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One call site is ill-typed, but GHC reports a type error at every call site, not just the problematic one. In the original code, the problem occurred with a very common function (Data.Lens.view), so the avalanche of error messages made it difficult to find the problematic call site.

{-# 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 ()

-- 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'

The spurious error disappears if illTyped is commented out or moved after wellTyped.

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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