Prolog has an amazing syntax, thanks to it both being homoiconic and supporting custom operators. However, writing conditionals in Prolog is often a drag.
Fortunately, we don't have to convince or wait for Prolog implementations to implement a nicer syntax. Instead, we can create our own if-else syntax today!
Example Usage
%% %% Before %% foo(X) :- ( X > 50 -> Y is X * 2, bar(X, blah) ; baz(10, 20) ), last(X). %% %% After %% foo(X) :- if (X > 50) then ( Y is X * 2, bar(X, blah) ) else ( baz(10, 20) ), last(X). %% %% Before %% shorter_foo(X) :- (X > 10 -> bar(X); baz(X)). %% %% After %% shorter_foo(X) :- if X > 10 then bar(X) else baz(X). Example Usage (or operator)
I also threw in a bonus or operator so you can do "or" logic without introducing additional choice points, and not have to worry about catching multiple terms (due to it having a stronger precedence than the comma , operator):
%% %% Before %% multi_foo(X, Y) :- (foo(X), !; bar(Y)). %% %% After %% multi_foo(X) :- foo(X) or bar(Y). %% %% Before %% multi_foo(X, Y) :- ((foo(X), !; bar(X)) -> Y = true ; Y = false ). %% %% After %% multi_foo(X) :- if (foo(X) or bar(X)) then ( Y = true ) else ( Y = false ). The Code
Here's the code you can copy/paste into your own project to make use of this sweet, sweet syntax.
There may even be a better way using term_expansion. If you find one, leave it in the comments!
:- op(990, fx, if). :- op(985, yfx, then). :- op(985, yfx, else). :- op(985, yfx, elseif). :- op(800, xfx, or). if(X) :- (if_then(X, Then) -> call(Then); true). if_then(then(elseif(Try, Cond), MaybeThen), Then) :- !, (if_then(Try, Then) -> true; call(Cond), Then = MaybeThen). if_then(then(Cond, Then), Then) :- !, call(Cond). if_then(else(Try, MaybeThen), Then) :- !, (if_then(Try, Then) -> true; Then = MaybeThen). or(X,Y) :- call(X) -> true; call(Y). 
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