Hi all, I have an idea about type classes that I have been experimenting. It appears to be a generalization to Haskell’s type classes and seems to be doable. It seems to related the three ideas: type classes, implicit parameters, and (typed) dynamic scoping. But I don't know whether it is good or not. I hope to get some opinions before going further. Basically, Haskell’s type classes passes dictionaries around. Each dictionary contains one or more “methods”. When “names” which belong to a dictionary are called, we invoke functions that match its principle type in the call site's scope. I have an experimental system which “decouples” the dictionary. Instead of passing on a dictionary, it passes individual “implicit parameters" around. Those implicit parameters are type inferenced and they can contain type parameters just as methods in a type class. Similarly, they are resolved by their types in the call site's scope. The convenience of this approach compared to Haskell’s type classes is that we no longer require a user of a type class to define ALL the methods in a type class. For example, a user could just define a method + without defining other methods in the Num class: -, *, … He can use the method + independently. For example, if + is defined on the String type to be concatenation, we can use + in another function: weirdConcat x y = x + y + y This has a utility, because the methods in the Num class don’t “make sense” for Strings except +, but the current type class design requires us to define them. Note here that weirdConcat will not have the type (Num a) => a -> a -> a, since we no longer have the Num class, it is decoupled into separate methods. There is another benefit of this decoupling: it can subsume the functionality of MPTC. Because the methods are no longer grouped, there is no “common” type parameter to the methods. Thus we can easily have more than one parameter in the individual methods and conveniently use them as MPTC methods. SOME IMPLEMENTATION DETAILS Here is how it can be implemented. When we see an “undefined” variable in a function definition which has been declared as “overloaded function”, we store the function name, and the type variables that are associated with it. For example, overload g — (explicitly declare g as an overloaded function) f x y (String s) = … … let z = g x s y in … … We don’t know what x and y are, but we know from the body of f that their types satisfy this pattern: g ’a String ’b. Thus we store this pattern constraint as an extra (implicit) argument in the type of f: f :: a → b → String (exist g: g a String b) We may have multiple such arguments. At the call sites of f, we look for a function g in the scope that satisfies the pattern g ‘a String ’b, but we don’t pass on the substitution, so they remain polymorphic. Once found, the function is passed as an extra parameter to f. This is essentially dictionary passing, but without grouping. It can be also more efficient because the parameters may be stored in registers. Here g is explicitly declared as “overloaded”, although my experimental system doesn’t need this. Any undefined variable inside function body automatically becomes overloaded. This may cause unintended overloading and it catches bugs late. That’s why we need the “overload” declarations. But the automatic overloading of the undefined may be useful in certain situations. For example, if we are going to use Haskell as a shell language. Every “command” must be evaluated when we type them. If we have mutually recursive definitions, the shell will report “undefined variables” either way we order the functions. The automatic overloading may solve this problem. The undefined variables will temporarily exist as automatic overloaded functions. Once we actually define a function with the same name AND satisfies the type constraints, they become implicit parameters to the function we defined before. If we call a function whose implicit parameters are not associated, the shell reports error very similar to Haskell’s “type a is not of class Num …” RELATIONSHIP TO DYNAMIC SCOPING It seems to be helpful to think of the “method calls” as referencing dynamically scoped variables. They are dispatched depending on the bindings we have in the call site's scope (and not the scope where the method is defined!). So it is very much similar to the much-hated dynamic scoping. But the dispatching is more disciplined — it doesn't just match the name. It must match both the name and the inferred principle type. This intuition also explains why local instances shouldn't be allowed, because if we capture the variables at the definition site, the method call becomes "statically scoped". -- yin