[clean-list] Parser, why three type variables

[Pieter Koopman]

Hi,

my parser library has parsers with 3 type parameters: Parser s r t.
s is the type of the symbols parsed (the tokens)
r is the result of this parser. The type r can be identical to s (for 
instance for the combinator symbol), but often is it a (part of) syntax tree
t is the final result. Rather than constructing some intermediate data 
type, my parser combinators use continuations to handle the parser 
results. The success continuation transforms the parse results of type r 
to elements of type t. Basically the success continuation acts like a 
function of type r->t.
That is the reason I needed those 3 type arguments, see 
http://www.st.cs.ru.nl/papers/1999/koop98-EffCombParsIFL98.ps.gz for 
details.

Hope this helps,

Pieter Koopman

Erik Zuurbier wrote:
> Carlos,
>
> I wrote those parser combinators based on Pieter Koopman's work with 
> only two type parameters. Pieter also has a version with three type 
> parameters, necessary for the XOR-combinator and the way he improved 
> the library's efficiency. I did not like that approach and found a 
> different way. See the help file that comes with the library.
>
> But after that Pieter found a bug that appeared in both of his 
> libraries but solved it only for his 3-parameter library. I could not 
> solve it for the 2-parameter library so I reworked my parser library 
> based on his improved 3-parameter library.
>
> It is true, the parser library that emits error messages does not need 
> the third parameter, but to my opinion it is not trivial to take it 
> out. Hope Pieter reads this and will once find the time to debug his 
> 2-parameter library as well, after which I can base the error-message 
> emitting library on it.
>
> Regards
> Erik Zuurbier
> ----- Original Message ----- From: "Carlos Aya" <carlosayam at yahoo.com.au>
> To: <clean-list at science.ru.nl>
> Sent: Sunday, May 04, 2008 5:48 AM
> Subject: [clean-list] Parser, why three type variables
>
>
>> Hello,
>>
>> In the parsers library one finds...
>>
>> :: Parser s r t
>>
>> and
>>
>> parse :: !(Parser s r r) [s] String String -> Result r
>>
>> Still not clear for me, from a software design point of view, why 
>> three type variables in 'Parse' if 'parse' use only 2 in the type? 
>> Why we want the type engine to have that free 't' around? I saw the 
>> combinators and none of them touch it... hope I didn't overlooked 
>> something... any help appreciated.
>>
>> regards
>> Carlos
>>
>>
>>
>>      Get the name you always wanted with the new y7mail email address.
>> www.yahoo7.com.au/y7mail
>>
>>
>>
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