haskell program to remove part of list and print the rest

Question

How do I remove a part of a list in Haskell? This is what I have done so far. Please tell me what are the changes that can be made:

import Data.List
import Data.List.Split

removePrefix :: Eq t => [t] -> [t] -> Maybe [t]
removePrefix [] [] = Nothing
removePrefix ts [] = Just ts 
removePrefix (t:ts) (y:ys) = 
  if inTnFixOf (y:ys) (t:ts) == True
  then SrtipPrefix (y:ys) (t:ts)
  else Just [(x:xs)]

Example: input "toyboat" output "boat"

Can you please edit your code, so that it compiles? Some typos and indentation errors. — kaan
– kaan, Commented Sep 1, 2013 at 0:38
i'm rather green to haskell, don't quite know how to handle these errors. So, any help would suffice. — Ratan Kumar
– Ratan Kumar, Commented Sep 1, 2013 at 0:40
try: cabal install split in your terminal; do you want to remove all sublists from a list or only prefixes? — jev
– jev, Commented Sep 1, 2013 at 0:51

wit · Accepted Answer · 2013-09-01 17:28:57Z

2

Already exists function stripPrefix from Data.List

 stripPrefix "foo" "foobar" == Just "bar"
 stripPrefix "foo" "foo" == Just ""
 stripPrefix "foo" "barfoo" == Nothing
 stripPrefix "foo" "barfoobaz" == Nothing

is defined as

stripPrefix :: Eq a => [a] -> [a] -> Maybe [a]
stripPrefix [] ys = Just ys
stripPrefix (x:xs) (y:ys)
 | x == y = stripPrefix xs ys
stripPrefix _ _ = Nothing

So, you could just declare:

removePrefix [] [] = Nothing
removePrefix xs ys = stripPrefix xs ys

answered Sep 1, 2013 at 17:28

wit

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Comments

jev · Accepted Answer · 2013-09-01 01:25:50Z

1

the following removes all sublists from a given list:

import Data.List.Split

rmSublist :: Eq a => [a] -> [a] -> [a]
rmSublist [] _ = []
rmSublist _ [] = []
rmSublist xs ys = concat $ filter (/=[]) (splitOn xs ys)

alternatively, if you want to remove only the prefix:

rmPrefix :: Eq a => [a] -> [a] -> [a]
rmPrefix [] ys = ys
rmPrefix _ [] = []
rmPrefix xs ys =
  if xs == take n ys
  then drop n ys
  else ys
  where n = length xs

edited Sep 1, 2013 at 1:25

answered Sep 1, 2013 at 1:06

jev

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