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Conditional structures: Difference between revisions
→{{header|PureBasic}}: Adding Prolog.
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=={{header|Prolog}}==
A "pure" Prolog program by its very nature is one very long, very complicated boolean test. Absolutely every executable portion of Prolog is a test that succeeds or fails. Here are some examples, thus, of using conditionals in Prolog:
<lang Prolog>go :- write('Hello, World!'), nl.</lang>
While operationally this looks like a program that when go/0 is executed will print "Hello, World!" and exit, it is actually a predicate, in the strict logical sense of the term, that tests conditions. Denotationally we'd describe it as "go/0 succeeds iff write/1 succeeds with its passed-in argument, and if nl/0 subsequently succeeds." (The fact that write/1 and nl/0 **always** succeed and that we use them for their side effects only doesn't matter to the Prolog view of a program.)
<lang Prolog>fact(foo).
fact(bar).
fact(baz).
go :- fact(booger).
go :- fact(bar).</lang>
This example shows a few features of Prolog's testing and, specifically, shows nondeterminism and backtracking in action. In this we have a predicate fact/1 (so named because in this format, without an executable body, it is termed a "fact" in the literature). It has two clauses asserting both "bar" and "baz" as facts. go/0 also has two clauses. If we execute go/0, the runtime will tell us "true" (or, in some implementations, "yes") to indicate that the predicate call was successful. Denotationally we would say "fact(X) succeeds iff X unifies with foo, X unifies with bar, or X unifies with baz". We would also say "go/0 succeeds iff fact(booger) succeeds or if fact(bar) succeeds". When running, the first clause of go/0 will be executed and fact(booger) will be tested. fact(booger) does not match fact(bar) nor does it match fact(baz) so it fails. This leads the runtime to go back and try again with the **second** go/0 clause. In this one fact(bar) does, in fact, match fact(bar), so the overall test passes. A Prolog program is, thus, a very complicated tree of if/then statements, in effect.
<lang Prolog>fact(X) :-
( X = foo
; X = bar
; X = baz ).
go :-
( fact(booger)
; fact(bar) ).</lang>
This version is semantically the same as the previous one. (In actual execution, because of some runtime optimizations, there are some minor differences in outcome, but nothing that would change the logical interpretation of the program.) Here we're showing more explicitly the various "or" conditions. In Prolog "," is roughly equivalent to "and" (conjunction) while ";" is roughly equivalent to "or" (disjunction). Because of this, and because of the fact we've taken separate clauses now and put them into explicit disjunctions it is clearer that we're performing a series of if/then tests in effect.
That being said, Prolog does have something that's very akin to real if/then statements (or, more accurately, similar to the ternary operator of languages like C):
<lang Prolog>fact(X) :-
( X = bar -> write('You got me!'), nl
; write(X), write(' is not right!'), nl, fail ).
go :-
( fact(booger)
; fact(bar) ).</lang>
In this version of fact/1, the -> operator is used to perform a more traditional if/then/else. The general construct is ( condition -> succeed_branch ; fail_branch ). In this case if the parameter passed in unifies with 'bar', a message is written (recall that write/1 and nl/0 always succeed!) and the whole predicate exists with a success. If, on the other hand, the unification fails (you pass anything other than 'bar') it writes a snarky message and then calls fail/0, a predicate that, as its name suggests, always fails. There are more implications to using the conditional expression in Prolog; it is generally considered code smell. Other operators also exist for handling conditionals (like *->) that lack the "smell" of the conditional operator. The reasons for this are out of scope, however, for this article. Just know that the fact/1 predicate could have used *-> in place of -> and been more "sound" as a result.
=={{header|PureBasic}}==
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