[Haskell-cafe] Is it possible to make lazy combinators for IO?

[David Turner]

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 at 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?
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