Re: [Haskell-cafe] List spine traversal

seq allows you to distinguish (undefined) from (const undefined) case allows you to distinguish (undefined) from the pair (undefined, undefined) isFun f = f `seq` True isPair p = case p of (_,_) -> True isFun undefined -> undefined isFun (const undefined) -> True isPair undefined -> undefined isPair (undefined, undefined) -> True I don't think either of these violate equational reasoning or referential transparency, though. You cannot treat a call to seq as "flip const", though, it has to be a bit more magic than that. You do get different bottoms, however; (length xs1 + length xs1) isn't terminating, while (length xs2 + length xs2) will probably consume your heap. But that just means that you can distinguish them in IO, which is allowed :) -- ryan On Tue, Jun 30, 2009 at 1:38 PM, Don Stewart wrote:

andrewhhunter:

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On Mon, Jun 29, 2009 at 11:30 PM, Ryan Ingram wrote:

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There can't be a way to do so that is pure, because such a function could distinguish between

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xs1 = () : xs1 and xs2 = f () where f () = () : f ()

But doesn't seq and friends cause many of our normal referential transparency guarantees to be invalid?

Equational reasoning, in the presence of bottoms, anyway.

-- Don