Commit f3d376e1 authored by simonpj's avatar simonpj
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[project @ 2003-03-07 13:36:59 by simonpj]

Update docs on class method types
parent 230850a2
......@@ -959,38 +959,19 @@ classes: exploring the design space</ULink > (Simon Peyton Jones, Mark
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,
......@@ -1005,11 +986,15 @@ in GHC, you can give the foralls if you want. See <xref LinkEnd="universal-quan
<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>
......@@ -1064,10 +1049,6 @@ territory free in case we need it later.
</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
......@@ -1154,56 +1135,14 @@ be acyclic</emphasis>. So these class declarations are OK:
</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>
......
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