atze at cs.uu.nl
Thu Jul 8 04:12:32 EDT 2010
For expressions operator sections will also be difficult to deal with, i.e. the expression (!x) could either be a parenthesized prefix !, or a section for binary !. Some disambiguation mechanism would be required. Currently such disambiguation is hardcoded for - as a special case in the grammar.
On 8 Jul, 2010, at 09:09 , Simon Peyton-Jones wrote:
> Yes, I somewhat hacked up the rules for ! in an ad-hoc way. I really wanted to allow
> f !x !y = (x,y)
> which meant a bit of fiddling, because LHSs are parsed as terms, so this is parsed as
> (f ! x) ! y
> (ie as infix operators) and I have to squizzle around to re-interpret them as prefix operators. Not very cool. Something unified would be a Good Thing.
> | -----Original Message-----
> | From: haskell-prime-bounces at haskell.org [mailto:haskell-prime-
> | bounces at haskell.org] On Behalf Of John Meacham
> | Sent: 08 July 2010 00:59
> | To: haskell-prime at haskell.org
> | Subject: prefix operators
> | It occurred to me the other day that Haskell (w/ bang patterns) now has
> | 3 prefix operators, all of which are defined independently and follow
> | their own special rules for parsing. we have (-), (!) and (~).
> | It would seem to me that we should somehow be able to unify the
> | mechanism behind parsing these, as in practice, it seems that prefix
> | operators are useful in haskell.
> | We have some similarities, - and ! are both infix and prefix operators,
> | ~ is not. ! and ~ can only be in patterns as prefix, (-) can be in both
> | patterns and expressions.
> | But it seems like we may be able to come up with a common way of parsing
> | them all, prolog has had user defined infix, prefix, and postfix
> | operators (sharing the same name even) and is still able to parse things
> | properly so I don't think there will be a technical issue.
> | My first impulse is to treat application as just another binary operator
> | with a certain precedence and find appropriate precedences for !,~,- in
> | the new framework.
> | note: I am not proposing user defined prefix operators, just musing
> | about whether we can unify the rules behind parsing the current three
> | prefix operators, perhaps folding them into the fixity resolution
> | algorithm.
> | John
> | --
> | John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/
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