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Consider the following valid Haskell 98 function:

f x =
  do let apply p r = mapM p (r x)
     a <- apply id     fst
     b <- apply return snd
     return (a ++ b)

It compiles fine with both ghc 6.12.x and ghc 7. However, if I add the -XTypeFamilies flag to ghc 7 it is rejected with:

mono.hs:5:17:
    Occurs check: cannot construct the infinite type: a0 = m0 a0
    Expected type: m0 a0 -> m0 a0
      Actual type: m0 a0 -> m0 (m0 a0)
    In the first argument of `apply', namely `return'
    In a stmt of a 'do' expression: b <- apply return snd

If one adds a type signature, the error message gets more intimidating.

A project of mine uses Type Families in some modules so TypeFamilies is listed as an extension in cabal's configuration file; therefore, it is being passed to every module. That is how I stumbled upon this problem.

Trac metadata

Trac field	Value
Version	7.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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