global keyword in inner scopes

[amckinlay]

global does not always refer to the global scope.

In [1]: for i = 1:1
   ...:     local z = 1
   ...:     for i = 1:1
   ...:         global z = 1
   ...:     end
   ...: end

In [2]: z


z not defined
at In[2]:1

Another example:

In [17]: function foo(z)
    ...:     global z = 2
    ...: end
Out[17]: foo (generic function with 1 method)

In [18]: foo(3)
Out[18]: 2
In [19]: z

z not defined
at In[19]:1

Version information:

In [3]: versioninfo()
Julia Version 0.2.0

Commit 05c6461 (2013-11-16 23:44 UTC)
Platform Info:
  System: Windows (x86_64-w64-mingw32)
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY)
  LAPACK: libopenblas
  LIBM: libopenlibm

Note: I believe this same behavior occurs on the latest snapshot.

[elextr]

This appears to be a symptom of the REPL having to use a "fake" global scope for code typed into it.

Both examples work fine from files.

The manual obliquely refers to the fact that in the REPL global has slightly different semantics.

[amckinlay]

@elextr All the manual says is that global scope in the REPL is inherited with local variable semantics into inner scopes. So, you can assign to a global from an inner scope without having to use the global keyword.

Edit: Sorry, this comment is a little confusing. What I should have said is that it is inherited "like" a local variable, but is still global and should be accessible via the global keyword no matter what inner scope you're in.

[elextr]

The part of the manual I was referring to is in http://docs.julialang.org/en/release-0.2/manual/variables-and-scoping/

"Due to the special identification of the prompt’s scope block with the global scope, it is not necessary to declare global y inside the loop. However, in code not entered into the interactive prompt this declaration would be necessary in order to modify a global variable."

This implies that at the REPL the "global" is not the true global scope. Indeed having the REPL use the true global scope would be dangerous, a user typing at the REPL could make a (un)lucky choice of name and affect the REPL or Julia itself.

But it seems that Julia doesn't hide the differing semantics well enough.

[amckinlay]

So, the global keyword should refer to the enclosing scope even if the
enclosing scope is not global? That doesn't make sense.
On Jun 15, 2014 10:19 PM, "elextr" [email protected] wrote:

The part of the manual I was referring to is in
http://docs.julialang.org/en/release-0.2/manual/variables-and-scoping/

"Due to the special identification of the prompt’s scope block with the
global scope, it is not necessary to declare global y inside the loop.
However, in code not entered into the interactive prompt this declaration
would be necessary in order to modify a global variable."

This implies that at the REPL the "global" is not the true global scope.
Indeed having the REPL use the true global scope would be dangerous, a user
typing at the REPL could make a (un)lucky choice of name and affect the
REPL or Julia itself.

But it seems that Julia doesn't hide the differing semantics well enough.

—
Reply to this email directly or view it on GitHub
#7264 (comment).

[elextr]

In the REPL, global refers to a hidden local scope that "surrounds everything you type". It is indeed not the global scope. As the referred manual section notes, its semantics may therefore be slightly different.

In 0.3 a lot of the REPL has been changed, but I don't know if this has been fixed.

[JeffBezanson]

No, in the REPL global variables are the same as any other global variables. If I type x=1 then Main.x exists, like any other global. It's just that the REPL doesn't always require the global keyword for assigning to globals; in the REPL, globals can be automatically inherited like locals from an enclosing scope, but they are still real globals.

[amckinlay]

@JeffBezanson So, this issue is a bug.