Re: [Sbcl-devel] type inference question

[Christophe R. <cs...@ca...>]

"Robert E. Brown" <bb...@sp...> writes:

> The following code:
>
>     (defvar *foo* nil)
>     (declaim (type (or null function) *foo*))
>
>     (defun a () (when *foo* (funcall *foo*)))
>     (defun b () (if *foo* (funcall *foo*)))
>     (defun c () (and *foo* (funcall *foo*)))
>
> generates 3 compiler messages, one for each of A, B, and C:
>
>     ; note: unable to
>     ;         optimize away possible call to FDEFINITION at runtime
>     ;       due to type uncertainty:
>     ;         The first argument is a (OR NULL FUNCTION), not a FUNCTION.
>
> In each case, SBCL fails to deduce that *FOO* can only be invoked when it is
> not NIL.  I would have guessed that Python's type inferencer could handle
> this sort of thing.

In a threaded sbcl with a shared binding of *foo*, something can
mutate the current binding of *foo* between the test of the
conditional and the execution of the FUNCALL.  So the type derivation
would lead to the consequences of an incorrect declaration -- probably
a bus error, in this case, if such a race were to trigger.

Cheers,

Christophe
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