The avoiding /try/ is a good part of Parsec golf. Because turning
natural literals into fractions is easy (%1) it is simple to use the
/option/ parser to parse a suffix or return a default.
/symbol/ is also a valuable parser, often preferable to /char/ or
/string/ as it chomps trailing white space.
symbol = P.symbol lexer
fraction :: Parser Rational
fraction = do
num <- integer
den <- option 1 (symbol "/" >> natural)
return (num % den)
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