[Haskell-cafe] applicative challenge
ketil at malde.org
Tue May 5 10:06:52 EDT 2009
Thomas Hartman <tphyahoo at gmail.com> writes:
> That's slick, but is there some way to use interact twice in the same program?
> t10 =
> let f = unlines . takeWhile (not . blank) . lines
> in do putStrLn "first time"
> interact f
> putStrLn "second time"
> interact f
> this results in *** Exception: <stdin>: hGetContents: illegal
> operation (handle is closed) -}
Yes. Interacting uses hGetContents, and hGetContents semi-closes (or
fully-closes) the handle. If you do it from GHCi, you only get to run
your program once.
> I also tried
> t15 =
> let grabby = unlines . takeWhile (not . blank) . lines
> top = ("first time: " ++) . grabby . ("second time: " ++) . grabby
> in interact top
> but that didn't work either:
> thartman at ubuntu:~/haskell-learning/lazy-n-strict>runghc sequencing.hs
> first time: second time: a
Well - the input to the leftmost grabby is "second time" prepended to
the input from the first, and then you prepend "first time" - so this
Something like this, perhaps:
interact (\s -> let (first,second) = span (not . null) (lines s)
in unlines ("first":first++"second":takeWhile (not.null) second))
> If someone can explain the subtleties of using interact when you run
> out of stdio here, it would be nice to incorporate this into
hGetContents - there can only be one.
If I haven't seen further, it is by standing in the footprints of giants
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