Function linkages in ARexx are established at the time of the function call. A specific search order is followed until a function matching the name symbol or string is found. If the specified function cannot be located, an error is generated and the expression evaluation is terminated. The full search order is:
The program source is examined for a label that matches the function name. If a match is found, a new storage environment is created and control is transferred to the label.
The built-in function library is searched for the specified name. All of these functions are defined by uppercase names.
Function Libraries and Function Hosts
The available function libraries and function hosts are maintained in the Library List, which is searched starting at the highest priority until the requested function is found or the end of the list is reached. Function hosts are called using a message-passing protocol similar to that used for commands and may be used as gateways for remote procedure calls to other machines in a network.
The final search step is to check for an external ARexx program file by sending an invocation message to the ARexx resident process. The search always begins in the current directory and follows the same search path as the original ARexx program invocation. The name matching process is not case-sensitive.
The function name-matching procedure may be case-sensitive for some of the search steps, but not for others. The matching procedure used in a function library r function host is design dependent. Functions defined with mixed-case names must be called using a string token, since symbol names are always translated to uppercase.
The full search order is followed whenever the function name is defined by a symbol token. However, the search for internal functions is bypassed if the name is specified by a string token. This allows internal functions to usurp the names of external functions, as in the following example:
CENTER: /*internal "CENTER"*/
ARG string, length /*get arguments*/
length = MIN(length,60) /*compute length*/
return `CENTER' (string, length)
Here the built-in function CENTER() has been replaced by an internal function after modifying the length argument.
