[Haskell-cafe] Lazy Evaluation in Monads

[Scott Lawrence]

Apparently:

Prelude> let r = (fmap (1:) r) :: IO [Integer]
Prelude> fmap (take 5) r
*** Exception: stack overflow

Thanks - I'll just have to stay out of IO for this, then.

On Tue, May 31, 2011 at 17:05, Stephen Tetley <stephen.tetley at gmail.com> wrote:
> 2011/5/31 Scott Lawrence <bytbox at gmail.com>:
>
>> Evaluation here also doesn't terminate (or, (head $ unfoldM (return .
>> head)) doesn't), although I can't figure out why. fmap shouldn't need to
>> fully evaluate a list to prepend an element, right?
>
> I'm afriad fmap doesn't get to choose - if the monad is strict then
> both definitions are equivalent (probably...).
>
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-- 
Scott Lawrence