Jones, Erik Meijer).
</para>
-<para>
-I'd like to thank people who reported shorcomings in the GHC 3.02
-implementation. Our default decisions were all conservative ones, and
-the experience of these heroic pioneers has given useful concrete
-examples to support several generalisations. (These appear below as
-design choices not implemented in 3.02.)
-</para>
-
-<para>
-I've discussed these notes with Mark Jones, and I believe that Hugs
-will migrate towards the same design choices as I outline here.
-Thanks to him, and to many others who have offered very useful
-feedback.
-</para>
-<sect3>
+<sect3 id="type-restrictions">
<title>Types</title>
<para>
-There are the following restrictions on the form of a qualified
-type:
-</para>
-
-<para>
+GHC imposes the following restrictions on the form of a qualified
+type, whether declared in a type signature
+or inferred. Consider the type:
<programlisting>
forall tv1..tvn (c1, ...,cn) => type
</programlisting>
-</para>
-
-<para>
(Here, I write the "foralls" explicitly, although the Haskell source
language omits them; in Haskell 1.4, all the free type variables of an
explicit source-language type signature are universally quantified,
<para>
<emphasis>Each universally quantified type variable
-<literal>tvi</literal> must be mentioned (i.e. appear free) in <literal>type</literal></emphasis>.
+<literal>tvi</literal> must be reachable from <literal>type</literal></emphasis>.
+A type variable is "reachable" if it it is functionally dependent
+(see <xref linkend="functional-dependencies">)
+on the type variables free in <literal>type</literal>.
The reason for this is that a value with a type that does not obey
this restriction could not be used without introducing
-ambiguity. Here, for example, is an illegal type:
+ambiguity.
+Here, for example, is an illegal type:
<programlisting>
</para>
-<para>
-These restrictions apply to all types, whether declared in a type signature
-or inferred.
-</para>
<para>
Unlike Haskell 1.4, constraints in types do <emphasis>not</emphasis> have to be of
</para>
</listitem>
-<listitem>
-
-<para>
- <emphasis>In the signature of a class operation, every constraint
-must mention at least one type variable that is not a class type
-variable</emphasis>.
-
-Thus:
-
-
-<programlisting>
- class Collection c a where
- mapC :: Collection c b => (a->b) -> c a -> c b
-</programlisting>
-
-
-is OK because the constraint <literal>(Collection a b)</literal> mentions
-<literal>b</literal>, even though it also mentions the class variable
-<literal>a</literal>. On the other hand:
-
-
-<programlisting>
- class C a where
- op :: Eq a => (a,b) -> (a,b)
-</programlisting>
-
-
-is not OK because the constraint <literal>(Eq a)</literal> mentions on the class
-type variable <literal>a</literal>, but not <literal>b</literal>. However, any such
-example is easily fixed by moving the offending context up to the
-superclass context:
-
-<programlisting>
- class Eq a => C a where
- op ::(a,b) -> (a,b)
-</programlisting>
-
-
-A yet more relaxed rule would allow the context of a class-op signature
-to mention only class type variables. However, that conflicts with
-Rule 1(b) for types above.
-
-</para>
-</listitem>
<listitem>
<para>
- <emphasis>The type of each class operation must mention <emphasis>all</emphasis> of
-the class type variables</emphasis>. For example:
+ <emphasis>All of the class type variables must be reachable (in the sense
+mentioned in <xref linkend="type-restrictions">)
+from the free varibles of each method type
+</emphasis>. For example:
<programlisting>
instance Stateful (ST s) (MutVar s) where ...
</programlisting>
-
-The "at least one not a type variable" restriction is to ensure that
-context reduction terminates: each reduction step removes one type
-constructor. For example, the following would make the type checker
-loop if it wasn't excluded:
-
-
-<programlisting>
- instance C a => C a where ...
-</programlisting>
-
-
-There are two situations in which the rule is a bit of a pain. First,
-if one allows overlapping instance declarations then it's quite
-convenient to have a "default instance" declaration that applies if
-something more specific does not:
-
-
-<programlisting>
- instance C a where
- op = ... -- Default
-</programlisting>
-
-
-Second, sometimes you might want to use the following to get the
-effect of a "class synonym":
-
-
-<programlisting>
- class (C1 a, C2 a, C3 a) => C a where { }
-
- instance (C1 a, C2 a, C3 a) => C a where { }
-</programlisting>
-
-
-This allows you to write shorter signatures:
-
-
-<programlisting>
- f :: C a => ...
-</programlisting>
-
-
-instead of
-
-
-<programlisting>
- f :: (C1 a, C2 a, C3 a) => ...
-</programlisting>
-
-
-I'm on the lookout for a simple rule that preserves decidability while
-allowing these idioms. The experimental flag
-<option>-fallow-undecidable-instances</option><indexterm><primary>-fallow-undecidable-instances
-option</primary></indexterm> lifts this restriction, allowing all the types in an
-instance head to be type variables.
-
+See <xref linkend="undecidable-instances"> for an experimental
+extension to lift this restriction.
</para>
</listitem>
<listitem>
</programlisting>
-is not OK. Again, the intent here is to make sure that context
-reduction terminates.
+is not OK. See <xref linkend="undecidable-instances"> for an experimental
+extension to lift this restriction.
+
-Voluminous correspondence on the Haskell mailing list has convinced me
-that it's worth experimenting with a more liberal rule. If you use
-the flag <option>-fallow-undecidable-instances</option> can use arbitrary
-types in an instance context. Termination is ensured by having a
-fixed-depth recursion stack. If you exceed the stack depth you get a
-sort of backtrace, and the opportunity to increase the stack depth
-with <option>-fcontext-stack</option><emphasis>N</emphasis>.
</para>
</listitem>
</sect2>
+<sect2 id="undecidable-instances">
+<title>Undecidable instances</title>
+
+<para>The rules for instance declarations state that:
+<itemizedlist>
+<listitem><para>At least one of the types in the <emphasis>head</emphasis> of
+an instance declaration <emphasis>must not</emphasis> be a type variable.
+</para></listitem>
+<listitem><para>All of the types in the <emphasis>context</emphasis> of
+an instance declaration <emphasis>must</emphasis> be type variables.
+</para></listitem>
+</itemizedlist>
+These restrictions ensure that
+context reduction terminates: each reduction step removes one type
+constructor. For example, the following would make the type checker
+loop if it wasn't excluded:
+<programlisting>
+ instance C a => C a where ...
+</programlisting>
+There are two situations in which the rule is a bit of a pain. First,
+if one allows overlapping instance declarations then it's quite
+convenient to have a "default instance" declaration that applies if
+something more specific does not:
+
+
+<programlisting>
+ instance C a where
+ op = ... -- Default
+</programlisting>
+
+
+Second, sometimes you might want to use the following to get the
+effect of a "class synonym":
+
+
+<programlisting>
+ class (C1 a, C2 a, C3 a) => C a where { }
+
+ instance (C1 a, C2 a, C3 a) => C a where { }
+</programlisting>
+
+
+This allows you to write shorter signatures:
+
+
+<programlisting>
+ f :: C a => ...
+</programlisting>
+
+
+instead of
+
+
+<programlisting>
+ f :: (C1 a, C2 a, C3 a) => ...
+</programlisting>
+
+
+Voluminous correspondence on the Haskell mailing list has convinced me
+that it's worth experimenting with more liberal rules. If you use
+the experimental flag <option>-fallow-undecidable-instances</option>
+<indexterm><primary>-fallow-undecidable-instances
+option</primary></indexterm>, you can use arbitrary
+types in both an instance context and instance head. Termination is ensured by having a
+fixed-depth recursion stack. If you exceed the stack depth you get a
+sort of backtrace, and the opportunity to increase the stack depth
+with <option>-fcontext-stack</option><emphasis>N</emphasis>.
+</para>
+<para>
+I'm on the lookout for a less brutal solution: a simple rule that preserves decidability while
+allowing these idioms interesting idioms.
+</para>
+</sect2>
+
<sect2 id="implicit-parameters">
<title>Implicit parameters
</title>
ESOP 2000, Berlin, Germany, March 2000, Springer-Verlag LNCS 1782,
.
</para>
-
<para>
+Functional dependencies are introduced by a vertical bar in the syntax of a
+class declaration; e.g.
+<programlisting>
+ class (Monad m) => MonadState s m | m -> s where ...
+
+ class Foo a b c | a b -> c where ...
+</programlisting>
There should be more documentation, but there isn't (yet). Yell if you need it.
</para>
</sect2>
<sect2 id="inline-pragma">
<title>INLINE pragma
-<indexterm><primary>INLINE pragma</primary></indexterm>
+<indexterm><primary>INLINE and NOINLINE pragmas</primary></indexterm>
<indexterm><primary>pragma, INLINE</primary></indexterm></title>
<para>
GHC (with <option>-O</option>, as always) tries to inline (or “unfold”)
functions/values that are “small enough,” thus avoiding the call
overhead and possibly exposing other more-wonderful optimisations.
-</para>
-
-<para>
-You will probably see these unfoldings (in Core syntax) in your
-interface files.
-</para>
-
-<para>
Normally, if GHC decides a function is “too expensive” to inline, it
will not do so, nor will it export that unfolding for other modules to
use.
{-# INLINE key_function #-}
#endif
</programlisting>
-
(You don't need to do the C pre-processor carry-on unless you're going
to stick the code through HBC—it doesn't like <literal>INLINE</literal> pragmas.)
</para>
</para>
<para>
-An <literal>INLINE</literal> pragma for a function can be put anywhere its type
+Syntactially, an <literal>INLINE</literal> pragma for a function can be put anywhere its type
signature could be put.
</para>
</para>
-</sect2>
-
-<sect2 id="noinline-pragma">
-<title>NOINLINE pragma
-</title>
+<sect3 id="noinline-pragma">
+<title>The NOINLINE pragma </title>
<indexterm><primary>NOINLINE pragma</primary></indexterm>
<indexterm><primary>pragma</primary><secondary>NOINLINE</secondary></indexterm>
<literal>NOINLINE</literal> (<literal>NOTINLINE</literal> is specified
by Haskell 98 as the standard way to disable inlining, so it should be
used if you want your code to be portable).</para>
+</sect3>
+
+
+<sect3 id="phase-control">
+<title>Phase control</title>
+
+<para> Sometimes you want to control exactly when in GHC's pipeline
+the INLINE pragma is switched on. Inlining happens only during runs of
+the <emphasis>simplifier</emphasis>. Each run of the simplifier has a different
+<emphasis>phase number</emphasis>; the phase number decreases towards zero.
+If you use <option>-dverbose-core2core</option>
+you'll see the sequence of phase numbers for successive runs of the simpifier.
+In an INLINE pragma you can optionally specify a phase number, thus:
+<itemizedlist>
+<listitem> <para>You can say "inline <literal>f</literal> in Phase 2 and all subsequent
+phases":
+<programlisting>
+ {-# INLINE [2] f #-}
+</programlisting>
+</para></listitem>
+
+<listitem> <para>You can say "inline <literal>g</literal> in all phases up to, but
+not including, Phase 3":
+<programlisting>
+ {-# INLINE [~3] g #-}
+</programlisting>
+</para></listitem>
+
+<listitem> <para>If you omit the phase indicator, you mean "inline in all phases".
+</para></listitem>
+</itemizedlist>
+You can use a phase number on a NOINLINE pragma too:
+<itemizedlist>
+<listitem> <para>You can say "do not inline <literal>f</literal> until Phase 2; in
+Phase 2 and subsequently behave as if there was no pragma at all":
+<programlisting>
+ {-# NOINLINE [2] f #-}
+</programlisting>
+</para></listitem>
+
+<listitem> <para>You can say "do not inline <literal>g</literal> in Phase 3 or any subsequent phase;
+before that, behave as if there was no pragma":
+<programlisting>
+ {-# NOINLINE [~3] g #-}
+</programlisting>
+</para></listitem>
+
+<listitem> <para>If you omit the phase indicator, you mean "never inline this function".
+</para></listitem>
+</itemizedlist>
+</para>
+<para>The same phase-numbering control is available for RULES (<xref LinkEnd="rewrite-rules">).</para>
+</sect3>
+
+
</sect2>
+<sect2 id="rules">
+<title>RULES pragma</title>
+
+<para>
+The RULES pragma lets you specify rewrite rules. It is described in
+<xref LinkEnd="rewrite-rules">.
+</para>
+
+</sect2>
+
+
<sect2 id="specialize-pragma">
<title>SPECIALIZE pragma</title>
</sect2>
-<sect2 id="rules">
-<title>RULES pragma</title>
-
-<para>
-The RULES pragma lets you specify rewrite rules. It is described in
-<xref LinkEnd="rewrite-rules">.
-</para>
-
-</sect2>
-
<sect2 id="deprecated-pragma">
<title>DEPRECATED pragma</title>
<listitem>
<para>
+ There may be zero or more rules in a <literal>RULES</literal> pragma.
+</para>
+</listitem>
+
+<listitem>
+
+<para>
Each rule has a name, enclosed in double quotes. The name itself has
no significance at all. It is only used when reporting how many times the rule fired.
</para>
</listitem>
-<listitem>
+<listitem>
<para>
- There may be zero or more rules in a <literal>RULES</literal> pragma.
+A rule may optionally have a phase-control number (see <xref LinkEnd="phase-control">),
+immediately after the name of the rule. Thus:
+<programlisting>
+ {-# RULES
+ "map/map" [2] forall f g xs. map f (map g xs) = map (f.g) xs
+ #-}
+</programlisting>
+The "[2]" means that the rule is active in Phase 2 and subsequent phases. The inverse
+notation "[~2]" is also accepted, meaning that the rule is active up to, but not including,
+Phase 2.
</para>
</listitem>
+
+
<listitem>
<para>
enclosing definitions.
</para>
</listitem>
+
<listitem>
<para>
<listitem>
<para>
- The defintion of (say) <function>build</function> in <FileName>PrelBase.lhs</FileName> looks llike this:
+ The defintion of (say) <function>build</function> in <FileName>GHC/Base.lhs</FileName> looks llike this:
<programlisting>
build :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a]
<listitem>
<para>
- In <filename>ghc/lib/std/PrelBase.lhs</filename> look at the rules for <function>map</function> to
+ In <filename>libraries/base/GHC/Base.lhs</filename> look at the rules for <function>map</function> to
see how to write rules that will do fusion and yet give an efficient
-program even if fusion doesn't happen. More rules in <filename>PrelList.lhs</filename>.
+program even if fusion doesn't happen. More rules in <filename>GHC/List.lhs</filename>.
</para>
</listitem>
</sect2>
+<sect2 id="core-pragma">
+ <title>CORE pragma</title>
+
+ <indexterm><primary>CORE pragma</primary></indexterm>
+ <indexterm><primary>pragma, CORE</primary></indexterm>
+ <indexterm><primary>core, annotation</primary></indexterm>
+
+<para>
+ The external core format supports <quote>Note</quote> annotations;
+ the <literal>CORE</literal> pragma gives a way to specify what these
+ should be in your Haskell source code. Syntactically, core
+ annotations are attached to expressions and take a Haskell string
+ literal as an argument. The following function definition shows an
+ example:
+
+<programlisting>
+f x = ({-# CORE "foo" #-} show) ({-# CORE "bar" #-} x)
+</programlisting>
+
+ Sematically, this is equivalent to:
+
+<programlisting>
+g x = show x
+</programlisting>
+</para>
+
+<para>
+ However, when external for is generated (via
+ <option>-fext-core</option>), there will be Notes attached to the
+ expressions <function>show</function> and <VarName>x</VarName>.
+ The core function declaration for <function>f</function> is:
+</para>
+
+<programlisting>
+ f :: %forall a . GHCziShow.ZCTShow a ->
+ a -> GHCziBase.ZMZN GHCziBase.Char =
+ \ @ a (zddShow::GHCziShow.ZCTShow a) (eta::a) ->
+ (%note "foo"
+ %case zddShow %of (tpl::GHCziShow.ZCTShow a)
+ {GHCziShow.ZCDShow
+ (tpl1::GHCziBase.Int ->
+ a ->
+ GHCziBase.ZMZN GHCziBase.Char -> GHCziBase.ZMZN GHCziBase.Cha
+r)
+ (tpl2::a -> GHCziBase.ZMZN GHCziBase.Char)
+ (tpl3::GHCziBase.ZMZN a ->
+ GHCziBase.ZMZN GHCziBase.Char -> GHCziBase.ZMZN GHCziBase.Cha
+r) ->
+ tpl2})
+ (%note "foo"
+ eta);
+</programlisting>
+
+<para>
+ Here, we can see that the function <function>show</function> (which
+ has been expanded out to a case expression over the Show dictionary)
+ has a <literal>%note</literal> attached to it, as does the
+ expression <VarName>eta</VarName> (which used to be called
+ <VarName>x</VarName>).
+</para>
+
+</sect2>
+
</sect1>
<sect1 id="generic-classes">
instance (Bin a, Bin b) => Bin (a,b)
instance Bin a => Bin [a]
</programlisting>
-That is, just leave off the "where" clasuse. Of course, you can put in the
+That is, just leave off the "where" clause. Of course, you can put in the
where clause and over-ride whichever methods you please.
</para>
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