!CHAPTER Conventions

The *::* symbol is used inside AQL as the namespace separator. Using 
the namespace separator, users can create a multi-level hierarchy of 
function groups if required.

Examples:

``` js
RETURN myfunctions::myfunc()

RETURN myfunctions::math::random()
```

**Note**: As all function names in AQL, user function names are also
case-insensitive.

Built-in AQL functions reside in the namespace *_aql*, which is also
the default namespace to look in if an unqualified function name is
found. Adding user functions to the *_aql* namespace is disallowed and 
will fail.

User functions can take any number of input arguments and should 
provide one result. They should be kept purely functional and thus free of
side effects and state. 

Modification of global variables is unsupported, as is changing
the data of a collection from inside an AQL user function. 

User function code is late-bound, and may thus not rely on any variables
that existed at the time of declaration. If user function code requires
access to any external data, it must take care to set up the data by
itself.

User functions must only return primitive types (i.e. *null*, boolean
values, numeric values, string values) or aggregate types (lists or
documents) composed of these types.
Returning any other Javascript object type from a user function may lead 
to undefined behavior and should be avoided.

Internally, user functions are stored in a system collection named
*_aqlfunctions*. That means that by default they are excluded from dumps
created with [arangodump](../Arangodump/README.md). To include AQL user functions in a dump, the
dump should be started with the option *--include-system-collections true*.