% experimental functional programming feature... Use 'es' for 'is'...
%
% as a side effect 'S es T' will unify S with the result of evaluating parts of T
%
% Declare functions and macros using 'function(f(X), Expr):-condition'
:-op(700, xfx, es).
:-op(700, xfx, are).
% are is a special case for vectors:
[] are [].
[X|R] are [Y|S] :-!, X es Y, R are S.

% The following are commented out until tested.
% X es Y:-macro(Y,E), !, X=E.
% X es Y:-macro(Y,E), !, X es E.

% Can not evaluate an uninstantiated variable.
X es Y:-var(Y),!, fail.
% If it is an atomic constant then copy it.
X es X:-atomic(X),!.
% X es +Y:-!,X es Y.
function(+X,X).
% X es -Y:- !,MY es Y, X is -MY.
function(-X,Y):- Y is -Y.
% X es Y+Z:-!,VY es Y, VZ es Z, X is VY+VZ.
% X es Y*Z:-!,VY es Y, VZ es Z, X is VY*VZ.
% X es Y-Z:-!,VY es Y, VZ es Z, X is VY-VZ.
% X es Y/Z:-!,VY es Y, VZ es Z, X is VY/VZ.
% X es Y mod Z:-!,VY es Y, VZ es Z, X is VY mod VZ.
% The next two define the value of a vector as a vector of values
% and vectors of vectors!
% They are needed to handle the general case
X es []:-!,X=[].
[X|RX] es [Y|RY]:-!,X es Y, RX es RY.

% This is the general case - Y = F(Args).
X es Y  :-Y=..[F|Args],
	 ArgVals es Args,
	 FY=..[F|ArgVals],
	 (function(FY,E),!,X es E ;  X = FY).
% The last alternative leaves functors (undeclared functions) unevaluated.
% while evaluating any arguments that can be evaluated.

:- write(''), nl.
:- write('X es Y loaded. Define functions like this: function(f(...), nl,e).').
doit:-prompt(_, 'Expression=? '), repeat, read(Term), Val es Term,
	prin('      Value='),  write(Val), nl.
:- write('Input 'doit?'... end each expression with a period.'), nl.

examples('function(sq(X), X*X).').
examples('function(factorial(0), 1).').
examples('function(factorial(N), N*factorial(N-1)).').
function:-print('Define a function by inputting 'function(Form, Value).''),
	print('or   'function(Form, Expression):-ConditionsEtc.'').