On Jan 19, 2012, at 1:51 PM, Ryan Culpepper wrote: > The Racket class system doesn't have such a thing because it doesn't need it. > The idiomatic Racket alternative to "class variables" or "static members" is > to define the variable/constant/function at the top-level of the enclosing > module. > > This scope-based solution also eliminates what in other OOPLs is a distinct > class of errors: attempting to refer from static members to instance members, > which syntactically appear to be in scope. In Racket, it's obvious that the > instance members aren't in scope. To properly implement a form like > 'class-variable' in Racket, one would have to implement error-checks for "in > scope but not accessible" references (a naive implementation would give > misleading "unbound identifier" errors). Or perhaps restrict the > initialization expressions to immediate values. > > I'm not sure that helps you with your goal, but perhaps it explains why > Racket doesn't have what you're looking for. > > Ryan > > On 01/19/2012 12:41 PM, Christian Wagenknecht wrote: >> I expected to find some special-form, like 'class-variable' or something >> like that. >> >> For pedagogical reasons I'd prefer to implement two syntactically >> different programs representing the oop thinking style quite obvious: >> the first one makes absolutely no use of the bindings provided by the >> 'class' library that comes with Racket whereas the second one is mainly >> limited to take them. The first one helps the students to understand >> lots behind the scene of how oop works and why. The second one abstracts >> of that and really allows for oo programming. >> >> What I mean is that the terms describing the basic concepts of object >> oriented programming should be mapped to related code. Using 'let' to >> get the right variable scope causes to mix both levels of thinking / >> abstraction. Thats exactly what I like to avoid.
This may be obvious at this point, but it appears that the underlying issue is the conflict between a) the view that object-oriented languages are an *alternative* to other languages, and b) the view that objects and classes are a useful *addition* to a language. I believe that Shriram argues this point pretty clearly in his "Teaching Programming Languages in a Post-Linnean World" www.sigplan.org/pl-workshop/contributions/8.pdf ... and that Racket is (by and large) in the (b) camp. In this context, then: you're switching from "implementing classes" to "using classes." However, I claim that there are a sizable family of language features that are orthogonal to these, and that it's reasonable that these language features appear in both languages. To take a more clearly delineated example: I'm guessing that both of these two languages/projects include the "quote" syntax for introducing lists of symbols and numbers, and that this doesn't concern you; I claim that lexical bindings also fall into this category. Naturally, I speak for no one but myself. John >> >> Am 19.01.2012 18:09, schrieb Matthias Felleisen: >>> >>> On Jan 19, 2012, at 11:57 AM, Christian Wagenknecht wrote: >>> >>>> How class variables/methods (instead of instance variables/methods) >>>> can be implemented be means of Racket's class definition expressions? >>>> An unsatisfying way I found is by using a let expression enclosing >>>> the whole definition of the class. >>> >>> What's unsatisfying about it? >> ____________________ >> Racket Users list: >> http://lists.racket-lang.org/users > ____________________ > Racket Users list: > http://lists.racket-lang.org/users
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