counting indentations in alex

gistfile1.hs

getLexerIndentLevel :: Alex Int
getLexerIndentLevel = Alex $ \s@AlexState{alex_ust=ust} -> Right (s, indentation ust)

setLexerIndentLevel :: Int -> Alex ()
setLexerIndentLevel i = Alex $ \s@AlexState{alex_ust=ust} -> Right (s{alex_ust=(AlexUserState i)}, ())

collectIndent :: AlexInput -> Int -> Alex Lexeme
collectIndent input@(p, _, _, str) i = do
    lastIndent <- getLexerIndentLevel
    currIndent <- countIndent (drop 1 str) 0
    if (lastIndent < currIndent) then
        do setLexerIndentLevel currIndent
           mkL LIndent input i
    else if (lastIndent > currIndent) then
        do setLexerIndentLevel currIndent
           mkL LDedent input i
    else alexMonadScan
  where
    countIndent str total
        | take 1 str == "\t" = do skip input 1
                                  countIndent (drop 1 str) (total+1)
        | take 4 str == "    " = do skip input 4
                                    countIndent (drop 4 str) (total+1)
        | otherwise = return total