Functions

A function is a procedure which returns a value. We have already seen a number of built-in Darwin functions in Chapter : op, union, member, subset, length, mod amongst others. The difference between procedures and functions in Darwin is entirely conceptual. The exact same syntax is used for defining both and, actually, procedures do return a value. Recall our procedure SetAverage from §.

> SetAverage := proc( s : set )
>   local total;
>   description 'This procedure takes a set as an argument and
>                calculates the average value in the set';
>   total := 0:
>   for i from 1 to length(s) do
>     total := total + s[i];
>   od;
>   lprint('The average is ', total/length(s));
> end:

The procedure SetAverage returns the value NULL.

> x := SetAverage({1,2,3}):
> if (x=NULL) then print('It is NULL'); fi;
It is NULL

Let us re-write SetAverage to be a function.

> NewSetAverage := proc( s : set(real) )
>   local total;
>   description 'This procedure takes a set of real values as an 
>                argument and calculates the average value in the set';
>   total := 0:
>   for i from 1 to length(s) do
>     total := total + s[i];
>   od;
>   ave := total/length(s);
>   ave;
> end:

Now the last statement executed when NewSetAverage is invoked is the bottom-most (trivial) statement ave;. The result of this expression is simply the value assigned to this variable. We say that the value of the entire function NewSetAverage is assigned the value of ave and it is this value which Darwin returns to the point where the function was called.

> x := NewSetAverage({3, 5}):            # the value of function new_SetAverage 
> print(x);                              #   is assigned to variable x
4