polymorphism - Scheme define-macro and/or define-syntax -

-

i want create overloaded scheme macro simple form of polymorphism. is, macro smart enough expand differently when given params of different types, (look-up key container) "right" thing different kinds of containers.

(define-macro (look-up key container)   (cond     ((table? container) `(table-ref ,key ,container))     ((pair? container) `(assoc ,container ,key))       etc.     (else `(error "unknown type look-up))))

ideas?

i think chris right in not job macros. simple procedure might you're looking for:

(define (lookup key container)   (cond ((type1? container)            (type1-lookup key container))          .                                 ; repeat whichever types..          .         ((typen? container)            (typen-lookup key container))         (else 'undefined-lookup)))         ; or default value or ...

or maybe need find out you're dealing once can build build more dedicated procedure on fly. make-lookup procedure might similar code above, except return procedure, rather call lookup right away:

(define (make-lookup container)   (cond ((type1? container)            type1-lookup)          .                                 ; repeat supported types..          .         ((typen? container)            typen-lookup)         (else default-lookup-procedure)))  (define lookup (make-lookup container))

you add optional argument make-lookup take procedure , use rather 1 of type-specific lookups defined.


Comments

Post a Comment

Popular posts from this blog

python - TypeError: start must be a integer -

-
i trying draw 5 turtles, getting typeerror on line 25. here code: import turtle wn = turtle.screen() redrose = turtle.turtle() color = input("what background color be?") fillcolor_f = input("what color of rose be?") redrose.hideturtle() redrose.speed(30) redrose.penup() redrose.left(180) redrose.forward(175) redrose.right(90) redrose.forward(30) redrose.right(90) redrose.pendown() def drawrose(red): redrose.color("pink") redrose.fillcolor(fillcolor_f) redrose.fill(true) in range(red): redrose.forward(i) redrose.right(49) in range(5): drawrose(redrose) redrose.penup() redrose.forward(350) redrose.right(144) redrose.pendown() redrose.fill(false) drawrose(50) wn.bgcolor(color) i trying draw 5 roses, produces errors. doing in interactivepython.org. you recursively calling drawrose wrong parameter. on line 23 ( for in range(red): ) expect red integer...
Read more

c# - DevExpress RepositoryItemComboBox BackColor property ignored -

-
i have following code almost works properly; private void cbstatus_drawitem(object sender, listboxdrawitemeventargs e) { string item = e.item string; if (item != null) { switch (item) { case "1": e.appearance.forecolor = color.green; e.appearance.backcolor = color.green; break; case "2": e.appearance.forecolor = color.orange; e.appearance.backcolor = color.orange; break; case "3": e.appearance.forecolor = color.red; e.appearance.backcolor = color.red; break; } } } when dropdown shown, items forecolor correct, backcolor remains whatever backcolor of theme is; i.e. if i've got set dark theme, backcolor dark, cells in gridview, rather being green/orange/red. i've tried setting e.appearance.options.usebackcolor trying set e....
Read more

django - Creating multiple model instances in DRF3 -

-
1) create multiple model instances 1 api call, asked here: how create multiple model instances django rest framework? tried solution named in link, no success. i trying upload multiple files in 1 api call. result: files uploaded (only once overwrote perform_create) 1 instance created (if send 2 files, latter created instance). my code: class fileuploadserializer(serializers.modelserializer): def __init__(self, *args, **kwargs): many = kwargs.pop('many', true) super(fileuploadserializer, self).__init__(many=many, *args, **kwargs) class meta: model = fileupload read_only_fields = ('created', 'datafile', 'owner') class fileuploadviewset(viewsets.modelviewset): queryset = fileupload.objects.all() serializer_class = fileuploadserializer parser_classes = (multipartparser, formparser, ) def perform_create(self, serializer): file_list = self.request.data.getlist('file') ...
Read more