On Tue, Mar 08, 2005 at 11:02:23AM +0800, Shu-chun Weng wrote: > Hello, this mail is an analysis of implementing operator ternary > operator "?? ::" in pugs.
Greetings. Thanks for your extensive survey, which helped a lot
on pruning dead ends in my attempt in getting this implemented.
The key insight was provided a few minutes ago by kosmikus on #haskell:
00:52 < kosmikus> could you somehow view the whole thing from ? to :
as an infix binary operator?
00:52 < kosmikus> the binary operators can be parametrized over
arbitrary parsers, can't they?
and, to keep a long story short, ?? :: is now parsed correctly as
(ternOp "??" "::" "if") in Parser.hs, with ternOp defined thus:
ternOp pre post syn = (`Infix` AssocRight) $ do
symbol pre
y <- parseTightOp
symbol post
return $ \x z -> Syn syn [x, y, z]
that is, it reads everything between ?? and ::, allowing only
tight operators (i.e. "=" and everything tighter than it, but not
"and" etc), and bind the left and right inside an "if" syntactic
form. The AssocRight lets us parse (x ?? y :: z ?? v :: w) as
(x ?? y :: (z ?? v :: w)) instead of the other way around.
So, I consider this case closed. Let me know if you manage to
uncover any subtle errors in ?? :: parsing. Thanks again for your
investigation work and unit tests!
Thanks,
/Autrijus/
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