Question

In a similar problem as the one showed in this question, is it possible to have a Haskell expression of “generic” type a? Something like,

myExpression :: a

I’m new to Haskell, but from what i’ve been seeing this cannot be achieved.

Answer 1

No, there is nothing that satisfies that other than bottom, e.g.

myExpression = myExpression

-- or,

myExpression = undefined

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Is there any reasonable way that it could be possible? Some non-bottom expression that is both an Integer and of type Maybe (String -> IO ()) (for example).

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Furthermore, given the question you mention, one answer of which proved that the only non-bottom function with type signature a -> a is id, we have a proof that there can’t be a non-bottom expression with type a. If there was, then

f _ = myExpression

could have type a -> a, and this is neither id nor bottom, i.e. a contradiction.