Boolean And

rascal-0.40.17

Synopsis

Boolean and operator.

Syntax

Exp₁ && Exp₂

Types

//

Exp₁	Exp₂	Exp₁ && Exp₂
bool	bool	bool

Description

The and operator on Boolean values defined as follows:

Exp₁	Exp₂	Exp₁ && Exp₂
true	true	true
true	false	false
false	true	false
false	false	false

Boolean operators have short circuit semantics: only those operands are evaluated that are needed to compute the result. In the case of the && operator, the result is false if Exp₁ evaluates to false, otherwise Exp₂ is evaluated to determine the result.

Note that && backtracks over its argument expressions until it can find an evaluation that yields true unless there is none. This may happen if the left or right expression is a non-deterministic pattern match or a value generator.

Variable assignments as a result of matching or generator expressions under a && are visible outside the context of the operator, but only if the context is conditional, such as an if-then-else or a for loop. Note that if one of the argument expressions evaluates to false, then no binding is done either.

Examples

rascal>true && false;
bool: false
rascal>i <- [1,2,3] && (i % 2 == 0)
bool: true
rascal>import IO;
ok
rascal>if (i <- [1,2,3] && (i % 2 == 0))
>>>>>>>  println("<i> % 2 == 0");
2 % 2 == 0
ok
rascal>for (i <- [1,2,3,4] && (i % 2 == 0)) 
>>>>>>>  println("<i> % 2 == 0");
2 % 2 == 0
4 % 2 == 0
list[void]: []

Benefits

• The backtracking && allows one to express searching for a computational solution in concise manner.

Pitfalls

• Side effects to global variables or IO in the context of a backtracking && can lead to more effects than you bargained for.

rascal>import IO;
ok
rascal>int i = 0;
int: 0
rascal>bool incr() { i += 1; return true; }
bool (): function(|prompt:///|(0,36,<1,0>,<1,36>))
rascal>for (int j <- [1,2,3] && incr() && (i % 2 == 0)) 
>>>>>>>  println("once true for <j>");
once true for 2
list[void]: []
rascal>i;
int: 3