# __MAIN__ macro

Source: https://ref.dm-lang.org/DM/preprocessor/__MAIN__

The __MAIN__ macro is defined in the main `.dme` file being compiled. In all other files included by this file, __MAIN__ is not defined.

The purpose of this is for library writers to package a small demo of their library directly in the library source code. When users compile the library directly, the library's own `.dme` is the main file and can include extra files that are not normally part of the library.

```dm
#ifdef __MAIN__
#include "demo.dm"
#include "demo.dmm"
#endif
```

If the demo requires a lot of extra resources, it is probably better to package the demo as a separate project. Small demos, however, are nice and convenient using this "auto-demo" technique—especially since Dream Seeker automatically launches Dream Maker after installing a library containing a `.dme` file.

### See also
> [preprocessor](https://ref.dm-lang.org/DM/preprocessor/index.md)