Name Rascal/Expressions/Values/Boolean/Equivalence

Synopsis The equivalence operator on Boolean values.

Syntax Exp1 <==> Exp2

Types

Exp1	Exp2	Exp1 <==> Exp2
bool	bool	bool

Description The equivalence operator on Boolean values defined as follows:

Exp1	Exp2	Exp1 <==> Exp2
true	true	true
true	false	false
false	true	false
false	false	true

Boolean operators have short circuit semantics: only those operands are evaluated that are needed to compute the result. However, in the case of the <==> operator both operands have to be evaluated to determine the result.

Note that the <==> operator backtracks over its arguments until it finds an evaluation that is true, unless there is none. Variable bindings that are the effect of matching operators in its arguments are not visible outside the scope of the <==>.

Examples

rascal>import IO;
ok
rascal>false <==> false;
bool: true
rascal>false <==> true;
bool: false

We should add a more meaningful example of backtracking over <==> than this old one: (i <- [1,2]) <==> (j <- [1,2,3]); for ((i <- [1,2]) <==> (j <- [1,2,3])) println("true!"); (i <- [1,2] && (i % 2 == 0)) <==> (j <- [1,2,3] && (j % 3 == 0)) for ((i <- [1,2] && (i % 2 == 0)) <==> (j <- [1,2,3] && (j % 3 == 0))) println("true!");

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