For the Parsec parser
(many1 (string "a") <|> many1 (string "b")) >> string "c"
we have an equivalent Parsed parser given by:
(choice "many 'match a'" "many 'match b'" | match c)
Verifying it accepts your test case:
$ (choice "many 'match a'" "many 'match b'" | match c) <<< 'bc'
bc
$ echo $?
Indeed, you are correct about the slightly different semantics of
choice. It essentially automatically wraps its arguments in "try".
For those wanting the more traditional Alternative interface, I've
just uploaded a combinator called (<|>). We can use this combinator
exactly as you would in haskell (except that bash requires lots of
escaping):
$ (\(\<\|\>\) 'match ab' 'match a') <<< "a"
$ echo $?
2
$ (\(\<\|\>\) 'match ab' 'match a') <<< "ab"
ab
$ echo $?
0
Thanks for the bug report!
On Wed, Apr 1, 2015 at 4:48 AM, Tillmann Rendel
Hi again,
I wrote:
In parsec,
(many1 (string "a") <|> many1 (string "b")) >> string "c"
accepts "bc", but I don't see how the corresponding grammar can be par-séd.
Sorry, I think I was confused because your implementation looked like an attempt at unlimited backtracking to me. To compare with Parsec, it is better to treat your implementation as an attempt to implement Parsec's semantics. In that case, we should note that in Parsec,
(string "ab" <|> string "a")
rejects "a", but maybe your implementation would accept it?
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