On Nov 7, 2006, at 5:49 PM, Robert Dockins wrote: [On operator precedence]
Ha! Well, as long as we're being pedantic, surely we wouldn't need any setlarger than the rationals (which does have a decidable ordering)?Also, since I'm commenting anyway, I rather like the idea of specifying operator precedences via a partial order. However, I also feel that there needs to be some work done to make sure there aren't gremlins hiding in the details. Has anyone worked out the theory on this? How does associating to the right vs left play into the picture? How does it fit into the parsingtechnology?
Actually, we *do* use a DAG on operator precedence in the Fortress programming language (my day job). Our goal is to require parentheses when it is not blatantly obvious what is going on (eg because we're using operators from two libraries written in isolation). We can only use operators together in an expression without parentheses if there is an edge between them in the graph. Graph nodes are sets of operators with the "same" precedence (eg addition and subtraction). Among other things this means that if ++ has higher precedence than ==, and == has higher precedence than &&, we can't necessarily mix ++ and && in the same expression without parentheses.
That said, this would be a pretty big change for Haskell, and would break existing code unless you somehow wired in the transitive closure of all the existing operators. As another message in this discussion (from Simon M?) mentioned, you might want to be able to specify the relationship between operators imported from different modules, because you *do* know that a well-known relationship exists.
-Jan-Willem Maessen
--
Rob Dockins
Talk softly and drive a Sherman tank.
Laugh hard, it's a long way to the bank.
-- TMBG
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