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This program:

foo :: a -> a
bar :: a -> a
(foo, bar) = (\x -> x, \y -> y)

main = print $ foo $ bar 1

Is trying to pattern-bind a polymorphic function. The error message is dreadful:

/Users/mbolingbroke/Junk/PolyPatBinds.hs:5:15:
    Couldn't match expected type `t -> t1'
                with actual type `forall a. a -> a'
    The lambda expression `\ x -> x' has one argument one argument,
    but its type `forall a. a -> a' has none
    In the expression: \ x -> x
    In the expression: (\ x -> x, \ y -> y)

The issues are:

1. It repeats itself: "one argument one argument"
2. It is patently incorrect: the type forall a. a -> a DOES have one argument in the standard nomenclature
3. It does not point the user towards the fix (-XNoMonoPatBinds)

Trac metadata

Trac field	Value
Version	7.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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