TabをSpaceに展開

tabToSpace tabSize s = tabToSpace' tabSize s
  where tabToSpace' _ ""        = ""
        tabToSpace' 0 s         = tabToSpace' tabSize s
        tabToSpace' i ('\t':cs) = (take i $ repeat ' ') ++ (tabToSpace' tabSize cs)
        tabToSpace' i (c:cs)    = c:(tabToSpace' (i-1) cs)