export vs export_ok in perl

Question

I can not understand what is the difference/use case of `EXPORT_OK` vs `EXPORT`. Most resources mentions something in the lines of: @Export allows to export the functions and variables of modules to user’s namespace using the standard import method. This way, we don’t need to create the objects for the modules to access its members. @EXPORT_OK does export of symbols on demand basis for selective list of symbols (subroutines and variables) of the module. But I really don't see the difference/meaning here. Can someone please provide a small fundamental example of the difference/usage of these 2 symbols?

Accepted Answer

Let's say I have a package `MyPackage` that uses `@EXPORT`.

```
#this is MyPackage.pm
package MyPackage;
@EXPORT = qw(do_awesome_thing);

sub do_awesome_thing { ... }

sub be_awesome { ... }
```

Now, when I use `MyPackage` in my code,

```
#this is myscript.pl
use MyPackage;

do_awesome_thing(); #works

be_awesome(); #doesn't work
MyPackage::be_awesome(); #works
```

`do_awesome_thing` gets automatically exported to my code from `MyPackage`, without me having to say "give this to me". `be_awesome` isn't exported (and it won't be exported with `@EXPORT_OK` either, I'm just showing that part to get you clear on what "exporting" gives us).

On the other hand, if I have a package `MyOtherPackage` that uses `@EXPORT_OK`,

```
#this is MyOtherPackage.pm
package MyOtherPackage;
@EXPORT_OK = qw(do_awesome_thing);

sub do_awesome_thing { ... }

sub be_awesome { ... }
```

and then try

```
#this is mynewscript.pl
use MyOtherPackage;

do_awesome_thing(); #doesn't work
MyOtherPackage::do_awesome_thing(); #works, as always
```

the line calling `do_awesome_thing` directly won't work. This is because putting something in `@EXPORT_OK` says "give this to my users only if they ask for it". Since we've just said `use MyOtherPackage` without explicitly asking for `do_awesome_thing` to be imported here, it doesn't get imported, and is accessible only by specifying the package name.

The way you ask for `do_awesome_thing` to be imported is to say `use MyOtherPackage qw(do_awesome_thing)` in the second line of `mynewscript.pl` above. This says import that module and make `do_awesome_thing` available directly. After that, the fourth line in `mynewscript.pl` above will start working.

Note that the user can specify `use MyPackage qw(do_awesome_thing)` with the first package also, and in that case, anything else in the `@EXPORT` list won't be exported, only `do_awesome_thing` will be. So, except for the default case of `use PackageName;`, `@EXPORT` and `@EXPORT_OK` behave similarly. In the default case, anything in `@EXPORT` gets thrown into the user's script automatically, while `@EXPORT_OK` is more polite and doesn't export anything.

export vs export_ok in perl

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