Functions and Actions


NLP++ supports function calls, which have a similar form to other programming languages (such as C/C++):

    functionName(arg1, .... argN)

where functionName is constrained to be alphabetic and where each argument to the function may be an NLP++ expression.

Only call-by-value arguments are supported at present.  That means that when a variable is used as an argument, its value is not changed by a change to the local variable that it becomes associated with in the function.

Functions have no template or declaration -- only a definition.  Even so, you may use a function in one pass and define it in a subsequent pass (forward reference).  The parameters to functions take arbitrary data types that may vary dynamically.  Return statements similarly return arbitrary data types.  The same function could return a numeric value in one call and a string in another.  We discourage the abuse of this extreme flexibility, however!  The new L variable reference has been introduced to support formal function parameters as well as local variables within a function body.  Recursive functions are also possible.  The example below illustrates the definition and use of user-defined functions in NLP++.




"output.txt" << L("var") << "\n";

L("local") = 3;   # create a local variable.

"output.txt" << L("local") << "\n";

return 1+2;



facto(L("num"))  # recursive factorial function.


if (L("num") <= 0)

return 1;

return L("num") * facto(L("num") - 1);





G("x")[2] = 3;

"output.txt" << G("x") << "\n";


"output.txt" << "4! = " << facto(4) << "\n";



The code above prints out

0 0 3

0 0 3


4! = 24


Actions, e.g., single, are specialized functions that don't take generalized NLP++ expressions as arguments.  Though some actions have been upgraded and now take NLP++ arguments (e.g. group), some actions have not been upgraded and the argument must be a literal value.

A unique feature of actions in the PRE Region is that they have a mandatory range.  The syntax is as follows:

<num,num> pre_action_name(args....)

The action applies to each rule element in the <num, num> range individually.

See Actions as a starting point for per-page descriptions for actions.

Case Sensitivity

NLP++ function and action names are case-sensitive.  

Defining New Functions

In VisualText 1.2 and later, user-defined NLP++ functions are fully supported, including parameter lists, return statements, recursive calls, and compilation of these.  Functions are specified in the @DECL region, which must appear first within a pass file, if present.

New NLP++ functions can also be defined with C++ code, within an application's user project. User-defined function calls must be preceded by the 'user' scope qualifier, e.g., user::myfunction. (Interpreted NLP++ will find the correct function and issue a warning, but user functions without the scope qualifier will be compiled incorrectly.)

See Also