Re[2]: [Haskell-cafe] Re: Haskell and C++ program

Hello Eugene, Thursday, January 15, 2009, 3:27:59 PM, you wrote: but at least "length.map show" is eq to length :)

Well, your program is not equivalent to the C++ version, since it doesn't bail on incorrect input.

2009/1/15 Apfelmus, Heinrich :

...

Jonathan Cast wrote:

...

reverseDouble = unlines . intro . map show . reverse . map (read :: String -> Double) . takeWhile (/= "end") . words where intro l = ("read " ++ show (length l) ++ " elements") : "elements in reversed order" : l

This can be simplified to

... . map show . reverse . map read . ...

= { map f . reverse = reverse . map f }

... . reverse . map show . map read . ...

= { map f . map g = map (f . g) }

... . reverse . map (show . read) . ...

= { show . read = id }

... . reverse . map id . ...

= { map id = id }

... . reverse . ...

In other words,

reverseDouble = unlines. intro . reverse . takeWhile (/= "end") . words where intro xs = ("read " ++ show (length xs) ++ " elements") : "elements in reversed order" : xs

And the doubles disappeared completely. :)

Regards, H. Apfelmus

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