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This code

{-# LANGUAGE UnicodeSyntax #-}

id' :: ∀ a. a → a
id' a = a

produces following error

/tmp/id.hs:4:8: parse error on input `∀'

However, equivalent code without unicode syntax gives a much better error message:

id' :: forall a. a → a
id' a = a

/tmp/id.hs:4:16:
    Illegal symbol '.' in type
    Perhaps you intended -XRankNTypes or similar flag
    to enable explicit-forall syntax: forall <tvs>. <type>

Trac metadata

Trac field	Value
Version	7.6.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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