Peter Theissen wrote:
I´m progarmming a parser for functional programs. Now I want to implement the infixL and infixR feature to increase the readability of the code. I would be very glad if anyone can send me some information about the implementation of this feature of the Haskell parser or where I can find something about it.
Different Haskell implementations may differ, but I think the easiest way to handle this is to have your parser treat all infix operators as though they had the same precedence and associativity, and then go back and fix up the infix expressions later. Otherwise you have to search for infix directives before you've finished parsing, which is probably a bad idea. The fix-up process can be done in two passes: first parenthesize things according to their precedence, and then according to their associativity. If you're not familiar with the algorithms for the first pass, try to find a hand-coded parser for arithmetic expressions.
Futher qurestion: Is it possible to define priorities for functions in Haskell? Any hints about that?
If you mean use the infix* directives with ordinary function names like "foo", then yes, it's perfectly okay to write
infixr 3 `foo`
The difference between functions and infix operators is only lexical. Your lexer should recognize "+" as infix (+) and "`foo`" as infix foo. These two cases shouldn't need to be treated separately except in the lexer.
-- Ben
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