[Haskell-cafe] Polymorphic algebraic type constructors

Adrian Hey ahey at iee.org
Tue Jun 22 15:52:36 EDT 2004

On Tuesday 22 Jun 2004 6:20 pm, MR K P SCHUPKE wrote:
> ahh but in this example:
>  f :: [Int] -> [Bool]
>  f (i:is) = even i : f is
>  f e@[]   = e
> e is an empty list of Ints not an empty list of Bools!

If the difference is significant (I don't believe it is)
then consistency demands that this expression should
give a type error..

 let e=[] in (length e : e, null e : e)

It doesn't, so clearly e can be both an empty list of
Ints and an empty list of Bools :-)

Adrian Hey

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