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

David Feuer david.feuer at gmail.com
Mon Apr 18 20:18:57 UTC 2016


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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