prefix operators

Simon Peyton-Jones simonpj at
Thu Jul 8 03:09:29 EDT 2010

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 [mailto:haskell-prime-
| bounces at] On Behalf Of John Meacham
| Sent: 08 July 2010 00:59
| To: haskell-prime at
| 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 - ⑆⑆john⑈ -
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