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Editorial Team
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Asked: 2026-05-24T10:15:15+00:00

I’m maintaining a large code base in J, and a few rare functions and

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I’m maintaining a large code base in J, and a few rare functions and variables end with __ (two underscores).

Normally, __ is used to reference something in a locale.

barObj =: conew 'Bar'
Foo__barObj

However, I have this:

Foo__

Declared as:

Foo__=: 3 : 0

Is that another syntax for a static function? Is it accessible to all locales?

EDIT:

From what I can gather, it’s not local. A variable declared thus:

FooVar__ =.

can be referenced by another function, despite =. making it seem local. The same way any “into locale” declarations are always “global to the locale” (Foo__Bar =. and Foo__Bar =: are the same).

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1 Answer

  1. Editorial Team

    Ending with double-underscore is an old technique for referencing a name in the base locale. Here are details from the relevant part of the Locale lab:

    A locative with no locale name between the bracketing _ characters is
    treated as if it had the name base.

    abc__ is the same as abc_base_

    Treating __ as _base_ is for compatibility with earlier versions, and
    in general you should use _base_ .

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