I don't control class creation. I need to make a wrapper around make-object and attach contract to the wrapper.
Now I have (provide/contract [ read-object ( ->i ([ binary-class ( implementation?/c binary<%> )] [ port input-port? ]) #:rest [ args list? ] [ result ( binary-class ) ( is-a?/c binary-class )])]) I cannot control number of args. Now, when error encountered I have confusing error message mentioning "instantiate". Thu, 5 Jun 2014 12:13:26 -0400 от Matthias Felleisen <matth...@ccs.neu.edu>: > >Here is the pattern I recommend: > >Welcome to Racket v6.0.1.11. >> (define (create-c #:x [x 0]) (new c% [x x])) >> (define c% (class object% (init-field x) (super-new))) > >That is, a class comes with a 'factory' definition, a function that creates >instances and uses keywords similar to those used by the class initializer. If >you then export these factories, you can enforce invariants and also probe the >factory for the information you want: > >> (create-c) >(object:c% ...) >> (create-c #:x 10) >(object:c% ...) >> (procedure-arity create-c) >0 >> (procedure-keywords create-c) >'() >'(#:x) > > >Yes, one could argue that this is a poor man's substitute for missing class >reflection. -- Matthias > > > > > > >On Jun 5, 2014, at 11:32 AM, Roman Klochkov < kalimeh...@mail.ru > wrote: > >> For any procedure I can use procedure-arity. But how to get the number of >> init arguments for a class? >> >> Or maybe there are any other way to make a contract, like in Guide 7.3.9, >> where one can compare number of arguments and arity of the function, but >> when using (make-object someclass ...) >> instead of the function. >> >> >> -- >> Roman Klochkov >> ____________________ >> Racket Users list: >> http://lists.racket-lang.org/users > -- Roman Klochkov
____________________ Racket Users list: http://lists.racket-lang.org/users
