parsing - Monadic parse with uu-parsinglib -

i'm trying create monadic parser using uu_parsinglib. thought had covered, i'm getting unexpected results in testing

a cut down example of parser is:

ptype :: parser asttype ptype = addlength 0 $   (amb n_list) <- pname      let r_list = filter attributefilter n_list      case r_list of         (astname_idname : [] )   -> return (asttype a)         (astname_typename : [] ) -> return (asttype a)         _                          -> pfail      namefilter :: astname' -> bool            namefilter =               case of                  (astname_idname _)   -> true                  (astname_typename _) -> true                  _                    -> false  data asttype  = asttype astname  data astname  = amb [astname']  data astname' =    astname_idname     astname    astname_typename   astname    astname_othername  astname    astname_simple     string 

pname ambiguous parser. want type parser apply post filter, , return alternatives satisfy namefilter, wrapped asttype.

if there none, should fail.

(i realise example i've given fail if there more 1 valid match in list, example serves purpose)

now, works far can see. problem lies when use in more complicated grammars, odd matches seem occur. suspect problem addlength 0 part

what separate out monadic , applicative parts. create monadic parser filtering component, , apply pname using <**> operator.

alternatively

i'd settle explanation of addlength doing.

i've put fudge/workaround use monadic parsing uu-parsinglib. way ever use monadic parsers analysis overly generous initial parser, , selectively fail results.

bind' :: parser -> (a -> parser b) -> parser b bind' a@(p _ _ _ l') b = let (p t nep e _) = (a >>= b) in p t nep e l' 

the important thing remember when using parser

a -> m b 

must consume no input. must either return transformed version of a, or fail.

warning

testing on minimal currently, , behaviour not enforced type. fudge.