[chaley]

28 Feb 2021 (in calibre source): Addition of common logical and arithmetic operators

The template language now supports the common logical operators ('&&, '||', and '!), and arithmetic operators (unary '+', unary '-', and binary '+', '-', '*', and '/').

The operator precedence in the template language is now:

• Function calls, constants, parenthesized expressions, statement expressions, assignment expressions. Remember that in the template language, 'if', 'for', and assignment return a value.
• Unary plus (+) and minus (-). These operators evaluate right to left. These and the other arithmetic operators return integers if the expression doesn't produce a fractional part.
• Multiply (*) and divide (/). These operators are associative and evaluate left to right. Use parentheses if you want to change the order of evaluation.
• Add (+) and subtract (-). These operators are associative and evaluate left to right.
• Numeric and string comparisons (these already existed). These operators return '1' if the comparison is True, otherwise ''. Comparisons are not associative: a < b < c produces a syntax error. Comparisons return '1' if True and '' if False.
• Unary logical not (!). This operator returns '1' if the expression is False (evaluates to the empty string), otherwise ''.
• Logical and (&&). This operator returns '1' if both the left-hand and right-hand expressions are True, the empty string '' if either is False, is associative, evaluates left to right, and does short-circuiting.
  
  Spoiler:

  Regarding short-circuiting: for example this program produces the answer '4'. Because of short-circuiting the right-hand expression, the assignment, is evaluated because the left-hand expression is True. The assignment is done.
  

  program:
  	a = 5;
  	'a' && (a = 4);
  	a

  This program produces '5' because the the left-hand expression is False so because of short-circuiting the right-hand expression is not evaluated. The assignment is not done.
  

  program:
  	a = 5;
  	'' && (a = 4);
  	a

• Logical or (||). This operator returns '1' if either the left-hand or right-hand expression is True, '' if both are False, is associative, evaluates left to right, and does short-circuiting. It does an inclusive or, returning '1' if both the left- and right-hand expressions are True.