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-- Write a version of globToRegex that uses the type signature earlier.
{-- From examples/examples/ch08/GlobRegex.hs modified according to the assignment --}
import Text.Regex.Posix ((=~))
type GlobError = String
globToRegex :: String -> Either GlobError String
globToRegex cs = propagateError regex
where regex = globToRegex' cs
propagateError (Left e) = Left e
propagateError (Right s) = Right ('^' : s ++ "$")
globToRegex' :: String -> Either GlobError String
globToRegex' "" = Right ""
globToRegex' ('*':cs) = propagateError regex
where regex = globToRegex' cs
propagateError (Left e) = Left e
propagateError (Right s) = Right (".*" ++ s)
globToRegex' ('?':cs) = propagateError regex
where regex = globToRegex' cs
propagateError (Left e) = Left e
propagateError (Right s) = Right ("." ++ s)
globToRegex' ('[':'!':c:cs) = propagateError charCls
where charCls = charClass cs
propagateError (Left e) = Left e
propagateError (Right s) = Right ("[^" ++ c : s)
globToRegex' ('[':c:cs) = propagateError charCls
where charCls = charClass cs
propagateError (Left e) = Left e
propagateError (Right s) = Right ('[' : c : s)
globToRegex' ('[':_) = Left "unterminated character class"
globToRegex' (c:cs) = propagateError regex
where regex = globToRegex' cs
propagateError (Left e) = Left e
propagateError (Right s) = Right ((escape c) ++ s)
escape :: Char -> String
escape c | c `elem` regexChars = '\\' : [c]
| otherwise = [c]
where regexChars = "\\+()^$.{}]|"
charClass :: String -> Either GlobError String
charClass (']':cs) = propagateError regex
where regex = globToRegex' cs
propagateError (Left e) = Left e
propagateError (Right s) = Right (']' : s)
charClass (c:cs) = propagateError charCls
where charCls = charClass cs
propagateError (Left e) = Left e
propagateError (Right s) = Right (c : s)
charClass [] = Left "unterminated character class"
{-- End of code from examples --}
-- ghci> :l 8_c_1.hs
-- [1 of 1] Compiling Main ( 8_c_1.hs, interpreted )
-- Ok, one module loaded.
-- ghci> globToRegex "["
-- Left "unterminated character class"
-- ghci> globToRegex "[]"
-- Left "unterminated character class"
-- ghci> globToRegex "a?b*c[!DE][FG]+"
-- Right "^a.b.*c[^DE][FG]\\+$"
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