On 2006 mei 30, at 17:33, Brian Hulley wrote:
But the buffer will nearly always be incomplete as you're editing it.
I was kind of hoping that the syntax of Haskell could be changed so that for any sequence of characters there would be a unique parse that had a minimum number of "gaps" inserted by the editor to create a complete parse tree, and moreover that this parse could be found by deterministic LL1 recursive descent.
If you use my parsercombinators, and are willing to work on the grammar I think this can in principle be done. The combinators automatically "correct" incorrect (i.e. in this case incomplete) input, but: - you may really need some time to tune the grammar so the corrections are what a user might expect (there are many hooks for doing so, but it will takje some effort, since this s also a human interface issue) - making a Haskell grammar that parsers top-down is not an easy task, and making it LL1 is definitely impossible, but also not needed if you use my combinators - we could in principle provide you with a complete parser for Haskell using our combinators that was tested by replacing the GHC parser with this parser, and that worked (contact athurb@cs.uu.nl to get a copy of his code) - did you think about how to handle the offside rule? If not, the good news is that we have combinators for that too. Doaitse Swierstra
Haskell already seems so very close to having this property - its a real pity about =>
Not only the =>, but e.g. the commonality between patterns and expressions makes left factorisation a not so simple task.
The problem is: when Haskell was designed, no one seems to have thought about the syntax from the point of view of making it easy to parse or ensuring that it would have this nice property for editing.
foo :: {SomeClass a} a->a a Prelude.+ b -- no spaces in the qvarsym a `Prelude.(+)` b -- a little generalization a `Prelude.(+) b -- no need for closing `
I would not like an editor that forces me to use a special coding style (like using brackets where not strictly necessary). Even less would I like to use one that introduces non-standard syntax.
My humble opinion is that you'll have to bite the bullet and implement your syntax recognizer so that it conforms to Haskell'98 (including the approved addenda) [and addtionally however many of the existing extensions you'll manage to support]. It may be more difficult but in the end will also be a lot more useful. And you'll have to find a way to (unobtrusively!) let the user know whether some piece of code does not yet have enough context to parse it unambigously.
Thanks for the feedback - I suppose I was being overly optimistic to think I could just change the language to suit my editor ;-). I'll have to find a balance between what I'm able to implement and what is specified in Haskell98 (or Haskell' etc) - just as GHCi has done with qvarsym, and perhaps have different levels of conformance to H98 so people could choose their preferred balance between conformity and instantaneousness of feedback (everything would of course still be loaded/saved as H98).
Regards, Brian.
-- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us.
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