[Haskell-beginners] Simple Continuation question

[David McBride]

It's because the type of f is not (String -> String) -> String

It is (String -> Identity String) -> Identity String

Do a replacement manually and you'll see that f has to be of type ->  ContT
String Identity String --> ContT (String -> Identity String) -> Identity
String)

You can see that in the error message Expected type: ContT String Identity
String, Actual type: ContT Char [] String.  The reason why it looks weird
is that it is assuming that your monad instead of being identity is [], and
that the r in "m r" must be a Char in order for that to work.  I'm not
really sure why it chose list, probably type defaulting rules.


On Sat, Jul 12, 2014 at 6:24 AM, martin <martin.drautzburg at web.de> wrote:

> Hello all,
>
> I just started trying to understand Continuations, but my very first
> exercise already left me mystified.
>
> import Control.Monad.Cont
>
> resultIs :: Int -> Cont String String
> resultIs i = ContT $ f
>         where
>             f :: (String -> a) -> a
>             f k = k ("result=" ++ show i)
>
> If resultIs returns a Cont String String, then f should be
> (String->String)->String, but that doesn't compile. Why is
> that so?
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