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[prolog/en] Corrected statement about unifying two free terms (#3033)

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* Corrected statement about unifying two free terms

While the intricacies of unification would bring us too far, stating that assigning two free 'sides' is wrong. I tried to give a small description about how this works (without going into the details of occurrence checks or unification of more complex structures).

* Fixed indentation

* Replaced old style of structured comments
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Evert Heylen 2018-02-28 12:41:09 +01:00 committed by Pratik Karki
parent 7d303e5042
commit eefc0a9c92
1 changed files with 62 additions and 54 deletions
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prolog.html.markdown
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@ -66,7 +66,10 @@ magicNumber(42).
% follows:
% If both sides are bound (ie, defined), check equality.
% If one side is free (ie, undefined), assign to match the other side.
% If both sides are free, abort because this can't be resolved.
% If both sides are free, the assignment is remembered. With some luck,
% one of the two sides will eventually be bound, but this isn't
% necessary.
%
% The = sign in Prolog represents unification, so:
?- 2 = 3. % False - equality test
@ -86,6 +89,10 @@ magicNumber(42).
?- 5 is X+2. % Error. Unlike =, the right hand side of IS
% must always be bound, thus guaranteeing
% no attempt to solve an equation.
?- X = Y, X = 2, Z is Y + 3. % X = Y, Y = 2, Z = 5.
% X = Y are both free, so Prolog remembers
% it. Therefore assigning X will also
% assign Y.
% Any unification, and thus any predicate in Prolog, can either:
% Succeed (return True) without changing anything,
@ -230,9 +237,9 @@ nearby3(X,Y) :- nearby2(X,Y).
% Here is the structured comment declaration for nearby3:
%% nearby3(+X:Int, +Y:Int) is semideterministic.
%% nearby3(+X:Int, -Y:Int) is multi.
%% nearby3(-X:Int, +Y:Int) is multi.
%! nearby3(+X:Int, +Y:Int) is semideterministic.
%! nearby3(+X:Int, -Y:Int) is multi.
%! nearby3(-X:Int, +Y:Int) is multi.
% For each variable we list a type. The + or - before the variable name
% indicates if the parameter is bound (+) or free (-). The word after
@ -267,8 +274,8 @@ character(darthVader). % Creates atom value darthVader
% Note that below, writeln is used instead of print because print is
% intended for debugging.
%% countTo(+X:Int) is deterministic.
%% countUpTo(+Value:Int, +Limit:Int) is deterministic.
%! countTo(+X:Int) is deterministic.
%! countUpTo(+Value:Int, +Limit:Int) is deterministic.
countTo(X) :- countUpTo(1,X).
countUpTo(Value, Limit) :- Value = Limit, writeln(Value), !.
countUpTo(Value, Limit) :- Value \= Limit, writeln(Value),
@ -281,7 +288,7 @@ countUpTo(Value, Limit) :- Value \= Limit, writeln(Value),
% IF test. If Value = Limit fails the second declaration is run.
% There is also a more elegant syntax.
%% countUpTo2(+Value:Int, +Limit:Int) is deterministic.
%! countUpTo2(+Value:Int, +Limit:Int) is deterministic.
countUpTo2(Value, Limit) :- writeln(Value),
Value = Limit -> true ; (
NextValue is Value+1,
@ -294,14 +301,15 @@ countUpTo2(Value, Limit) :- writeln(Value),
% called a "failure-driven loop" to do this, but newer ones use a higher
% order function.
%% countTo2(+X:Int) is deterministic.
%! countTo2(+X:Int) is deterministic.
countTo2(X) :- forall(between(1,X,Y),writeln(Y)).
?- countTo2(10). % Outputs 1 to 10
% Lists are given in square brackets. Use memberchk to check membership.
% A group is safe if it doesn't include Joker or does include Batman.
%% safe(Group:list(atom)) is deterministic.
%! safe(Group:list(atom)) is deterministic.
safe(Group) :- memberchk(joker, Group) -> memberchk(batman, Group) ; true.
?- safe([robin]). % True