blob: 84e8fa95925f488e10fc9b38a459a1d2d25d0f15 (
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
|
{-# LANGUAGE FlexibleInstances, FunctionalDependencies, MultiParamTypeClasses #-}
module Bug where
class FunDep lista a | lista -> a
instance FunDep [a] a
singleton :: FunDep lista a => a -> lista
singleton _ = undefined
-- this error is expected:
-- Couldn't match type 'Char' with '()'
-- arising from a functional dependency between
-- constraint 'FunDep [Char] ()' arising from a use of 'singleton'
-- instance 'FunDep [a] a'
illTyped :: [Char]
illTyped = singleton ()
{- [W] FunDep [Char] () -}
-- but this one is not:
-- Couldn't match type '()' with 'Char'
-- arising from a functional dependency between constraints:
-- 'FunDep [Char] Char' arising from a use of 'singleton' (in 'wellTyped')
-- 'FunDep [Char] ()' arising from a use of 'singleton' (in 'illTyped')
wellTyped :: [Char]
wellTyped = singleton 'a'
{- [W] FunDep [Char] Char -}
|
