Re: [Haskell-cafe] coerce (safe!)

On 03/03/2008, at 8:30 AM, Luke Palmer wrote:

2008/3/2 Roman Cheplyaka :

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* Krzysztof Skrzętnicki [2008-03-02 01:21:42 +0100]

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Well, it is simply

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coerce :: a -> b coerce _ = undefined

so coerce is simply empty function. But still, it is possible to write a function of type (a->b). Well, possibly I didn't write anything particularly new, but please excuse me for I'm still in sort of a shock after I've discovered it.

Also there's nice possibility of defining Maybe a without ADT. type Maybe a = (a, Bool) just x = (x, True) nothing = (undefined, False)

That's a hack. This is my favorite:

type Maybe a = forall b. (a -> b) -> b -> b just x = \j n -> j x nothing = \j n -> n

For something slightly different, I've always enjoyed lists (or integer indexed structures) as functions: type List a = Integer -> Maybe a You've got to watch out for non-contiguous lists. It's a good challenge to write head, tail, nil and cons for this type. Then write conversions to/ from normal lists. Bernie.