blob: d530cfd6d020adc5d3ff6ee8937f1f96ac98658c (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
|
{-# LANGUAGE MultiParamTypeClasses #-}
-- This code defines a default method with a highly dubious type,
-- because 'v' is not mentioned, and there are no fundeps
--
-- However, arguably the instance declaration should be accepted,
-- beause it's equivalent to
-- instance Baz Int Int where { foo x = x }
-- which *does* typecheck
-- GHC does not actually macro-expand the instance decl. Instead, it
-- defines a default method function, thus
--
-- $dmfoo :: Baz v x => x -> x
-- $dmfoo y = y
--
-- Notice that this is an ambiguous type: you can't call $dmfoo
-- without triggering an error. And when you write an instance decl,
-- it calls the default method:
--
-- instance Baz Int Int where foo = $dmfoo
--
-- I'd never thought of that. You might think that we should just
-- *infer* the type of the default method (here forall a. a->a), but
-- in the presence of higher rank types etc we can't necessarily do
-- that.
module Foo1 where
class Baz v x where
foo :: x -> x
foo y = y
instance Baz Int Int
|
