>The equality test in an overloaded numeric pattern
uses whatever <literal>(==)</literal> is in scope.
</para> </listitem>
<listitem><para>The subtraction operation, and the
greater-than-or-equal test, in <literal>n+k</literal> patterns
use whatever <literal>(-)</literal> and <literal>(>=)</literal> are in scope.
</para></listitem>
<listitem>
<para>Negation (e.g. "<literal>- (f x)</literal>")
means "<literal>negate (f x)</literal>", both in numeric
patterns, and expressions.
</para></listitem>
<listitem>
<para>"Do" notation is translated using whatever
functions <literal>(>>=)</literal>,
<literal>(>>)</literal>, and <literal>fail</literal>,
are in scope (not the Prelude
versions). List comprehensions, mdo (<xref linkend="mdo-notation"/>), and parallel array
comprehensions, are unaffected. </para></listitem>
<listitem>
<para>Arrow
notation (see <xref linkend="arrow-notation"/>)
uses whatever <literal>arr</literal>,
<literal>(>>>)</literal>, <literal>first</literal>,
<literal>app</literal>, <literal>(|||)</literal> and
<literal>loop</literal> functions are in scope. But unlike the
other constructs, the types of these functions must match the
Prelude types very closely. Details are in flux; if you want
to use this, ask!
</para></listitem>
</itemizedlist>
In all cases (apart from arrow notation), the static semantics should be that of the desugared form,
even if that is a little unexpected. For emample, the
static semantics of the literal <literal>368</literal>
is exactly that of <literal>fromInteger (368::Integer)</literal>; it's fine for
<literal>fromInteger</literal> to have any of the types:
<programlisting>
fromInteger :: Integer -> Integer
fromInteger :: forall a. Foo a => Integer -> a
fromInteger :: Num a => a -> Integer
fromInteger :: Integer -> Bool -> Bool
</programlisting>
</para>
<para>Be warned: this is an experimental facility, with
fewer checks than usual. Use <literal>-dcore-lint</literal>
to typecheck the desugared program. If Core Lint is happy
you should be all right.</para>
</sect2>
<sect2 id="postfix-operators">
<title>Postfix operators</title>
<para>
GHC allows a small extension to the syntax of left operator sections, which
allows you to define postfix operators. The extension is this: the left section
<programlisting>
(e !)
</programlisting>
is equivalent (from the point of view of both type checking and execution) to the expression
<programlisting>
((!) e)
</programlisting>
(for any expression <literal>e</literal> and operator <literal>(!)</literal>.
The strict Haskell 98 interpretation is that the section is equivalent to
<programlisting>
(\y -> (!) e y)
</programlisting>
That is, the operator must be a function of two arguments. GHC allows it to
take only one argument, and that in turn allows you to write the function
postfix.
</para>
<para>Since this extension goes beyond Haskell 98, it should really be enabled
by a flag; but in fact it is enabled all the time. (No Haskell 98 programs
change their behaviour, of course.)
</para>
<para>The extension does not extend to the left-hand side of function
definitions; you must define such a function in prefix form.</para>
</sect2>
<sect2 id="disambiguate-fields">
<title>Record field disambiguation</title>
<para>
In record construction and record pattern matching
it is entirely unambiguous which field is referred to, even if there are two different
data types in scope with a common field name. For example:
<programlisting>
module M where
data S = MkS { x :: Int, y :: Bool }
module Foo where
import M
data T = MkT { x :: Int }
ok1 (MkS { x = n }) = n+1 -- Unambiguous
ok2 n = MkT { x = n+1 } -- Unambiguous
bad1 k = k { x = 3 } -- Ambiguous