can't find Parsec modules in GHCi

Question

I've been working on Question 67A of 99 Haskell Questions. The question is constructing a tree from a given string: "x(y,a(,b))" => Branch 'x' (Branch 'y' Empty Empty) (Branch 'a' Empty (Branch 'b' Empty Empty))
One solution using Parsec is as below:

import Text.Parsec.String
import Text.Parsec hiding (Empty)

pTree :: Parser (Tree Char)
pTree = do
   pBranch <|> pEmpty

pBranch = do
  a <- letter
  char '('
  t0 <- pTree 
  char ','
  t1 <- pTree
  char ')'
  return $ Branch a t0 t1

pEmpty =
  return Empty

stringToTree str =
  case parse pTree "" str of
     Right t -> t
     Left e  -> error (show e)

However, my GHCi could neither find Text.Parsec.String nor Text.Parsec. Are those modules obsolete? My GHCi version is 6.12.3

Answer 1

Text.Parsec and Text.Parsec.String are modules in the parsec package from version 3 on. The old parsec-2 interface is available from the compatibility modules with the traditional names Text.ParserCombinators.Parsec.*, but there's no *.String module, that's new in parsec-3. If you have parsec-2 or no parsec at all installed, I recommend installing parsec-3 with the canonical cabal install parsec.

Edit:

If you want to parse a less rigid syntax for the trees, supporting your example input,

pBranch = do
  a <- letter
  do char '('
     t0 <- pTree
     char ','
     t1 <- pTree
     char ')'
     return $ Branch a t0 t1
   <|> return (Branch a Empty Empty)

defaults to two empty children if the letter is not followed by an opening parenthesis.