Procedures

Abstract

Consider the task of processing a set of data consisting of a header and a sequence of N similar individual units. It might be generally described as

• Read Header;

• ProcessHeader;

• WriteHeader;

• FOR i ; = 1 TO N DO ReadUnit; ProcessUnit; Write(i); WriteUnit

• END

Clearly, the description of the original task has been made in terms of subtasks, emphasizing the dominant structure and supressing details. Of course, the subtasks ReadHeader, ProcessHeader, etc. must now be further described with all the necessary details. Instead of replacing these descriptive English words with elaborate Modula programs, we may consider these words as identifiers and define the details of the subtasks by textually separate pieces of program, called procedures (or subroutines); These definitions are called procedure declarations, because they define the actions of the procedure and give it a name. The identifiers in the main program referring to these declarations are said to be procedure calls, and their action is to invoke the procedure. Syntactically, the procedure call is a statement.