To be fair: As a new user, it's possible to have the intuition that `define` is just a way to avoid indentation -- that it "writes a `let` for you, from the point of the define to 'the end of the enclosing scope'".
And it's possible for that intuition to seem correct for a very long time -- until you hit an example like Brian did. And then you need to learn about `letrec`. (As a non-new user, 99.9% of the time that I use a local `define` I actually wish it were "like `let`" not `letrec`, but I use it anyway and try to be careful.) On Mon, Mar 11, 2019 at 12:29 PM Matthias Felleisen <[email protected]> wrote: > > > > > On Mar 11, 2019, at 11:21 AM, Brian Adkins <[email protected]> wrote: > > > > I just discovered that define will fail at runtime, where let would fail at > > compile time. Besides helping to keep the indentation level from marching > > to the right "too much", what are the benefits of define over let? > > > > --- snip --- > > #lang racket > > > > (define (f n) (+ n 1)) > > > > (define (foo) > > (define b (f a)) > > (define a 7) > > > > b) > > > > (define (bar) > > (let ([b (f a)] > > [a 7]) > > > > b)) > > --- snip --- > > > > I think your characterization is a bit misleading here. > > In ‘bar’ ‘a’ is not bound, something that Racket (and DrRacket) properly > signal at compile time. > > In ‘foo’ ‘a’ *is* bound, because you’ve set up a mutually recursive scope. > But, when Racket evaluates (foo) it notices that ‘a’ is bound but > uninitialized, which is two different things. > > If you want to compare apples to apples, use a ‘letrec' instead of a ‘let' in > ‘bar'. Then you have (1) the same semantics and (2) the same error. > > — Matthias > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
