18!:5 '' +----+ |loc3| +----+

The Shared Locale 'z'

It is a J convention, universally adhered to, that every locale's search path must end with the locale 'z' . Any name in the 'z' locale can then be referred to by a simple name from any locale, making names in the 'z' locale truly global names.

Using Locales

You have my sympathy for having to read through that detailed description of name processing; I refuse to apologize, though, because you really can't write programs if you don't know what names mean. But, you wonder, How do I use locales?

You won't go far wrong to think of a locale as akin to a class in C++. When you have a set of functions that go together as a module, define all their verbs and nouns in a single locale. The easiest way to do this is to put a line

coclass <'localename'

at the beginning of each source file for the module (coclass is like cocurrent but it supports inheritance). Then, every public assignment in the file will automatically be made in the locale localename .

The names defined in the locale are the equivalent of the private portion of class. To provide the public interface to the class, you need to put those names in a place where they can be found by other modules. The traditional way to do this is to define them in the locale 'z' by ending each file with lines like

epname_z_ =: epname_localename_

Here epname is the name of a verb, and localename is the name of the module's locale. Take a minute to see what happens when some other locale invokes epname . The name search for epname will end in the locale 'z' where this definition is found. Execution of this definition immediately results in execution of epname_localename_ which switches the current locale to localename and runs the definition of epname found there. The benefit is that the calling module doesn't need to know the locale that epname is going to be executed in.

If you tire of writing out the public definitions one by one, I have included in jforc.ijs a verb to do it for a list of entry points using a sentence like

PublishEntryPoints 'public1 public2 public3'

If you want to create multiple copies of objects derived from a class, you should consult the Lab on Object Oriented Programming. There you will learn about numbered locales and how to create and destroy objects. We will not discuss these topics here.

By following the guidelines given above you will be able to emulate the class facilities of C++. Because J is interpreted, you can do much more if you want: you can change search paths dynamically; you can use locales and paths to create a highperformance network database; you can pass locales as data and use them to direct

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processing; you can peek at a module's private data. You can even modify a module's private data from outside the module, but if you are struck by lightning after doing so the coroner will find it was suicide.

Using locale-names as data allows for dynamic separation of namespaces. For example, the processing of forms in J requires definition of verbs for a great many events. You may let these verbs all share the same locale; but if you want to segregate them, the Window Driver will remember what locale was running when each form was displayed, and direct events for a form to the locale handling the form.

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