Advanced Programming Guide

104 • Chapter 3: Programming with Modules> if nargs 2 then> error "real and imaginary parts are required"> end if;> module()> description "a complex number";> local real_part, imag_part;> export re, im, abs, arg;>> real_part, imag_part := real, imag;> re := () -> real_part;> im := () -> imag_part;> abs := () -> sqrt( re()^2 + im()^2 );> arg := () -> arctan( im(), re() );> end module> end proc:To create the complex number 1 + i, use the constructor.> z := MakeComplex( 1, 1 );z := module()local real _part, imag_part;export re, im, abs, arg;description “a complex number”;end moduleThe procedure MakeComplex is a constructor for complex number objects.The value returned by the procedure is the instantiation of themodule whose definition appears in the body of MakeComplex.The local state of the complex number is represented by the localvariables of the module, real_part and imag_part. The behavior is representedby the exported procedures re, im, abs, and arg.The exports of a module that represents an object are sometimesviewed also as messages. Objects respond to these messages by exhibitingthe behavior that the messages elicit.> z:-re(), z:-im();1, 1> z:-abs();√2> z:-arg();