[clean-list] Parser, why three type variables

[Erik Zuurbier]

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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