2014-03-05 19:12 GMT+01:00 Jan Wedekind <j...@wedesoft.de>: >> Besides, I think that Scheme + OOP has its own flaws. In most OOP >> languages, you have this notation object.property or object->method(), >> which also allows for chain calls, i.e. object->getChild()->method(), >> or -- if you have nested objects -- to use >> object.property.propertys_property. > > Somewhat related I have seen an interesting project proposal for generics in > Racket [1]. The generics use predicate functions instead of classes. E.g.: > > (defmethod add ((x number?) (y number?)) > (+ (x y)) > > where 'number?' is a function returning '#t' for numbers. > > The type hierarchy (specialisation) is declared using 'defsubtype'. E.g.: > > (defsubtype zero? integer?)
I don't know whether it's better to provide a separate entity for types/classes or to use bare predicates. I think that an advantage of explicit first class types (such as the ones used in GOOPS) is that one could easily provide a method to generate exemplars of objects of those types, which could be useful for unit tests performed by systems such as QuickCheck. I think it should be fairly easy to implement such system portably for Scheme. Even better, I believe it would be nice to employ the Wright-Shinn pattern matcher (known as (ice-9 match) for Guilers) in signatures of functions, so that one could write e.g. (generic factorial) (specialize (factorial 0) 1) (specialize (factorial (? number? n)) (* n (factorial (- n 1))) [I am using the name 'specialize' here in order to avoid confuision, but the concept is the same] It's a nice idea to stipulate the ordering relation for predicates (although it obviously would be better if the interpreter could infer it automatically, at least to some extent). What would be needed for the "match"-based implementation is a partial order of the "match" patterns, so that they could be ordered from the most specific to the least specific (which seems an interesting issue by itself). It would also be nice if that could integrate with a full-blown type system, so that apart from types for arguments, one could declare types for the values being returned by a function, and if a tool for static type analysis could be provided, because that would allow to increase the type-safety of Scheme programs. But I agree that this is an interesting and important issue. Best regards, M.
