[Haskell-beginners] Compilation error parsing arithmetic expressions with parsec-3.1.2

[j.romildo at gmail.com]

Hello.

I want a parser to parse arithmetic expressions using
Parsec-3.1.2. Therefore I wrote the attached Haskell module.

But its compilation with ghc-7.4.1 fails with the message:


Expr1.hs:22:22:
    No instance for (Stream s0 m0 Char)
      arising from a use of `pLexeme'
    Possible fix: add an instance declaration for (Stream s0 m0 Char)
    In the second argument of `(<$>)', namely
      `pLexeme (many1 (satisfy isDigit))'
    In the expression: read <$> pLexeme (many1 (satisfy isDigit))
    In an equation for `pNum':
        pNum = read <$> pLexeme (many1 (satisfy isDigit))


Any clues on how to fix this?

Romildo
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