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Simple Unix tools

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(can be improved a bit more)
(Try to keep the *clean* qualities of the original article. Type signatures are *not* included, since we want to emphasies type inference)
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Simple Unix commandline tools written in Haskell.
 
Simple Unix commandline tools written in Haskell.
   
This is intended as a beginner's tutorial for learning Haskell from a
+
This is intended as a beginner's tutorial for learning Haskell from a
 
"Let's just solve things already!" point of view. The examples should
 
"Let's just solve things already!" point of view. The examples should
 
help give a flavor of the beauty and expressiveness of Haskell
 
help give a flavor of the beauty and expressiveness of Haskell
 
programming.
 
programming.
   
These functions can be executed as one liners from a shell. For example, to use the Haskell version of 'wc':
+
These functions can be executed as one liners from a shell. For example,
  +
to use the Haskell version of 'wc':
   
$ cat file.txt | ghc -e 'wc_l' UnixTools.hs
+
$ cat file.txt | ghc -e 'wc_l' UnixTools.hs
   
Or, one could define 'main' to be a chosen tool/function (add a line to the effect that "main = wc_l") and then compile the tool with
+
Or, one could define 'main' to be a chosen tool/function (add a line to
$ ghc --make UnixTools.hs
+
the effect that "main = wc_l") and then compile the tool with
   
The given Haskell codes presents yet a third way of doing things: much like the BusyBox[http://en.wikipedia.org/wiki/BusyBox] suite of Unix tools, it is possible to compile a single monolithic binary and have it detect what name it is run by and then act appropriately. This is the approach the following code takes: you can compile it and then make symbolic links (like "ln -s UnixTools echo") and then run those commands ("echo foo | ./echo" would produce output of "foo").
+
$ ghc --make UnixTools.hs
  +
  +
The given Haskell codes presents yet a third way of doing things: much
  +
like the [http://en.wikipedia.org/wiki/BusyBox BusyBox] suite of Unix
  +
tools, it is possible to compile a single monolithic binary and have it
  +
detect what name it is run by and then act appropriately. This is the
  +
approach the following code takes: you can compile it and then make
  +
symbolic links (like <code>"ln -s UnixTools echo"</code>) and then run
  +
those commands (<code>"echo foo | ./echo"</code> would produce output of
  +
"foo").
   
 
<haskell>
 
<haskell>
-- Some Unix-like tools written in simple, clean Haskell
 
module Main
 
where
 
import Data.List ( sort, nub, intersperse, delete, isPrefixOf, foldl' )
 
import Data.Char ( toUpper, isSpace, ord )
 
import Text.Printf ( printf )
 
import System.Environment ( getProgName )
 
   
main = do
+
import Data.List
calledBy <- getProgName
+
import Data.Char
case calledBy of
+
import Data.Maybe
"blank" -> io blank
+
import Text.Printf
"cksum" -> interact (showln . cksum)
+
import System.Environment
"clean" -> io clean''
 
"echo" -> interact id -- not perfect. what if echoed thing is an argument and not on stdin?
 
"drop" -> interact drop'
 
"grep" -> io grep
 
"grep -v" -> io grep_v
 
"head" -> io (return . head')
 
"join" -> io join
 
"num" -> io num
 
"remove" -> io (remove "str")
 
"revw" -> io revw
 
"reverse" -> io rev
 
"reverseword" -> io revw
 
"rpt" -> io rpt
 
"sort" -> interact sort
 
"space" -> io space
 
"tac" -> interact tac
 
"take" -> io take'
 
"tail" -> io (return . tail')
 
--"tr" -> interact tr
 
--"tr -d" -> interact (trd . unwords)
 
"unspace" -> io unspace
 
"upper" -> interact upper
 
"uniq" -> interact uniq
 
"wc_c" -> interact wc_c
 
"wc_l" -> interact wc_l
 
"wc_w" -> interact wc_w
 
   
 
-- First, two helpers
 
-- First, two helpers
io :: ([String] -> [String]) -> IO ()
 
 
io f = interact (unlines . f . lines)
 
io f = interact (unlines . f . lines)
   
showln :: Int -> String
 
 
showln = (++ "\n") . show
 
showln = (++ "\n") . show
 
-- Sort a file
 
sort' :: String -> String
 
sort' = sort
 
   
 
-- remove duplicate lines from a file (like uniq)
 
-- remove duplicate lines from a file (like uniq)
uniq :: String -> String
+
uniq = nub
uniq = nub
 
   
 
-- repeat the input file infinitely
 
-- repeat the input file infinitely
rpt :: [a] -> [a]
+
rpt = cycle
rpt = cycle
 
   
 
-- Return the head -10 line of a file
 
-- Return the head -10 line of a file
take' :: [String] -> [String]
 
 
take' = take 10
 
take' = take 10
   
 
-- Remove the first 10 lines of a file
 
-- Remove the first 10 lines of a file
drop' :: String -> String
 
 
drop' = drop 10
 
drop' = drop 10
   
 
-- Return the head -1 line of a file
 
-- Return the head -1 line of a file
head' :: [String] -> String
 
 
head' = head
 
head' = head
   
 
-- Return the tail -1 line of a file
 
-- Return the tail -1 line of a file
tail' :: [String] -> String
 
 
tail' = last
 
tail' = last
   
 
-- Reverse lines in a file (tac)
 
-- Reverse lines in a file (tac)
tac :: String -> String
+
tac = reverse
tac = reverse
 
   
 
-- Reverse characters on each line (rev)
 
-- Reverse characters on each line (rev)
rev :: [String] -> [String]
+
rev = map reverse
rev = map reverse
 
   
 
-- Reverse words on each line
 
-- Reverse words on each line
revw :: [String] -> [String]
+
rev_w = map (unwords. reverse . words)
revw = map (unwords. reverse . words)
 
   
 
-- Count number of characters in a file (like wc -c)
 
-- Count number of characters in a file (like wc -c)
wc_c :: String -> String
+
wc_c = showln . length
wc_c = showln . length
 
   
 
-- Count number of lines in a file, like wc -l
 
-- Count number of lines in a file, like wc -l
wc_l :: String -> String
+
wc_l = showln . length . lines
wc_l = showln . length . lines
 
   
 
-- Count number of words in a file (like wc -w)
 
-- Count number of words in a file (like wc -w)
wc_w :: String -> String
+
wc_w = showln . length . words
wc_w = showln . length . words
 
   
 
-- double space a file
 
-- double space a file
space :: [String] -> [String]
+
space = intersperse ""
space = intersperse ""
 
   
 
-- undo double space
 
-- undo double space
unspace :: [String] -> [String]
 
 
unspace = filter (not.null)
 
unspace = filter (not.null)
   
 
-- remove the first occurrence of the line "str"
 
-- remove the first occurrence of the line "str"
remove :: String -> [String] -> [String]
+
remove = delete
remove a = delete a
 
   
 
-- make a string all upper case
 
-- make a string all upper case
upper :: String -> String
 
 
upper = map toUpper
 
upper = map toUpper
   
 
-- remove leading space from each line
 
-- remove leading space from each line
clean :: [String] -> [String]
 
 
clean = map (dropWhile isSpace)
 
clean = map (dropWhile isSpace)
   
 
-- remove trailing whitespace
 
-- remove trailing whitespace
clean' :: [String] -> [String]
+
clean' = map (reverse . dropWhile isSpace . reverse)
clean' = (map f)
 
where f = reverse . dropWhile isSpace . reverse
 
   
 
-- delete leading and trailing whitespace
 
-- delete leading and trailing whitespace
clean'' :: [String] -> [String]
 
 
clean'' = map (f . f)
 
clean'' = map (f . f)
where f = reverse . dropWhile isSpace
+
where f = reverse . dropWhile isSpace
   
 
-- insert blank space at beginning of each line
 
-- insert blank space at beginning of each line
blank :: [String] -> [String]
 
 
blank = map (s ++)
 
blank = map (s ++)
where s = replicate 8 ' '
+
where s = replicate 8 ' '
   
 
-- join lines of a file
 
-- join lines of a file
join :: [String] -> [String]
 
 
join = return . concat
 
join = return . concat
   
 
-- Translate the letter 'e' to '*', like tr 'e' '*' (or y// in sed)
 
-- Translate the letter 'e' to '*', like tr 'e' '*' (or y// in sed)
tr :: Char -> Char -> IO ()
 
 
tr a b = interact (map f)
 
tr a b = interact (map f)
where f c = if c == a then b else c
+
where f c = if c == a then b else c
   
 
-- Delete characters from a string.
 
-- Delete characters from a string.
trd :: Char -> IO ()
+
tr_d a = tr a ' '
trd a = tr a ' '
 
   
 
-- grep lines matching "^foo" from a file
 
-- grep lines matching "^foo" from a file
grep :: [String] -> [String]
 
 
grep = filter (isPrefixOf "foo")
 
grep = filter (isPrefixOf "foo")
   
 
-- grep lines that don't match "^foo" (grep -v)
 
-- grep lines that don't match "^foo" (grep -v)
grep_v :: [String] -> [String]
 
 
grep_v = filter (not . isPrefixOf "foo")
 
grep_v = filter (not . isPrefixOf "foo")
   
 
-- number each line of a file
 
-- number each line of a file
num :: [String] -> [String]
 
 
num = zipWith (printf "%3d %s") [(1::Int)..]
 
num = zipWith (printf "%3d %s") [(1::Int)..]
   
 
-- Compute a simple cksum of a file
 
-- Compute a simple cksum of a file
cksum :: [Char] -> Int
 
 
cksum = foldl' k 5381
 
cksum = foldl' k 5381
 
where k h c = h * 33 + ord c
 
where k h c = h * 33 + ord c
  +
  +
-- And our main wrapper
  +
main = do
  +
who <- getProgName
  +
maybe (return ()) id $ lookup who $
  +
[("blank", io blank )
  +
,("cksum", interact (showln . cksum) )
  +
,("clean", io clean'' )
  +
,("echo" , interact id ) -- not perfect
  +
,("drop", interact drop' )
  +
,("grep", io grep )
  +
,("grep -v", io grep_v )
  +
,("head", io (return . head') )
  +
,("join", io join )
  +
,("num", io num )
  +
,("remove", io (remove "str") )
  +
,("revw", io rev_w )
  +
,("reverse", io rev )
  +
,("reverseword", io rev_w )
  +
,("rpt", io rpt )
  +
,("sort", interact sort )
  +
,("space", io space )
  +
,("tac", interact tac )
  +
,("take", io take' )
  +
,("tail", io (return . tail') )
  +
-- ,( "tr" , interact tr)
  +
-- ,( "tr -d", interact (tr_d . unwords))
  +
,("unspace", io unspace )
  +
,("upper", interact upper )
  +
,("uniq", interact uniq )
  +
,("wc_c", interact wc_c )
  +
,("wc_l", interact wc_l )
  +
,("wc_w", interact wc_w )
  +
]
 
</haskell>
 
</haskell>
   

Revision as of 10:53, 11 March 2007

Simple Unix commandline tools written in Haskell.

This is intended as a beginner's tutorial for learning Haskell from a "Let's just solve things already!" point of view. The examples should help give a flavor of the beauty and expressiveness of Haskell programming.

These functions can be executed as one liners from a shell. For example, to use the Haskell version of 'wc':

   $ cat file.txt | ghc -e 'wc_l' UnixTools.hs

Or, one could define 'main' to be a chosen tool/function (add a line to the effect that "main = wc_l") and then compile the tool with

   $ ghc --make UnixTools.hs

The given Haskell codes presents yet a third way of doing things: much like the BusyBox suite of Unix tools, it is possible to compile a single monolithic binary and have it detect what name it is run by and then act appropriately. This is the approach the following code takes: you can compile it and then make symbolic links (like "ln -s UnixTools echo") and then run those commands ("echo foo | ./echo" would produce output of "foo").

import Data.List
import Data.Char
import Data.Maybe
import Text.Printf
import System.Environment
 
-- First, two helpers
io f = interact (unlines . f . lines)
 
showln  = (++ "\n") . show
 
-- remove duplicate lines from a file (like uniq)
uniq    = nub
 
-- repeat the input file infinitely
rpt     = cycle
 
-- Return the head -10 line of a file
take'   = take 10
 
-- Remove the first 10 lines of a file
drop'   = drop 10
 
-- Return the head -1 line of a file
head'   = head
 
-- Return the tail -1 line of a file
tail'   = last
 
-- Reverse lines in a file (tac)
tac     = reverse
 
-- Reverse characters on each line (rev)
rev     = map reverse
 
-- Reverse words on each line
rev_w   = map (unwords. reverse . words)
 
-- Count number of characters in a file (like wc -c)
wc_c    = showln . length
 
-- Count number of lines in a file, like wc -l
wc_l    = showln . length . lines
 
-- Count number of words in a file (like wc -w)
wc_w    = showln . length . words
 
-- double space a file
space   = intersperse ""
 
-- undo double space
unspace = filter (not.null)
 
-- remove the first occurrence of the line "str"
remove  = delete
 
-- make a string all upper case
upper   = map toUpper
 
-- remove leading space from each line
clean   = map (dropWhile isSpace)
 
-- remove trailing whitespace
clean'  = map (reverse . dropWhile isSpace . reverse)
 
-- delete leading and trailing whitespace
clean'' = map (f . f)
    where f = reverse . dropWhile isSpace
 
-- insert blank space at beginning of each line
blank   = map (s ++)
     where s = replicate 8 ' '
 
-- join lines of a file
join = return . concat
 
-- Translate the letter 'e' to '*', like tr 'e' '*' (or y// in sed)
tr a b = interact (map f)
    where f c = if c == a then b else c
 
-- Delete characters from a string.
tr_d a = tr a ' '
 
-- grep lines matching "^foo" from a file
grep = filter (isPrefixOf "foo")
 
-- grep lines that don't match "^foo" (grep -v)
grep_v  = filter (not . isPrefixOf "foo")
 
-- number each line of a file
num  = zipWith (printf "%3d %s") [(1::Int)..]
 
-- Compute a simple cksum of a file
cksum   =  foldl' k 5381
   where k h c = h * 33 + ord c
 
-- And our main wrapper
main = do
    who <- getProgName
    maybe (return ()) id $ lookup who $
        [("blank",       io blank                  )
        ,("cksum",       interact (showln . cksum) )
        ,("clean",       io clean''                )
        ,("echo" ,       interact id               ) -- not perfect 
        ,("drop",        interact drop'            )
        ,("grep",        io grep                   )
        ,("grep -v",     io grep_v                 )
        ,("head",        io (return . head')       )
        ,("join",        io join                   )
        ,("num",         io num                    )
        ,("remove",      io (remove "str")         )
        ,("revw",        io rev_w                  )
        ,("reverse",     io rev                    )
        ,("reverseword", io rev_w                  )
        ,("rpt",         io rpt                    )
        ,("sort",        interact sort             )
        ,("space",       io space                  )
        ,("tac",         interact tac              )
        ,("take",        io take'                  )
        ,("tail",        io (return . tail')       )
    --  ,( "tr"  ,    interact tr)
    --  ,( "tr -d",   interact (tr_d . unwords))
        ,("unspace",     io unspace                )
        ,("upper",       interact upper            )
        ,("uniq",        interact uniq             )
        ,("wc_c",        interact wc_c             )
        ,("wc_l",        interact wc_l             )
        ,("wc_w",        interact wc_w             )
        ]


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