[Haskell-cafe] Data.Text performance problem

I experienced a following problem while dealing with some text processing. I have a text and want to get the same text with parts enclosed into {} or [] stripped away. Substituting them with a ' ' would also work. Here is the code I wrote (T is Data.Text): stripBrackets :: T.Text -> T.Text stripBrackets text = snd $ T.mapAccumL f 0 text where f depth c = let depth' = depth + d' c c' | depth > 0 || depth' > 0 = ' ' | otherwise = c in (depth', c') d' '{' = 1 d' '[' = 1 d' '}' = -1 d' ']' = -1 d' _ = 0 The only problem is that it takes about a minute to complete on 3GHz+ processor when text is a 30k chars long. Any ideas how to improve this code? -- Regards, Petr