[Haskell-cafe] Is it possible to make lazy combinators for IO?
david.feuer at gmail.com
Mon Apr 18 21:03:31 UTC 2016
Consider the implementation of `second` for pairs:
second f ~(a,b) = (a, f b)
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 at mythic-beasts.com>
> 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,
> On 18 Apr 2016 21:19, "David Feuer" <david.feuer at gmail.com> wrote:
>> 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 at haskell.org
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