On Thu, 2006-11-09 at 22:20 +0100, Benjamin Franksen wrote:
Henning Thielemann wrote:
Maybe making fixity declarations like type class instance declarations is good.
I thought so too at first but, having thought about it for a while, I now see this will cause /major/ problems. The precedence relations as declared explicitly by the programmer must form a DAG, with the vertices being the operator classes with equal precedence. There are two ways you can break the DAG: by introducing a 'smaller' or 'larger' relation when another module has already declared them to have equal precedence (resp. the other way around); or by introducing a cycle. Both can be caused simply by importing yet another module. I think it would be unacceptable not to provide some way for the programmer to resolve such conflicts.
[ ... possibilities for resolving conflicts omitted ... ] Another possibility is: If you have operators op1 and op2, where the compiler sees conflicting requirements for the precedence of op1 and op2, then they are treated as non-associative relative to each other: the expression a op1 b op2 c is illegal, and the programmer must instead write (a op1 b) op2 c or a op1 (b op2 c) Carl Witty