On Tue, May 24, 2011 at 11:16 AM, Richard Lawrence <[email protected]> wrote: > > Curiously, if I change `(assert s string?)' to `(string? s)', the > example passes the type check. So I guess maybe the short-circuiting > nature of `and' and `or' is a red herring here, and what I don't > understand is something about `assert'. What makes for the difference > between `(assert s string?)' and `(string? s)'? > > I thought that the difference was that `(assert s string?)' performs a > type check at macro expansion time on the type of the expression `s', in > addition to performing the dynamic check that `(string? s)' does at > runtime. But if that's right, then why doesn't the type check fail on > the `assert' form in the `and' expression, rather than on the second > form? That is, if the type of `s' is known to be (U String False) by > the time the second form is type checked, why doesn't the assertion in > the first form fail the type check?
(assert x p?) is equivalent to (if (p? x) x (error ...)) except that the result is known to have the appropriate type (here `String'). Nothing is done at macro expansion time, and `assert' doesn't change the typechecking of anything. It's just a function with a particular type. I would rewrite your expression like this: (and (string? s) (string=? s "") s) which I think is what you did, and which is how I'd write the program in Racket as well. -- sam th [email protected] _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
