error found hole: _ is not appropriate in the following case

f :: [Char]
f =  [x | x <- ['a', 'b'],
 y <- (undefined:undefined:undefined:[]),
 z <- [1,2,3,4],
 r <- [True,False,True,False,True]]

f' :: [Char]
f' =  [x | x <- ['a', 'b'],
 y <- (undefined:undefined:undefined:[]),
 z <- [1,2,3,4],
 r <- [True, False, True, False, True],
 s <- [_, _, _, _, _, _]]

main :: IO ()
main = do
    print f
    print f'

When the code is compiled with the function f the result is the following:\\

"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"

If we add in f the code s <- [_, _, _, _, _, _] to become the function f' and then compile the program, the following error is displayed.\\

test11.hs:5:131: error:
    * Found hole: _ :: t0
      Where: `t0' is an ambiguous type variable
    * In the expression: _
      In the expression: [_, _, _, _, ....]
      In a stmt of a list comprehension: s <- [_, _, _, _, ....]
    * Relevant bindings include
        r :: Bool (bound at test11.hs:5:86)
        z :: Integer (bound at test11.hs:5:70)
        y :: t1 (bound at test11.hs:5:29)
        x :: Char (bound at test11.hs:5:12)
        f' :: [Char] (bound at test11.hs:5:1)

test11.hs:5:134: error:
    * Found hole: _ :: t0
      Where: `t0' is an ambiguous type variable
    * In the expression: _
      In the expression: [_, _, _, _, ....]
      In a stmt of a list comprehension: s <- [_, _, _, _, ....]
    * Relevant bindings include
        r :: Bool (bound at test11.hs:5:86)
        z :: Integer (bound at test11.hs:5:70)
        y :: t1 (bound at test11.hs:5:29)
        x :: Char (bound at test11.hs:5:12)
        f' :: [Char] (bound at test11.hs:5:1)

test11.hs:5:137: error:
    * Found hole: _ :: t0
      Where: `t0' is an ambiguous type variable
    * In the expression: _
      In the expression: [_, _, _, _, ....]
      In a stmt of a list comprehension: s <- [_, _, _, _, ....]
    * Relevant bindings include
        r :: Bool (bound at test11.hs:5:86)
        z :: Integer (bound at test11.hs:5:70)
        y :: t1 (bound at test11.hs:5:29)
        x :: Char (bound at test11.hs:5:12)
        f' :: [Char] (bound at test11.hs:5:1)

test11.hs:5:140: error:
    * Found hole: _ :: t0
      Where: `t0' is an ambiguous type variable
    * In the expression: _
      In the expression: [_, _, _, _, ....]
      In a stmt of a list comprehension: s <- [_, _, _, _, ....]
    * Relevant bindings include
        r :: Bool (bound at test11.hs:5:86)
        z :: Integer (bound at test11.hs:5:70)
        y :: t1 (bound at test11.hs:5:29)
        x :: Char (bound at test11.hs:5:12)
        f' :: [Char] (bound at test11.hs:5:1)

test11.hs:5:143: error:
    * Found hole: _ :: t0
      Where: `t0' is an ambiguous type variable
    * In the expression: _
      In the expression: [_, _, _, _, ....]
      In a stmt of a list comprehension: s <- [_, _, _, _, ....]
    * Relevant bindings include
        r :: Bool (bound at test11.hs:5:86)
        z :: Integer (bound at test11.hs:5:70)
        y :: t1 (bound at test11.hs:5:29)
        x :: Char (bound at test11.hs:5:12)
        f' :: [Char] (bound at test11.hs:5:1)

test11.hs:5:146: error:
    * Found hole: _ :: t0
      Where: `t0' is an ambiguous type variable
    * In the expression: _
      In the expression: [_, _, _, _, ....]
      In a stmt of a list comprehension: s <- [_, _, _, _, ....]
    * Relevant bindings include
        r :: Bool (bound at test11.hs:5:86)
        z :: Integer (bound at test11.hs:5:70)
        y :: t1 (bound at test11.hs:5:29)
        x :: Char (bound at test11.hs:5:12)
        f' :: [Char] (bound at test11.hs:5:1)

I think that here the wildcard pattern _ is justified and does not have to give this error and must be used as undefined.\\ So this error makes no sense here.\\ I propose to make a correction in this case.

Trac metadata

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