arrows if command given too general a type

This is a reduced version of a function posted on Haskell Cafe:

{-# LANGUAGE Arrows #-}

testB :: not_bool -> (() -> ()) -> () -> not_unit
testB b f = proc () -> if b then f -< () else f -< ()

There are two problems with the type of this function

• the first argument can be anything but should be Bool and
• the result type can be anything but should be ().

Yet the function is accepted by GHC.

This function can be used to define a value of arbitrary type:

anythingYouWant :: anything
anythingYouWant = testB () (const ()) ()

Here are some example calls from my machine:

ghci> anythingYouWant :: Int
1098066529
ghci> anythingYouWant :: String
""
ghci> anythingYouWant :: Char
'\1098066529'
ghci> anythingYouWant () :: ()
()
ghci> anythingYouWant () :: [()]
[()Segmentation fault

Trac metadata

Trac field	Value
Version	7.0.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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