Yep, it really is a long-standing bug. Script-mode top-level locals are
treated as globals (module-scope bindings) by the compiler, but their
initial bindings are evaluated eagerly instead of lazily (as you’d want in
a script). Taken together, this means that you can get this completely
unsafe behavior.
So, why is ‘a’ accepted but ‘b’ not in your original example?
func foo() -> Int { return b }
let a = 1
let b = 2
print(foo())
The secret to the current behavior is that script mode is executed
interactively, instead of parsing it all up front. To make things a little
better, it *actually* parses any number of declarations until it sees
something it actually needs to execute—a statement or a declaration with an
initial value expression. This allows for recursive functions while still
being “live”.
The consequence here is that *one* top-level binding after a series of
functions may be visible. This is obviously not optimal.
To fix this, we should:
- Distinguish between script-mode top-level locals and module-scope
variables that happen to be declared. My personal preference is to treat
anything with explicit access control as a normal lazy global and anything
without access as a top-level local.
- Consider parsing everything up front, even if we don’t type-check it, so
that we can say “use of ‘b’ before it’s initialized” instead of “undeclared
name ‘b’”
Note that we *do* need to be conservative here. This code should continue
to be rejected, even though ‘f’ doesn’t refer to ‘local’ directly, because
*calling* ‘f' would be dangerous before the initialization of ‘local':
internal func f() -> Int {
return g()
}
*// more code here*
let local = 42
private func g() -> Int {
return local
}
Thanks for bringing this up, if only so I have an opportunity to write out
the issue. :-)
Jordan
On Sep 21, 2016, at 23:04, Jens Persson <jens@bitcycle.com> wrote:
Did you see the other code examples that came up in that twitter
conversations?
For example:
This worrying little program compiles:
func f() -> Int {
return a
}
let a = f()
It also compiles if you print(a) at the end, and it will print 0.
If we replace Int with [Int] it will still compile but crash when run.
And also this:
AnotherFile.swift containing:
func f() -> Int {
return a
}
let a = f()
main.swift containing
print(a)
Compile, run (for eternity, at 0% CPU).
/Jens
On Thu, Sep 22, 2016 at 3:13 AM, Joe Groff <jgroff@apple.com> wrote:
> On Sep 21, 2016, at 2:22 PM, Jens Persson via swift-users < >> swift-users@swift.org> wrote:
>
> // This little Swift program compiles (and runs) fine:
>
> func foo() -> Int { return a }
> let a = 1
> let b = 2
> print(foo())
>
> But if `foo()` returns `b` instead of `a`, I get this compile time
error:
> "Use of unresolved identifier `b`"
This looks like a bug to me (cc-ing Jordan, who's thought about global
scoping issues more than me). In "script mode", it shouldn't be possible to
refer to a variable before its initialization is executed. However, the way
this is currently modeled is…problematic, to say the least, among other
reasons because script globals are still visible to "library" files in the
same module.
-Joe
