parsing - Haskell Text Parser Combinators to parse a Range Greedily like Regex range notation -

in regex can acquire range of parse doing \d{1,5}, parses digit 1 5 times greedily. or \d{1,5}? make lazy.

how in haskell's text.parsercombinators.readp?

my attempt gave this:

rangeparse :: read => readp -> [int] -> readp [a] rangeparse parse ranges = foldr1 (<++) $ fmap (\n -> count n $ parse) ranges 

which if rangeparse (satisfy isdigit) ([5,4..1]) perform greedy parse of digits 1 5 times. while if swap number sequent [1..5], lazy parse.

is there better or more idiomatic way parser combinators?

update: below wrong - example rangegreedy 2 4 <* string "aab", equivalent of regexp a{2,4}aab, doesn't match. questioner's solution gets right. won't delete answer yet in case keeps else making same mistake.

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this isn't complete answer, possible way write greedy version. haven't found nice way lazy version.

define left-biased version of option returns maybes:

greedyoption :: readp -> readp (maybe a) greedyoption p = (just <$> p) <++ pure nothing 

then can n of replicatem of them:

uptogreedy :: int -> readp -> readp [a] uptogreedy n p = catmaybes <$> replicatem n (greedyoption p) 

to allow minimum count, mandatory part separately , append it:

rangegreedy :: int -> int -> readp -> readp [a] rangegreedy lo hi p = (++) <$> count lo p <*> uptogreedy (hi - lo) p 

the rest of test code in case it's useful anyone:

module main  import control.monad (replicatem) import data.maybe (catmaybes) import text.parsercombinators.readp  main :: io () main = mapm_ go ["aaaaa", "aaaab", "aaabb", "aabbb", "abbbb", "bbbbb"]         go = print . map fst . readp_to_s test  test :: readp [string] test = ((++) <$> rangegreedy 2 4 <*> many aorb) <* eof       = char 'a' *> pure "ay"     aorb = (char 'a' +++ char 'b') *> pure "ayorbee"