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{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ScopedTypeVariables #-}
 
class FooC a b where
  type FooE a b
  foob :: FooE a b -> b
 
foo :: forall a b . FooC a b => [FooE a b] -> b
foo = map (foob :: FooE a b -> b)
 
main :: IO ()
main = undefined

[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:10:7:
    Could not deduce (b ~ [b])
    from the context (FooC a b)
      bound by the type signature for foo :: FooC a b => [FooE a b] -> b
      at prog.hs:9:8-47
      `b' is a rigid type variable bound by
          the type signature for foo :: FooC a b => [FooE a b] -> b
          at prog.hs:9:17
    Expected type: [FooE a b] -> b
      Actual type: [FooE a0 b] -> [b]
    In the return type of a call of `map'
    In the expression: map (foob :: FooE a b -> b)
    In an equation for `foo': foo = map (foob :: FooE a b -> b)

Standard output is empty