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| author | simonpj <unknown> | 2004-04-05 10:53:52 +0000 |
|---|---|---|
| committer | simonpj <unknown> | 2004-04-05 10:53:52 +0000 |
| commit | 99655406c82829dfc9663fc545a0e134c49fb79f (patch) | |
| tree | b3bef2d70ec0040362856240b0f797be685f1fb9 /ghc/docs | |
| parent | 25ef3ade4257614cd966e29d0ed595c9db5587a7 (diff) | |
| download | haskell-99655406c82829dfc9663fc545a0e134c49fb79f.tar.gz | |
[project @ 2004-04-05 10:53:52 by simonpj]
Improve documentation about ambiguous types
Diffstat (limited to 'ghc/docs')
| -rw-r--r-- | ghc/docs/users_guide/glasgow_exts.sgml | 30 |
1 files changed, 23 insertions, 7 deletions
diff --git a/ghc/docs/users_guide/glasgow_exts.sgml b/ghc/docs/users_guide/glasgow_exts.sgml index 12e828495e..ceb4882ed2 100644 --- a/ghc/docs/users_guide/glasgow_exts.sgml +++ b/ghc/docs/users_guide/glasgow_exts.sgml @@ -1531,12 +1531,12 @@ in GHC, you can give the foralls if you want. See <xref LinkEnd="universal-quan <emphasis>Each universally quantified type variable <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. +A type variable <literal>a</literal> is "reachable" if it it appears +in the same constraint as either a type variable free in in +<literal>type</literal>, or another reachable type variable. +A value with a type that does not obey +this reachability restriction cannot be used without introducing +ambiguity; that is why the type is rejected. Here, for example, is an illegal type: @@ -1551,7 +1551,23 @@ would be introduced where <literal>tv</literal> is a fresh type variable, and applied to a dictionary for <literal>Eq tv</literal>. The difficulty is that we can never know which instance of <literal>Eq</literal> to use because we never get any more information about <literal>tv</literal>. - +</para> +<para> +Note +that the reachability condition is weaker than saying that <literal>a</literal> is +functionally dependendent on a type variable free in +<literal>type</literal> (see <xref +linkend="functional-dependencies">). The reason for this is there +might be a "hidden" dependency, in a superclass perhaps. So +"reachable" is a conservative approximation to "functionally dependent". +For example, consider: +<programlisting> + class C a b | a -> b where ... + class C a b => D a b where ... + f :: forall a b. D a b => a -> a +</programlisting> +This is fine, because in fact <literal>a</literal> does functionally determine <literal>b</literal> +but that is not immediately apparent from <literal>f</literal>'s type. </para> </listitem> <listitem> |