Re: [Haskell-cafe] Lazy Evaluation in Monads

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 wrote:

2011/5/31 Scott Lawrence :

...

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