, chompPrefix
, wrap
, trim
+ , defaultHead
+ , exitIfEmpty
+ , splitEithers
+ , recombineEithers
) where
import Data.Char (toUpper, isAlphaNum, isDigit, isSpace)
import Data.Function (on)
import Data.List
+import Control.Monad (foldM)
import Debug.Trace
(which is assumed to be a separator) to be absent from the string if the string
terminates there.
->>> chompPrefix "foo:bar:" "a:b:c"
+\>>> chompPrefix \"foo:bar:\" \"a:b:c\"
Nothing
->>> chompPrefix "foo:bar:" "foo:bar:baz"
-Just "baz"
+\>>> chompPrefix \"foo:bar:\" \"foo:bar:baz\"
+Just \"baz\"
->>> chompPrefix "foo:bar:" "foo:bar:"
-Just ""
+\>>> chompPrefix \"foo:bar:\" \"foo:bar:\"
+Just \"\"
->>> chompPrefix "foo:bar:" "foo:bar"
-Just ""
+\>>> chompPrefix \"foo:bar:\" \"foo:bar\"
+Just \"\"
->>> chompPrefix "foo:bar:" "foo:barbaz"
+\>>> chompPrefix \"foo:bar:\" \"foo:barbaz\"
Nothing
-}
chompPrefix :: String -> String -> Maybe String
-- strings.
trim :: String -> String
trim = reverse . dropWhile isSpace . reverse . dropWhile isSpace
+
+-- | A safer head version, with a default value.
+defaultHead :: a -> [a] -> a
+defaultHead def [] = def
+defaultHead _ (x:_) = x
+
+-- | A 'head' version in the I/O monad, for validating parameters
+-- without which we cannot continue.
+exitIfEmpty :: String -> [a] -> IO a
+exitIfEmpty _ (x:_) = return x
+exitIfEmpty s [] = exitErr s
+
+-- | Split an 'Either' list into two separate lists (containing the
+-- 'Left' and 'Right' elements, plus a \"trail\" list that allows
+-- recombination later.
+--
+-- This is splitter; for recombination, look at 'recombineEithers'.
+-- The sum of \"left\" and \"right\" lists should be equal to the
+-- original list length, and the trail list should be the same length
+-- as well. The entries in the resulting lists are reversed in
+-- comparison with the original list.
+splitEithers :: [Either a b] -> ([a], [b], [Bool])
+splitEithers = foldl' splitter ([], [], [])
+ where splitter (l, r, t) e =
+ case e of
+ Left v -> (v:l, r, False:t)
+ Right v -> (l, v:r, True:t)
+
+-- | Recombines two \"left\" and \"right\" lists using a \"trail\"
+-- list into a single 'Either' list.
+--
+-- This is the counterpart to 'splitEithers'. It does the opposite
+-- transformation, and the output list will be the reverse of the
+-- input lists. Since 'splitEithers' also reverses the lists, calling
+-- these together will result in the original list.
+--
+-- Mismatches in the structure of the lists (e.g. inconsistent
+-- lengths) are represented via 'Bad'; normally this function should
+-- not fail, if lists are passed as generated by 'splitEithers'.
+recombineEithers :: (Show a, Show b) =>
+ [a] -> [b] -> [Bool] -> Result [Either a b]
+recombineEithers lefts rights trail =
+ foldM recombiner ([], lefts, rights) trail >>= checker
+ where checker (eithers, [], []) = Ok eithers
+ checker (_, lefts', rights') =
+ Bad $ "Inconsistent results after recombination, l'=" ++
+ show lefts' ++ ", r'=" ++ show rights'
+ recombiner (es, l:ls, rs) False = Ok (Left l:es, ls, rs)
+ recombiner (es, ls, r:rs) True = Ok (Right r:es, ls, rs)
+ recombiner (_, ls, rs) t = Bad $ "Inconsistent trail log: l=" ++
+ show ls ++ ", r=" ++ show rs ++ ",t=" ++
+ show t