h4sh

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                              h4sh

Requirements: > make (GNU or BSD) > The Glasgow Haskell Compiler > Cabal > hs-plugins: darcs get http://www.cse.unsw.edu.au/~dons/code/hs-plugins > fps 0.5: darcs get http://www.cse.unsw.edu.au/~dons/code/fps

Building: > PREFIX=/tmp make > make install

also > make check (you may need to adjust the variable `GM4' in the toplevel Makefile, or in the environment)

Manifesto:

Unix is all about programs that do one thing, and one thing well. Unfortunately, over time, the common unix text processing commands have become bloated and silly, with rather arbitrary features for programs that should have simple semantics (consider uniq and wc outputting leading space, or cut indexing fields from 1).

On the other hand Haskell has a powerful and beautiful List library for processing text. By exposing the Haskell List library as a set of shell utilities, and utlising function composition via pipes, we can program in the shell using these precise, clean Haskell functions.

To this end, h4sh makes the functions of this library available as unix shell commands.

For example: take 100 data | map show.length | sort | reverse | head

Commands are generated from a description of their type. h4sh utilities thus have standard behaviour, for example, all functions read from stdin or file arguments (meaning that id' is equivalent cat'), and arguments are handled in the order they appear in the function type signature.

h4sh currently compiles functions of the following types:

    a      -> a
    a     -> [a]
    [a]   -> a
    [a]   -> Int
    [a]   -> [a]
    [[a]] -> [a]
    a     -> [a]  -> [a]
    a     -> [a]  -> [Int]
    Int   -> [a]  -> [a]
    [a]   -> [a]  -> [a]
    (a -> a)      -> a   -> a
    (a -> a)      -> a   -> [a]
    (a -> a)      -> [a] -> [a]
    (a -> a)      -> [a] -> a
    (a -> Bool)   -> [a] -> [a]
    (a -> a -> a) -> [a] -> a
    (a -> Maybe (a, a))  -> a -> [a]

The following functions are provided as shell commands:

    (!!) ($) (++) (:) (\\)
    concat concatMap cycle
    delete drop dropWhile
    elemIndices filter foldl
    foldr group head id init
    insert intersect intersperse
    iterate last length map
    maximum minimum nub repeat
    reverse show sort tail
    take takeWhile transpose
    unfoldr union words zip

Higher order functions like map are handled using runtime evaluation, provided by the hs-plugins library, allowing arbitrary Haskell code to be evaluated, e.g. for map and filter.

h4sh also provides a library H4SH.List of useful String and Regex functions.

h4sh was written during a cold, but sunny weekend in August 2005.