Commit 0f556c99 authored by simonpj@microsoft.com's avatar simonpj@microsoft.com
Browse files

Improvd documentation for overlapping instances

parent fea8c9e4
......@@ -2810,9 +2810,23 @@ Suppose that from the RHS of <literal>f</literal> we get the constraint
GHC does not commit to instance (C), because in a particular
call of <literal>f</literal>, <literal>b</literal> might be instantiate
to <literal>Int</literal>, in which case instance (D) would be more specific still.
So GHC rejects the program. If you add the flag <option>-XIncoherentInstances</option>,
So GHC rejects the program.
(If you add the flag <option>-XIncoherentInstances</option>,
GHC will instead pick (C), without complaining about
the problem of subsequent instantiations.
the problem of subsequent instantiations.)
Notice that we gave a type signature to <literal>f</literal>, so GHC had to
<emphasis>check</emphasis> that <literal>f</literal> has the specified type.
Suppose instead we do not give a type signature, asking GHC to <emphasis>infer</emphasis>
it instead. In this case, GHC will refrain from
simplifying the constraint <literal>C Int [Int]</literal> (for the same reason
as before) but, rather than rejecting the program, it will infer the type
<programlisting>
f :: C Int b => [b] -> [b]
</programlisting>
That postpones the question of which instance to pick to the
call site for <literal>f</literal>
by which time more is known about the type <literal>b</literal>.
</para>
<para>
The willingness to be overlapped or incoherent is a property of
......
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