On Mon, 2004-11-22 at 17:48 +0100, Frank-Andre Riess wrote:
Hi there folks,
once again, I've got a question related to Happy (I've got version 1.13 at the moment). Maybe, it's even more a question on formal languages, but well... How can I write a grammar that can cope with user-defined operators (of different precedences/associativities
One standard solution is to parse user defined operators as if they were all one precedence/associativity and then re-associate them later once you know what the precedence and associativity of each operator is.
That way the parser grammar does not need to be adjusted on the fly.
So you wold parse 1+2*3 as [LiteralInt 1, Op '+', LiteralInt 2, Op '*', LiteralInt 3] and then later turn that into BinOp '+' (LiteralInt 1) (BinOp '*' (LiteralInt 2) (LiteralInt 3)) using your mapping of operators to precedence/associativity.
Duncan
Thank you very much. What I did by now is more or less along the lines of your suggestion, but it doesn't work to my satisfication yet. Might be due to overloaded tokens, though (e.g. '|' is both a set union operator and the seperator in case statements). Maybe we'll need to redesign the syntax.