I think that something like op/3 might be a useful feature. I certainly think that Haskell is far more powerful because of its use of user-defined operators in the standard library. Haskell treats operators as functions, meaning that they can be exported and used in typeclass definitions. This means that you can have typeclasses like Ord, Eq and Num that provide operations like (<), (>), (==), (+) etc., which means that user-defined code can implement these type-classes, rather than leaving them a black box available only to the implementors of the compiler (it also could simplify the parser, as, rather than having to change the parser to support that new library function, you could just define a new op). User-defined operators are also used in high level GUIs like Fudgets and Fran, in order to make code that is concise but expressive, as well as in function composition (the (.) operator) and in Monads (including I/O). This is not currently available to Mercury developers. I can see that user-defined operators might be useful in specifying streams, in order to create a notation that is both compact and expressive. However, Haskell also has limitations on what can qualify as an operator, meaning that you cannot define arbitrary operators (I think they need to be all symbols, not just arbitrary words). This enables users to clearly distinguish between an infix operator and an infix function (with back-ticks). These infix functions can be given precedences and associativity, which makes them easier to use. Having an operator as a function makes it just as exportable as any other function. I think that user-defined operators lose much of their applicability if they can't be exported, as it means that you would need to define them in every module that uses them, giving you a classic double maintenance problem. It would also probably mean that they were never used in libraries, which would limit their usefulness. In short: I think that the user-defined operators ought to be able to be exportable from modules, and also part of the typeclass system. I think that they should be useful, but, for the reasons given in this thread, I do not think that they are ready for mercury-0.13. Jon