isDigit problem

Hi,
 
I've got a problem with parser primitives using Hugs. 
 
I have the standard code to define parsers for digits etc
 
sat              :: (Char -> Bool) -> Parser Char
sat p             =  do x <- item
                         if p x then return x else failure
 
digit            :: Parser Char
digit             =  sat isDigit

but when I try to run it I get the error 
 
Undefined variable "isDigit"
 
It does the same for any of the isXXXX predicates. I´m completely flummoxed, so any help would be greatly appreciated. :( 
 
Thanks,
 
Nik
 
Dr Nik Freydís Whitehead
University of Akureyri, Iceland
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Having the moral high ground is good.
Having the moral high ground and an FGMP-15 is better.
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