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Truthiness

In Jactl we often want to know whether an expression is true or not. The truthiness of an expression (a term stolen from Groovy) is used to determine which branch of an if statement to evaluate, or whether a for loop or a while loop should continue or not, for example. In any situation where a boolean true or false is expected we need to evaluate the given expression to determine whether it is true or not.

Obviously, if the expression is a simple boolean or boolean value then there is no issue with how to intepret the value: true is true, and false is false.

Other types of expressions can also be evalauted in a boolean context and return true or false.

The rules are:

  • false is false
  • 0 is false
  • null values are false
  • Empty list ([]) or empty map ([:]) is false
  • Empty String ('') is false
  • All other values are true

For example:

[] && true         // false
[[]] && true       // true
[:] && true        // false
[false] && true    // true
[a:1] && true      // true
'' && true         // false
'abc' && true      // true
1 && true          // true
0 && true          // false
null && true       // false