On Wed, Aug 18, 2010 at 16:52, Matthias Felleisen <matth...@ccs.neu.edu>wrote:
>
> Eduardo is providing a macro that helps you abbreviate complex
> selector and mutator chains. The BENFIT you get is that instead
> of writing
>
> (posn-x (bird-posn (sky-bird (world-sky some-world))))
>
> you can write -- in Racket --
>
> some-world.sky.bird.posn.x
>
> and make Racket programs look as short as C program selectors
> or Java dots or Javascript dots etc.
>
> The PRICE you pay is that you annotate variables and fields
> with struct information. That is, you have to say somewhere that
> some-world is a variable that always stands for a world struct.
> And the sky field in world has to come with information that it
> always is a sky-struct. And so on.
>
As describe in the HelpDesk about `prop:procedure' it is
also possible to use the struct id as a procedure, so that it can be
used as an accessor/mutator.
This is an intermediate solution, as it is nearly (but not quite) as
short as the dot notation without introducing global names.
Just a quick example that needs more error-checking and needs
to be abstracted (and, as Matthias and Shriram are pointing out,
this example has a f["baz"] flavor... I don't know if it is possible
to fix that if the procedure is not a macro) :
#lang racket
(struct foo (bar baz)
#:mutable
#:property prop:procedure
(case-lambda
[(self attr)
(case attr
[(bar) (foo-bar self)]
[(baz) (foo-baz self)]
[else (error "foo: No field named" attr)])]
[(self attr val)
(case attr
[(bar) (set-foo-bar! self val)]
[(baz) (set-foo-baz! self val)]
[else (error "foo: No field named" attr)])]))
(let ([f (foo 1 2)])
(f 'baz 3)
(list (f 'bar) (f 'baz)))
Laurent
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