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haskellpointfree

Point free style in haskell

I'm trying to change 

function (x:xs) = unwords (map reverse (words (x:xs)))

into point free style, and I can't do this.

isn't it simply 

function = unwords . map . reverse . words

Please help me to change this to point free style.

I tried to use pointfree after doing below command line

cabal install pointfree

Everything compiled and installed but when I type pointfree in terminal, it's keep saying there's no such thing.

Solution

  • Your definition is almost correct and can be tested in ghci interpreter, it should be

    ghci> let function = unwords . map reverse . words
ghci> function "The quick brown fox"
"ehT kciuq nworb xof"

    because reverse is an argument of map. Basically, to change pipeline-like function foo a3 = f a1 (g a2 (h a3)) (consisting of nested function calls) into pointfree, where the explicit argument (a3 in this case) is the last called, you need to remove it and replace nested parentheses with . composition operator: foo = f a1 . g a2 . h. However, this trick doesn't work when there are multiple arguments or the argument is used more than once. To meaningly operate with pointfree functions, you should understand combinatory logic.

    EDIT: oh, I got that you tried to use the command-line pointfree tool to auto-transform the function. Binaries compiled with cabal are placed in specific directory (for example, C:\Users\Username\AppData\Roaming\cabal\bin in Windows, AFAIR) which is probably isn't added to PATH by default and so command line interpreter can't find the executable.