RE: Xtract with hugs and Haskell implementation inconsistency

[Sigbjorn Finne (Intl Vendor)]

S. Alexander Jacobson <[EMAIL PROTECTED]> writes:
> 
> I am trying to get Malcolm Wallace's Xtract code to run with 
> Hugs and have noticed a few inconsistencies between whatever
> compiler he is using and hugs...
> 

Great - I'd encourage others to share their experiences of trying to
work with multiple Haskell systems. Cross-system portability
& Report compliance is not something that's at the very top of a
Haskell system implementor's checklist (i.e., give them a helping hand.)

> 
> 2. Allowing use of qualified names without importing qualified
> MW's compiler allows use of Prelude.elem without qualified import of
> the prelude.  Hugs does not like that.  I think Hugs is 
> right, but I am not sure which is "correct".
> 

NHC is right, but using left-of-centre features of the module system
is not something I would recommend if you want your code to work
with Hugs.

> 
> 3. Inadequacy of Haskell
> There are a few places where MW relies on the preprocessor rather than
> Haskell.  Xtract has this code:
> ---------From ParseSTXml.hs--------
> #define PARSEARG(tok)  P (\st inp -> case inp of { \
>                                       ((p,tok n):ts) -> [(n,st,ts)]; \
>                                       ts -> [] } )
> name :: Parser SymTab Token Name
> name =  PARSEARG(TokName)
> 
> string, freetext :: Parser SymTab Token String
> string =   PARSEARG(TokName)
> freetext = PARSEARG(TokFreeText)
> -------
> Is there clean a way of achieving the same functionality 
> without relying on the preprocessor?
> 

Your mileage may vary, but I'd probably do something like
this here:

mbParse :: (tok -> Maybe a) -> Parser state tok a
mbParse matcher = P $ \ st inp ->
   case inp of
    ((_,tok):ts) | Maybe.isJust mb_v -> [(v, st, ts)]
      where
       mb_v     = matcher tok
       (Just v) = mb_v
    _ -> []

name     = mbParse deTok where deTok (TokName x) = Just x ; deTok _ =
Nothing 
freetext = mbParse deTok where deTok (TokFreeText x) = Just x ; deTok _ =
Nothing
string   = name

If H98 had pattern guards, mbParse could have been expressed
with less effort,

       mbParse matcher = P $ \ st inp -> 
           case inp of
             ((_,tok):ts) | (Just v) <- matcher tok -> [(v,st,ts)]
             _ -> []


--sigbjorn