"Edward Z. Yang" wrote:

Excerpts from Luke Palmer's message of Tue Aug 10 01:04:04 -0400 2010:

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Except, of course, you want the signature

evalCont :: Cont r a -> a

Which is not possible. But I am not sure where all this discussion is coming from, Maybe and (r ->) cannot be broken out of. Isn't that example enough?

I'm confused... that's the type of evalCont, no?

There is no evalCont, there is runCont: runCont :: (a -> r) -> Cont r a -> r Note that Cont/ContT computations result in a value of type 'r': newtype Cont r a = Cont ((a -> r) -> r) Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

"Edward Z. Yang" wrote:

Excerpts from Ertugrul Soeylemez's message of Tue Aug 10 02:31:14 -0400 2010:

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There is no evalCont, there is runCont:

runCont :: (a -> r) -> Cont r a -> r

Note that Cont/ContT computations result in a value of type 'r':

newtype Cont r a = Cont ((a -> r) -> r)

Yes, but if you pass in 'id' as the continuation to 'runCont', the entire expression will result in 'a'. The continuation monad doesn't act globally...

Then you can only run evalCont, if r = a, which makes that function quite pointless: evalCont :: Cont r r -> r evalCont = runCont id Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

"Edward Z. Yang" wrote:

Excerpts from Ertugrul Soeylemez's message of Tue Aug 10 03:40:02 -0400 2010:

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Then you can only run evalCont, if r = a, which makes that function quite pointless:

evalCont :: Cont r r -> r evalCont = runCont id

Ah, yes, that was what I was imagining. I don't think the function is useless (though it is pointless ;-); it lets you transform continuation-style code into normal code. Also, r is usually not fixed (unless you use mapCont or similar), so it might be more accurately described as Cont a a -> a.

My point was, I would just write 'runCont id'. ;) The result type of the computation is fixed. It cannot change between (>>=). Note that 'a' is the result of one subcomputation, i.e. the result of one particular CPS-style function, while 'r' is the result of the entire computation. So runCont should give you an 'r', not an 'a'. In this case, they just happen to be the same. But of course this is really a matter of taste. =) Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/