option package:parsec-class

option x p tries to apply parser p. If p fails without consuming input, it returns the value x, otherwise the value returned by p.

priority  = option 0 (do{ d <- digit
; return (digitToInt d)
})

optionMaybe p tries to apply parser p. If p fails without consuming input, it return Nothing, otherwise it returns Just the value returned by p.

optional p tries to apply parser p. It will parse p or nothing. It only fails if p fails after consuming input. It discards the result of p.