String.hs

{-# LANGUAGE FlexibleContexts, ScopedTypeVariables #-}
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-- | Miscellaneous string manipulation functions.
module Hakyll.Core.Util.String
    ( trim
    , replaceAll
    , splitAll
    , needlePrefix
    , removeWinPathSeparator
    ) where


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import Data.Char (isSpace)
import Data.List (isPrefixOf)
import Text.Regex.TDFA ((=~~))


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-- | Trim a string (drop spaces, tabs and newlines at both sides).
trim :: String -> String
trim = reverse . trim' . reverse . trim'
  where
    trim' = dropWhile isSpace


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-- | A simple (but inefficient) regex replace funcion
replaceAll :: String              -- ^ Pattern
           -> (String -> String)  -- ^ Replacement (called on match)
           -> String              -- ^ Source string
           -> String              -- ^ Result
replaceAll pattern f source = replaceAll' source
  where
    replaceAll' ""  = ""
    replaceAll' src = case src =~~ pattern of
        Nothing                       -> src
        Just (before, capture, after) -> before ++ f capture ++ replaceAll' after


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-- | A simple regex split function. The resulting list will contain no empty
-- strings.
splitAll :: String    -- ^ Pattern
         -> String    -- ^ String to split
         -> [String]  -- ^ Result
splitAll pattern = filter (not . null) . splitAll'
  where
    splitAll' ""  = []
    splitAll' src = case src =~~ pattern of
        Nothing                         -> [src]
        Just (before, _::String, after) -> before : splitAll' after



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-- | Find the first instance of needle (must be non-empty) in haystack. We
-- return the prefix of haystack before needle is matched.
--
-- Examples:
--
-- > needlePrefix "cd" "abcde" = "ab"
--
-- > needlePrefix "ab" "abc" = ""
--
-- > needlePrefix "ab" "xxab" = "xx"
--
-- > needlePrefix "a" "xx" = "xx"
needlePrefix :: String -> String -> Maybe String
needlePrefix needle haystack = go [] haystack
  where
    go _   []                     = Nothing
    go acc xss@(x:xs)
        | needle `isPrefixOf` xss = Just $ reverse acc
        | otherwise               = go (x : acc) xs


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-- | Translate native Windows path separators '\\' to '/' if present.
removeWinPathSeparator :: String -> String
removeWinPathSeparator = concatMap (\c -> if c == '\\' then ['/'] else [c])