Trying to understand C declarations

CDeclarations.hs

import Data.Maybe (maybe)
import Data.List (intercalate)

data CType = CInt
           | CDouble
           | CChar
           | CVoid
           | CPointer CType
           | CArray CType (Maybe Int)
           | CFun CType [CType]
           deriving (Read)

toC :: CType -> String -> String
toC CInt a = "int " ++ a
toC CDouble a = "double " ++ a
toC CChar a = "char " ++ a
toC CVoid a = "void " ++ a
toC (CArray t n) a = toC t (a ++ '[':(maybe "" show n) ++ "]")
toC (CPointer t) a = toC t ("(*" ++ a ++ ")")
toC (CFun t ts) a = toC t (a ++ '(':(intercalate ", " $ map (flip toC "") ts) ++ ")")

main :: IO ()
main = do
  -- Test: Examples from chapter 5.12 of "The C Programming Language"
  putStrLn $ toC (CPointer CChar) "argv"
  putStrLn $ toC (CPointer (CArray CInt (Just 13))) "daytab"
  putStrLn $ toC (CArray (CPointer CInt) (Just 13)) "daytab"
  putStrLn $ toC (CPointer (CFun CVoid [])) "comp"
  putStrLn $ toC (CFun (CPointer (CArray (CPointer (CFun CChar [])) Nothing)) []) "x"
  putStrLn $ toC (CArray (CPointer (CFun (CPointer (CArray CChar (Just 5))) [])) (Just 3)) "x"