Consider the implementation of `second` for pairs:

second f ~(a,b) = (a, f b)

Now

fix $ second (3 :)

Will be (undefined, [3,3,....])

Translating this to IO, I'd want

lazyMap f undefined

to produce as much as possible of the result, although it cannot produce the final State# RealWorld token.

On Apr 18, 2016 4:47 PM, "David Turner" <dct25-561bs@mythic-beasts.com> wrote:

You can't know that the final result of the computation (x `seq` (3:...)) will begin with 3, because sometimes it doesn't! More specifically, it diverges (without yielding the 3) if x diverges.

I don't think this is anything special about mfix: (let x = x `seq` 3:... in x) also diverges for the same reason.

Hope that helps,

David

On 18 Apr 2016 21:19, "David Feuer" <david.feuer@gmail.com> wrote:

If

f :: a -> IO a

for some a, and I want to use

mfix f

then f must not inspect its argument in any way, or the computation
will get stuck. In some cases, this seems a bit harsh. For example,

mfix (\x -> fmap (3 :) (x `seq` readLn))

looks perfectly reasonable. There is no need to inspect the return []
action to know that the final result of the computation will begin
with 3:. Is there a lazy IO mapping function somewhere that can work
such magic?
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe