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I open this ticket instead of #13579 to restate it and I closed the other.\\ In fact a result of the compiler which I think is wrong misled me.\\ Therefore I close tickets #13602 and #13557.\\ In Haskell2010 Language Report it is said that :\\

1. underscore "_ " all by itself is a reserved identifier.\\
2. underscore "_" is treated as a lowercase letter, and can occur wherever a lowercase letter can.\\

So _e is an identifier like __\\

GHCi gives a bad result when he computed the code below.\\

Prelude> :set -fdefer-typed-holes
Prelude> let f = map (\x -> True) [_, _]

<interactive>:2:27: warning: [-Wtyped-holes]
    * Found hole: _ :: a0
      Where: `a0' is an ambiguous type variable
    * In the expression: _
      In the second argument of `map', namely `[_, _]'
      In the expression: map (\ x -> True) [_, _]
    * Relevant bindings include
        f :: [Bool] (bound at <interactive>:2:5)

<interactive>:2:30: warning: [-Wtyped-holes]
    * Found hole: _ :: a0
      Where: `a0' is an ambiguous type variable
    * In the expression: _
      In the second argument of `map', namely `[_, _]'
      In the expression: map (\ x -> True) [_, _]
    * Relevant bindings include
        f :: [Bool] (bound at <interactive>:2:5)
Prelude> f
[True,True]

The underscore "_" is alone , all by itself and yet is recognized as an identifier, and GHCi gives a result.\\ This is the bug.

Trac metadata

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