10. Function declarations

Functions can only be declared at the top level and all parameters must have a direction. P4 functions are modeled after functions as found in most other programming languages, but the language does not permit recursive functions.

functionDeclaration
    : annotations functionPrototype blockStatement
    | functionPrototype blockStatement
    ;

functionPrototype
    : typeOrVoid name optTypeParameters "(" parameterList ")"
    ;

Here is an example of a function that returns the maximum of two 32-bit values:

bit<32> max(in bit<32> left, in bit<32> right) {
return (left > right) ? left : right;
}

A function returns a value using the return statement. A function with a return type of void can simply use the return statement with no arguments. A function with a non-void return type must return a value of the suitable type on all possible execution paths.