Hi Sebastian,
I enjoyed this paper very much. Writing papers in the style of a play seems to
work very well! (although I think you should spice it up more if your want to
get it on Broadway)
It seems that only shift needs the reg field of the RegW datatype. So you can
also replace the reg field with a shift field. This makes the regexp parser
extensible, as there is no longer a dependence on the (closed) datatype Reg:
> data RegW w c = RegW { active :: !Bool,
> empty :: !w,
> final_ :: !w,
> shift :: w -> c -> RegW w c }
For example it is then easy to define the parser that matches nothing, which is
the identity element of alt:
> noMatch :: RegExp c
> noMatch = RegExp noMatchW
>
> noMatchW :: Semiring w => RegW w c
> noMatchW = RegW False zero zero $ \_ _ -> noMatchW
But otherwise I do wonder if the parser needs to be extensible. For example
some XML Schema implementations that are based on finite automata have special
cases for the xs:all construct, which matches a list of elements, each
occurring once in any order. But I tried a straightforward implementation and
it works fine:
> eachOnce :: [RegExp c] -> RegExp c
> eachOnce [] = eps
> eachOnce ps = eachOnce' ps [] where
> eachOnce' [] _ = noMatch
> eachOnce' (p:ps) qs = (p `seq_` eachOnce (ps ++ qs)) `alt` eachOnce' ps
> (p:qs)
*Main> accept (eachOnce (map char ['a'..'z'])) $ reverse ['a'..'z']
True
(0.05 secs, 8706356 bytes)
greetings,
Sjoerd
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