On Feb 13, 2013, at 12:47 PM, Dmitry Pavlov <[email protected]> wrote:
> Hello,
>
> I just figured that there is a thing in Racket coding style
> that bothers me from time to time. That thing have never arised
> in C-style languages.
Right: in C-style languages there's a special "dot" syntax for getting at the
fields of a struct object. If C++ allowed variable names to contain periods,
you could get an analogous ambiguity:
class posn { int x; int y;
posn(int xval, int yval) : x(xval), y(yval) {}
};
posn here(3,4);
int here.x = 5;
printf ("%d", here.x); // is the correct answer 3, referring to a field of the
"here" object, or 5, referring to the int variable named "here.x"?
Which is why C-family languages don't allow periods to appear in a variable
name.
> Say I want to have a "vehicle" struct. Also, I would like to keep
> its physical state in a separate structure. It seems natural
> to call it "vehicle-state". Also it seems natural to have a
> "state" member in the "vehicle" structure which refers to that
> separate structure. Here we go:
>
> (struct vehicle-state
> (position velocity))
>
> (struct vehicle (state color model))
>
> module: duplicate definition for identifier in: vehicle-state
Yes, that's the price you pay for the convenience of having getters defined
automatically. You can drop that convenience by using :omit-define-values or
make-struct-type, but then you lose the convenience for ALL your fields. Or
you could do something like
(struct vehicle-state (position velocity) #:constructor-name make-vs)
(struct vehicle (state color model))
which avoids the conflict.
Or, as you suggest, you could just name structs and their fields so that no
struct's name is another struct's name, a hyphen and one of the latter's fields.
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