CODEWITHSUNDEEP
haskell
Ratan Kumar
Ratan Kumar

Reputation: 11

haskell program to remove part of list and print the rest

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"

Upvotes: 0

Views: 1074

Answers (2)

wit
wit

Reputation: 1622

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

Upvotes: 2

jev
jev

Reputation: 2013

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

Upvotes: 1

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