How to split a string in Haskell

Question

Is there a standard way to split a string in Haskell   lines and words work great from splitting on a space or newline  but surely there is a standard way to split on a comma     I couldn t find it on Hoogle   To be specific  I m looking for something where split      my comma separated list  returns   my   comma   separated   list

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If you use Data Text  there is splitOn   http   hackage haskell org packages archive text 0 11 2 0 doc html Data-Text html v splitOn  This is built in the Haskell Platform   So for instance   import qualified Data Text as T main   print   T splitOn  T pack       T pack  this is a test     or    -  LANGUAGE OverloadedStrings  -   import qualified Data Text as T main   print   T splitOn      this is a test

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split    Eq a   gt  a - gt   a  - gt    a   split d         split d s   x   split d  drop 1 y  where  x y    span     d  s   E g   split      a bb ccc  d   gt    a   bb   ccc      d     A single trailing delimiter will be dropped   split      a bb ccc  d    gt    a   bb   ccc      d

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In the module Text Regex  part of the Haskell Platform   there is a function   splitRegex    Regex - gt  String - gt   String    which splits a string based on a regular expression  The API can be found at Hackage

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I started learning Haskell yesterday  so correct me if I m wrong but   split    Eq a   gt  a - gt   a  - gt    a   split x y   func x y          where         func x    z   reverse   map  reverse  z         func x  y ys   z zs    if y  x then              func x ys      z zs            else              func x ys   y z  zs    gives    Main gt  split      this is a test    this   is   a   test     or maybe you wanted   Main gt  splitWithStr    and    this and is and a and test    this   is   a   test     which would be   splitWithStr    Eq a   gt   a  - gt   a  - gt    a   splitWithStr x y   func x y          where         func x    z   reverse   map  reverse  z         func x  y ys   z zs    if  take  length x   y ys      x then             func x  drop  length x   y ys        z zs           else             func x ys   y z  zs

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Remember that you can look up the definition of Prelude functions   http   www haskell org onlinereport standard-prelude html  Looking there  the definition of words is   words      String - gt   String  words s    case dropWhile Char isSpace s of                          - gt                           s  - gt  w   words s                               where  w  s      break Char isSpace s    So  change it for a function that takes a predicate   wordsWhen         Char - gt  Bool  - gt  String - gt   String  wordsWhen p s    case dropWhile p s of                          - gt                           s  - gt  w   wordsWhen p s                               where  w  s      break p s    Then call it with whatever predicate you want   main   print   wordsWhen          break this string at commas

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Example in the ghci    gt   import qualified Text Regex as R  gt   R splitRegex  R mkRegex  x    2x3x777   gt     2   3   777

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There is a package for this called split   cabal install split   Use it like this   ghci gt  import Data List Split ghci gt  splitOn      my comma separated list    my   comma   separated   list     It comes with a lot of other functions for splitting on matching delimiters or having several delimiters

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I don   t know how to add a comment onto Steve   s answer  but I would like to recommend the  nbsp  nbsp GHC libraries documentation  and in there specifically the   nbsp  nbsp Sublist functions in Data List  Which is much better as a reference  than just reading the plain Haskell report   Generically  a fold with a rule on when to create a new sublist to feed  should solve it too

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Use Data List Split  which uses split    me localhost   ghci Prelude gt  import Data List Split Prelude Data List Split gt  let l   splitOn      1 2 3 4  Prelude Data List Split gt   t l l      Char   Prelude Data List Split gt  l   1   2   3   4   Prelude Data List Split gt  let   convert     String  - gt   Integer   convert   map read   Prelude Data List Split gt  let l2   convert l Prelude Data List Split gt   t l2 l2     Integer  Prelude Data List Split gt  l2  1 2 3 4

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Try this one   import Data List  unfoldr   separateBy    Eq a   gt  a - gt   a  - gt    a   separateBy chr   unfoldr sep where   sep      Nothing   sep l    Just   fmap  drop 1    break     chr    l   Only works for a single char  but should be easily extendable

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Without importing anything a straight substitution of one character for a space  the target separator for words is a space  Something like   words  if c        then     else c c  lt -  my comma separated list     or  words let f            f c   c in map f  my comma separated list    You can make this into a function with parameters  You can eliminate the parameter character-to-match my matching many  like in     if elem c      -        then     else c c  lt - my comma separated list

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In addition to the efficient and pre-built functions given in answers I ll add my own which are simply part of my repertory of Haskell functions I was writing to learn the language on my own time   -- Correct but inefficient implementation wordsBy    String - gt  Char - gt   String  wordsBy s c   reverse  go s     where     go s  ws   case  dropWhile   c  - gt  c     c  s   of            - gt  ws         rem - gt  go   dropWhile   c  - gt  c     c  rem     takeWhile   c  - gt  c     c  rem    ws   -- Breaks up by predicate function to allow for more complex conditions   c - gt  c           c         wordsByF    String - gt   Char - gt  Bool  - gt   String  wordsByF s f   reverse  go s     where     go s  ws   case   dropWhile   c  - gt  f c    s   of            - gt  ws         rem - gt  go   dropWhile   c  - gt   f c      False   rem     takeWhile   c  - gt   f c      False   rem    ws    The solutions are at least tail-recursive so they won t incur a stack overflow

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I find this simpler to understand   split    Char - gt  String - gt   String  split c xs   case break    c  xs of     ls      - gt   ls     ls  x rs  - gt  ls   split c rs

[string] How to split a string in Haskell?

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