[Haskell-cafe] parsec: problem combining lookAhead with many1 (bug?)

[Andreas Reuleaux]

While I haven't tried out your example in parsec, I can at least confirm
that in trifecta it does work that way you expect it, ie. there is no
difference between the error messages in both of your cases:
(parsec's many1 = trifecta's some)


Prelude > :m +Text.Trifecta
Prelude Text.Trifecta > :m +Text.Parser.LookAhead
Prelude Text.Trifecta Text.Parser.LookAhead >
...
Prelude Text.Trifecta Text.Parser.LookAhead > parseTest (lookAhead (char 'a') >> char 'b') "a"
...
Loading package reducers-3.10.2.1 ... linking ... done.
Loading package trifecta-1.5.1 ... linking ... done.
(interactive):1:1: error: expected: "b"
a<EOF> 
^      
Prelude Text.Trifecta Text.Parser.LookAhead > parseTest (lookAhead (some $ char 'a') >> char 'b') "a"
(interactive):1:1: error: expected: "b"
a<EOF> 
^      
Prelude Text.Trifecta Text.Parser.LookAhead > 


Hope this helps.

-Andreas




silly8888 <silly8888 at gmail.com> writes:

> Suppose that we have the following parser:
>
>   p = lookAhead (char 'a') >> char 'b'
>
> If we use it like so
>
>   parse p "" "a"
>
> we get the following error:
>
> Left (line 1, column 1):
> unexpected "a"
> expecting "b"
>
> What happened is that char 'a' succeeded by consuming the 'a' from the
> input and then lookAhead rewinded the input stream (as it does on
> success). Then, char 'b' tries to parse (again) the first character of
> the input and fails. Everything works as expected.
>
> Now let's slightly modify our parser:
>
>   p' = lookAhead (many1 $ char 'a') >> char 'b'
>
> I've only added a many1. I was expecting this parser to give the same
> error as the previous one: many1 $ char 'a' will succeed consuming one
> 'a' and then lookAhead will rewind the input (as it does on success).
> Thus when we call char 'b' we are going to be in the beginning of the
> input again. Well, that doesn't happen:
>
> Left (line 1, column 2):
> unexpected end of input
> expecting "b"
>
> As you can see, lookAhead did not rewind the input as it was supposed to.
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