It might help to know a bit more context about what you're trying to do. I think other Lisp-family languages use these terms in different ways, but in Racket it isn't usual to talk about a symbol being bound to something. A symbol is just a type of value (like an integer, a list, a vector, a function, or anything else) and exists only at the run-time of some code. Binding, on the other hand, is a compile-time notion. Functions like `identifier-binding` operate on identifiers, which are syntax objects that wrap symbols together with lexical context and source-location information: in other words, the first argument to `identifier-binding` must satisfy `identifier?`, which means it will never satisfy `symbol?`. Given renaming, lexical scope, and other features, a symbol isn't enough information to specify a binding, and I believe that at run-time symbolic names go away and are replaced by some form of de Bruijn indexes. (Take that last part with a grain of salt: I don't spend much time with low-level runtime details.) Also, in `#lang racket`, given that there isn't a static type system, the question of whether an identifier is bound to a procedure isn't answerable at compile-time: Matthew's macro thus expands to false for unbound identifiers, but expands to a `procedure?` check at runtime for all bound identifiers.
At the same time, if you explain what you are trying to do at a higher-level, there very well may be a way to do it that involves implementation techniques you wouldn't think to ask about. For example, if you want unbound identifiers to be self-quoting as symbols, the answer would involve defining a macro named `#%top`. Finally, I'm not sure that I understand your example: > [define [foo symbols) (bound-to-proc? sum)) > Ignoring the mismatched parentheses, this use of `bound-to-proc?` doesn't match the first clause of Matthew's `syntax-case` because there is no `quote`, and it looks like Matthew tried to implement your request to determine "if a quoted symbol has an associated procedure". (The implementation of `bound-to-proc?` would be more robust if `syntax-case` were given `quote` as a literal, but that way lies madness: what about `quasiquote`, or macros you don't know about that expand to symbols? Using `local-expand` could help … but, again, the best thing would be to know more about what you're ultimately trying to do.) -Philip On Mon, Sep 17, 2018 at 12:21 PM Kevin Forchione <[email protected]> wrote: > > > On Sep 16, 2018, at 10:07 PM, Matthew Butterick <[email protected]> wrote: > > #lang racket > (require rackunit) > > (define-syntax (bound-to-proc? stx) > (syntax-case stx () > [(_ 'x) > (and (identifier? #'x) (identifier-binding #'x)) > #'(procedure? x)] > [_ #'#f])) > > (define bound-to-proc +) > (define not-bound-to-proc 42) > > (check-true (bound-to-proc? 'bound-to-proc)) > (check-false (bound-to-proc? 'not-bound-to-proc)) > > > Thanks, Matthew. This approach works as long as the macro is being used > outside of a definition, but I suspect to build any sort of logic around it > they’d have to be macros as well. For instance, this fails because the > macro captures symptoms and not the value passed to the function: > > [define [foo symbols) (bound-to-proc? sum)) > > That seems to be the nature of macros, and I’m not sure what the solution > to that paradox is, apart from perhaps building a symbol/function hash > table as part of a define. Presumably Racke does something like that for > eva & namespaces, but I’m surprised there isn’t a symbol->procedure > function. > > Kevin > > -- > 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.
