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"
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