<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
+ url="http://www.haskell.org/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
</para>
<para>
-The web page: <ulink url="http://www.cse.ogi.edu/PacSoft/projects/rmb">http://www.cse.ogi.edu/PacSoft/projects/rmb</ulink>
+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>
branches.</para>
</sect2>
+
+ <!-- ===================== TRANSFORM LIST COMPREHENSIONS =================== -->
+
+ <sect2 id="generalised-list-comprehensions">
+ <title>Generalised (SQL-Like) List Comprehensions</title>
+ <indexterm><primary>list comprehensions</primary><secondary>generalised</secondary>
+ </indexterm>
+ <indexterm><primary>extended list comprehensions</primary>
+ </indexterm>
+ <indexterm><primary>group</primary></indexterm>
+ <indexterm><primary>sql</primary></indexterm>
+
+
+ <para>Generalised list comprehensions are a further enhancement to the
+ list comprehension syntatic sugar to allow operations such as sorting
+ and grouping which are familiar from SQL. They are fully described in the
+ paper <ulink url="http://research.microsoft.com/~simonpj/papers/list-comp">
+ Comprehensive comprehensions: comprehensions with "order by" and "group by"</ulink>,
+ except that the syntax we use differs slightly from the paper.</para>
+<para>Here is an example:
+<programlisting>
+employees = [ ("Simon", "MS", 80)
+, ("Erik", "MS", 100)
+, ("Phil", "Ed", 40)
+, ("Gordon", "Ed", 45)
+, ("Paul", "Yale", 60)]
+
+output = [ (the dept, sum salary)
+| (name, dept, salary) <- employees
+, then group by dept
+, then sortWith by (sum salary)
+, then take 5 ]
+</programlisting>
+In this example, the list <literal>output</literal> would take on
+ the value:
+
+<programlisting>
+[("Yale", 60), ("Ed", 85), ("MS", 180)]
+</programlisting>
+</para>
+<para>There are three new keywords: <literal>group</literal>, <literal>by</literal>, and <literal>using</literal>.
+(The function <literal>sortWith</literal> is not a keyword; it is an ordinary
+function that is exported by <literal>GHC.Exts</literal>.)</para>
+
+<para>There are five new forms of compehension qualifier,
+all introduced by the (existing) keyword <literal>then</literal>:
+ <itemizedlist>
+ <listitem>
+
+<programlisting>
+then f
+</programlisting>
+
+ This statement requires that <literal>f</literal> have the type <literal>
+ forall a. [a] -> [a]</literal>. You can see an example of it's use in the
+ motivating example, as this form is used to apply <literal>take 5</literal>.
+
+ </listitem>
+
+
+ <listitem>
+<para>
+<programlisting>
+then f by e
+</programlisting>
+
+ This form is similar to the previous one, but allows you to create a function
+ which will be passed as the first argument to f. As a consequence f must have
+ the type <literal>forall a. (a -> t) -> [a] -> [a]</literal>. As you can see
+ from the type, this function lets f "project out" some information
+ from the elements of the list it is transforming.</para>
+
+ <para>An example is shown in the opening example, where <literal>sortWith</literal>
+ is supplied with a function that lets it find out the <literal>sum salary</literal>
+ for any item in the list comprehension it transforms.</para>
+
+ </listitem>
+
+
+ <listitem>
+
+<programlisting>
+then group by e using f
+</programlisting>
+
+ <para>This is the most general of the grouping-type statements. In this form,
+ f is required to have type <literal>forall a. (a -> t) -> [a] -> [[a]]</literal>.
+ As with the <literal>then f by e</literal> case above, the first argument
+ is a function supplied to f by the compiler which lets it compute e on every
+ element of the list being transformed. However, unlike the non-grouping case,
+ f additionally partitions the list into a number of sublists: this means that
+ at every point after this statement, binders occuring before it in the comprehension
+ refer to <emphasis>lists</emphasis> of possible values, not single values. To help understand
+ this, let's look at an example:</para>
+
+<programlisting>
+-- This works similarly to groupWith in GHC.Exts, but doesn't sort its input first
+groupRuns :: Eq b => (a -> b) -> [a] -> [[a]]
+groupRuns f = groupBy (\x y -> f x == f y)
+
+output = [ (the x, y)
+| x <- ([1..3] ++ [1..2])
+, y <- [4..6]
+, then group by x using groupRuns ]
+</programlisting>
+
+ <para>This results in the variable <literal>output</literal> taking on the value below:</para>
+
+<programlisting>
+[(1, [4, 5, 6]), (2, [4, 5, 6]), (3, [4, 5, 6]), (1, [4, 5, 6]), (2, [4, 5, 6])]
+</programlisting>
+
+ <para>Note that we have used the <literal>the</literal> function to change the type
+ of x from a list to its original numeric type. The variable y, in contrast, is left
+ unchanged from the list form introduced by the grouping.</para>
+
+ </listitem>
+
+ <listitem>
+
+<programlisting>
+then group by e
+</programlisting>
+
+ <para>This form of grouping is essentially the same as the one described above. However,
+ since no function to use for the grouping has been supplied it will fall back on the
+ <literal>groupWith</literal> function defined in
+ <ulink url="../libraries/base/GHC-Exts.html"><literal>GHC.Exts</literal></ulink>. This
+ is the form of the group statement that we made use of in the opening example.</para>
+
+ </listitem>
+
+
+ <listitem>
+
+<programlisting>
+then group using f
+</programlisting>
+
+ <para>With this form of the group statement, f is required to simply have the type
+ <literal>forall a. [a] -> [[a]]</literal>, which will be used to group up the
+ comprehension so far directly. An example of this form is as follows:</para>
+
+<programlisting>
+output = [ x
+| y <- [1..5]
+, x <- "hello"
+, then group using inits]
+</programlisting>
+
+ <para>This will yield a list containing every prefix of the word "hello" written out 5 times:</para>
+
+<programlisting>
+["","h","he","hel","hell","hello","helloh","hellohe","hellohel","hellohell","hellohello","hellohelloh",...]
+</programlisting>
+
+ </listitem>
+</itemizedlist>
+</para>
+ </sect2>
<!-- ===================== REBINDABLE SYNTAX =================== -->
</sect3>
-<sect3>
-<title>Type classes</title>
+<sect3 id="existential-with-context">
+<title>Existentials and type classes</title>
<para>
An easy extension is to allow
generated by the call to <literal>elem</literal>, so that the type of
<literal>insert</literal> itself has no <literal>Eq</literal> constraint.
</para>
-<para>This behaviour contrasts with Haskell 98's peculiar treatment of
-contexts on a data type declaration (Section 4.2.1 of the Haskell 98 Report).
-In Haskell 98 the definition
-<programlisting>
- data Eq a => Set' a = MkSet' [a]
-</programlisting>
-gives <literal>MkSet'</literal> the same type as <literal>MkSet</literal> above. But instead of
-<emphasis>making available</emphasis> an <literal>(Eq a)</literal> constraint, pattern-matching
-on <literal>MkSet'</literal> <emphasis>requires</emphasis> an <literal>(Eq a)</literal> constraint!
-GHC faithfully implements this behaviour, odd though it is. But for GADT-style declarations,
-GHC's behaviour is much more useful, as well as much more intuitive.</para>
<para>
-For example, a possible application of GHC's behaviour is to reify dictionaries:
+For example, one possible application is to reify dictionaries:
<programlisting>
data NumInst a where
MkNumInst :: Num a => NumInst a
Here, a value of type <literal>NumInst a</literal> is equivalent
to an explicit <literal>(Num a)</literal> dictionary.
</para>
+<para>
+All this applies to constructors declared using the syntax of <xref linkend="existential-with-context"/>.
+For example, the <literal>NumInst</literal> data type above could equivalently be declared
+like this:
+<programlisting>
+ data NumInst a
+ = Num a => MkNumInst (NumInst a)
+</programlisting>
+Notice that, unlike the situation when declaring an existental, there is
+no <literal>forall</literal>, because the <literal>Num</literal> constrains the
+data type's univerally quantified type variable <literal>a</literal>.
+A constructor may have both universal and existential type variables: for example,
+the following two declarations are equivalent:
+<programlisting>
+ data T1 a
+ = forall b. (Num a, Eq b) => MkT1 a b
+ data T2 a where
+ MkT2 :: (Num a, Eq b) => a -> b -> T2 a
+</programlisting>
+</para>
+<para>All this behaviour contrasts with Haskell 98's peculiar treatment of
+contexts on a data type declaration (Section 4.2.1 of the Haskell 98 Report).
+In Haskell 98 the definition
+<programlisting>
+ data Eq a => Set' a = MkSet' [a]
+</programlisting>
+gives <literal>MkSet'</literal> the same type as <literal>MkSet</literal> above. But instead of
+<emphasis>making available</emphasis> an <literal>(Eq a)</literal> constraint, pattern-matching
+on <literal>MkSet'</literal> <emphasis>requires</emphasis> an <literal>(Eq a)</literal> constraint!
+GHC faithfully implements this behaviour, odd though it is. But for GADT-style declarations,
+GHC's behaviour is much more useful, as well as much more intuitive.
+</para>
<para>
The rest of this section gives further details about GADT-style data
A precise specification of the type rules is beyond what this user manual aspires to,
but the design closely follows that described in
the paper <ulink
-url="http://research.microsoft.com/%7Esimonpj/papers/gadt/index.htm">Simple
+url="http://research.microsoft.com/%7Esimonpj/papers/gadt/">Simple
unification-based type inference for GADTs</ulink>,
(ICFP 2006).
The general principle is this: <emphasis>type refinement is only carried out
<para>
These and many other examples are given in papers by Hongwei Xi, and
Tim Sheard. There is a longer introduction
-<ulink url="http://haskell.org/haskellwiki/GADT">on the wiki</ulink>,
+<ulink url="http://www.haskell.org/haskellwiki/GADT">on the wiki</ulink>,
and Ralf Hinze's
<ulink url="http://www.informatik.uni-bonn.de/~ralf/publications/With.pdf">Fun with phantom types</ulink> also has a number of examples. Note that papers
may use different notation to that implemented in GHC.
<para>
This section, and the next one, documents GHC's type-class extensions.
There's lots of background in the paper <ulink
-url="http://research.microsoft.com/~simonpj/Papers/type-class-design-space" >Type
-classes: exploring the design space</ulink > (Simon Peyton Jones, Mark
+url="http://research.microsoft.com/~simonpj/Papers/type-class-design-space/">Type
+classes: exploring the design space</ulink> (Simon Peyton Jones, Mark
Jones, Erik Meijer).
</para>
<para>
The type of <literal>elem</literal> is illegal in Haskell 98, because it
contains the constraint <literal>Eq a</literal>, constrains only the
class type variable (in this case <literal>a</literal>).
-GHC lifts this restriction.
+GHC lifts this restriction (flag <option>-XConstrainedClassMethods</option>).
</para>
</title>
<para> Functional dependencies are implemented as described by Mark Jones
-in “<ulink url="http://www.cse.ogi.edu/~mpj/pubs/fundeps.html">Type Classes with Functional Dependencies</ulink>”, Mark P. Jones,
+in “<ulink url="http://citeseer.ist.psu.edu/jones00type.html">Type Classes with Functional Dependencies</ulink>”, Mark P. Jones,
In Proceedings of the 9th European Symposium on Programming,
ESOP 2000, Berlin, Germany, March 2000, Springer-Verlag LNCS 1782,
.
is a type variable that occurs in the head.
</para>
<para>
-The <option>-fglasgow-exts</option> flag loosens these restrictions
+The <option>-XFlexibleInstances</option> flag loosens these restrictions
considerably. Firstly, multi-parameter type classes are permitted. Secondly,
the context and head of the instance declaration can each consist of arbitrary
(well-kinded) assertions <literal>(C t1 ... tn)</literal> subject only to the
</para></listitem>
<listitem><para>
-The standard defaulting rule (<ulink url="http://haskell.org/onlinereport/decls.html#sect4.3.4">Haskell Report, Section 4.3.4</ulink>)
+The standard defaulting rule (<ulink url="http://www.haskell.org/onlinereport/decls.html#sect4.3.4">Haskell Report, Section 4.3.4</ulink>)
is extended thus: defaulting applies when all the unresolved constraints involve standard classes
<emphasis>or</emphasis> <literal>IsString</literal>; and at least one is a numeric class
<emphasis>or</emphasis> <literal>IsString</literal>.
<sect2 id="type-restrictions">
<title>Type signatures</title>
-<sect3><title>The context of a type signature</title>
+<sect3 id="flexible-contexts"><title>The context of a type signature</title>
<para>
Unlike Haskell 98, constraints in types do <emphasis>not</emphasis> have to be of
the form <emphasis>(class type-variable)</emphasis> or
it has rank-2 types on the left of a function arrow.
</para>
<para>
-GHC allows types of arbitrary rank; you can nest <literal>forall</literal>s
-arbitrarily deep in function arrows. (GHC used to be restricted to rank 2, but
-that restriction has now been lifted.)
+GHC has three flags to control higher-rank types:
+<itemizedlist>
+<listitem><para>
+ <option>-XPolymorphicComponents</option>: data constructors (only) can have polymorphic argment types.
+</para></listitem>
+<listitem><para>
+ <option>-XRank2Types</option>: any function (including data constructors) can have a rank-2 type.
+</para></listitem>
+<listitem><para>
+ <option>-XRankNTypes</option>: any function (including data constructors) can have an arbitrary-rank type.
+That is, you can nest <literal>forall</literal>s
+arbitrarily deep in function arrows.
In particular, a forall-type (also called a "type scheme"),
including an operational type class context, is legal:
<itemizedlist>
<listitem> <para> As the type of an implicit parameter </para> </listitem>
<listitem> <para> In a pattern type signature (see <xref linkend="scoped-type-variables"/>) </para> </listitem>
</itemizedlist>
+</para></listitem>
+</itemizedlist>
Of course <literal>forall</literal> becomes a keyword; you can't use <literal>forall</literal> as
a type variable any more!
</para>
[a])</literal>.
</para>
<para>The technical details of this extension are described in the paper
-<ulink url="http://research.microsoft.com/%7Esimonpj/papers/boxy">Boxy types:
+<ulink url="http://research.microsoft.com/%7Esimonpj/papers/boxy/">Boxy types:
type inference for higher-rank types and impredicativity</ulink>,
which appeared at ICFP 2006.
</para>
<para>
In Haskell, a programmer-written type signature is implicitly quantified over
its free type variables (<ulink
-url="http://haskell.org/onlinereport/decls.html#sect4.1.2">Section
+url="http://www.haskell.org/onlinereport/decls.html#sect4.1.2">Section
4.1.2</ulink>
of the Haskel Report).
Lexically scoped type variables affect this implicit quantification rules
The Haskell Report specifies that a group of bindings (at top level, or in a
<literal>let</literal> or <literal>where</literal>) should be sorted into
strongly-connected components, and then type-checked in dependency order
-(<ulink url="http://haskell.org/onlinereport/decls.html#sect4.5.1">Haskell
+(<ulink url="http://www.haskell.org/onlinereport/decls.html#sect4.5.1">Haskell
Report, Section 4.5.1</ulink>).
As each group is type-checked, any binders of the group that
have
an explicit type signature are put in the type environment with the specified
polymorphic type,
and all others are monomorphic until the group is generalised
-(<ulink url="http://haskell.org/onlinereport/decls.html#sect4.5.2">Haskell Report, Section 4.5.2</ulink>).
+(<ulink url="http://www.haskell.org/onlinereport/decls.html#sect4.5.2">Haskell Report, Section 4.5.2</ulink>).
</para>
<para>Following a suggestion of Mark Jones, in his paper
-<ulink url="http://www.cse.ogi.edu/~mpj/thih/">Typing Haskell in
+<ulink url="http://citeseer.ist.psu.edu/424440.html">Typing Haskell in
Haskell</ulink>,
GHC implements a more general scheme. If <option>-XRelaxedPolyRec</option> is
specified:
on the latter. As a result, the specification of the language extension is
also still to some degree in flux. Hence, a more detailed description of
the language extension and its use is currently available
-from <ulink url="http://haskell.org/haskellwiki/GHC/Indexed_types">the Haskell
+from <ulink url="http://www.haskell.org/haskellwiki/GHC/Indexed_types">the Haskell
wiki page on type families</ulink>. The material will be moved to this user's
guide when it has stabilised.
</para>
Haskell.
The background to
the main technical innovations is discussed in "<ulink
-url="http://research.microsoft.com/~simonpj/papers/meta-haskell">
+url="http://research.microsoft.com/~simonpj/papers/meta-haskell/">
Template Meta-programming for Haskell</ulink>" (Proc Haskell Workshop 2002).
</para>
<para>
There is a Wiki page about
-Template Haskell at <ulink url="http://haskell.org/haskellwiki/Template_Haskell">
+Template Haskell at <ulink url="http://www.haskell.org/haskellwiki/Template_Haskell">
http://www.haskell.org/haskellwiki/Template_Haskell</ulink>, and that is the best place to look for
further details.
You may also
(!) f x = 3
</programlisting>
The semantics of Haskell pattern matching is described in <ulink
-url="http://haskell.org/onlinereport/exps.html#sect3.17.2">
+url="http://www.haskell.org/onlinereport/exps.html#sect3.17.2">
Section 3.17.2</ulink> of the Haskell Report. To this description add
one extra item 10, saying:
<itemizedlist><listitem><para>Matching
<literal>v</literal></para></listitem>
</itemizedlist>
</para></listitem></itemizedlist>
-Similarly, in Figure 4 of <ulink url="http://haskell.org/onlinereport/exps.html#sect3.17.3">
+Similarly, in Figure 4 of <ulink url="http://www.haskell.org/onlinereport/exps.html#sect3.17.3">
Section 3.17.3</ulink>, add a new case (t):
<programlisting>
case v of { !pat -> e; _ -> e' }
</programlisting>
</para><para>
That leaves let expressions, whose translation is given in
-<ulink url="http://haskell.org/onlinereport/exps.html#sect3.12">Section
+<ulink url="http://www.haskell.org/onlinereport/exps.html#sect3.12">Section
3.12</ulink>
of the Haskell Report.
In the translation box, first apply
<para>The major effect of an <literal>INLINE</literal> pragma
is to declare a function's “cost” to be very low.
The normal unfolding machinery will then be very keen to
- inline it.</para>
-
+ inline it. However, an <literal>INLINE</literal> pragma for a
+ function "<literal>f</literal>" has a number of other effects:
+<itemizedlist>
+<listitem><para>
+No funtions are inlined into <literal>f</literal>. Otherwise
+GHC might inline a big function into <literal>f</literal>'s right hand side,
+making <literal>f</literal> big; and then inline <literal>f</literal> blindly.
+</para></listitem>
+<listitem><para>
+The float-in, float-out, and common-sub-expression transformations are not
+applied to the body of <literal>f</literal>.
+</para></listitem>
+<listitem><para>
+An INLINE function is not worker/wrappered by strictness analysis.
+It's going to be inlined wholesale instead.
+</para></listitem>
+</itemizedlist>
+All of these effects are aimed at ensuring that what gets inlined is
+exactly what you asked for, no more and no less.
+</para>
<para>Syntactically, an <literal>INLINE</literal> pragma for a
function can be put anywhere its type signature could be
put.</para>
<indexterm><primary><option>-XNoMonomorphismRestriction</option></primary></indexterm>
<para>Haskell's monomorphism restriction (see
-<ulink url="http://haskell.org/onlinereport/decls.html#sect4.5.5">Section
+<ulink url="http://www.haskell.org/onlinereport/decls.html#sect4.5.5">Section
4.5.5</ulink>
of the Haskell Report)
can be completely switched off by
[x] = e -- A pattern binding
</programlisting>
Experimentally, GHC now makes pattern bindings monomorphic <emphasis>by
-default</emphasis>. Use <option>-XMonoPatBinds</option> to recover the
+default</emphasis>. Use <option>-XNoMonoPatBinds</option> to recover the
standard behaviour.
</para>
</sect2>
projects / ghc-hetmet.git / blobdiff