[Haskell-cafe] ANNOUNCE: Parsed 0.0.1

Mike Izbicki mike at izbicki.me
Wed Apr 1 15:01:37 UTC 2015

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'
    $ 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

    $ (\(\<\|\>\) 'match ab' 'match a') <<< "a"
    $ echo $?
    $ (\(\<\|\>\) 'match ab' 'match a') <<< "ab"
    $ echo $?

Thanks for the bug report!

On Wed, Apr 1, 2015 at 4:48 AM, Tillmann Rendel
<rendel at informatik.uni-tuebingen.de> wrote:
> 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?
>   Tillmann
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