2016-08-23

Implicit generic function creation

define-method would create a generic function implicitly if there's none defined yet. It's convenient and I've written libraries (probably only one) depending on this behaviour. (c.f. (binary data))

Convenience would usually be a trade-off  of consistency, at least in my case. For example, this is a long standing bug (though, I've just issued):
(import (clos user))

(let ()
  (define-generic foo))
foo
;; -> should be &undefined
This is because define-method would create a generic function during macro expansion, and it would be an unexpected result in this case if it didn't:
(begin
  (define-generic foo)
  (define-method foo (o) #t))
In this case, define-method should not make implicit generic function, but macro expansions are done in the same compilation time as define-generic. Thus, it's impossible to know if it should create or not. To let define-method know it shouldn't, define-generic also inserts binding into current environment (a library) during macro expansion.

Can't I do better? Creating global binding during macro expansion is rather ugly, but I don't I can get rid of it (or maybe the future?). Yet, I think I can avoid to create unwanted one like above example. It's still just an idea but if define-generic and define-method can see if they are used in a scope, then it seems there's a way. Since Sagittarius has current-usage-env and current-macro-env procedures, it is possible to access compile time environment during macro expansion. Thus, it should even be able to detect whether or not define-method should create an implicit generic function or not.

This should work, let's see.

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