[Haskell-cafe] Parsec Question

[Gerd M]

I'm trying to use parsec for parsing a custom input stream. As far as I 
understood the manual correctly I need to define the primitive parser:

type MyParser a   = GenParser (SourcePos,Tok) () a
mytoken :: (Tok -> Maybe a) -> MyParser a
mytoken test
  = token showToken posToken testToken
  where
    showToken (pos,tok)   = show tok
    posToken  (pos,tok)   = pos
    testToken (pos,tok)   = test tok

The problem is, since SourcePos is an abstract datatype, how can I actually 
run this parser without explicitly using values of type SourcePos in the 
input stream?

Many thanks in advance!

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