Request for comments: new template language operations

[chaley]

I am adding "standard" binary and unary logical and arithmetic operators to the template language. The question: "what is the precedence of the operators?"

In line with other languages, my own preferences, and the characteristics of the existing language; the current choice from highest to lowest (binding first to last) is:

• 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 (-). Although I can't see why it matters, 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 exist). 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.

[capink]

Those change are most welcome. I leave the discussion about the order of precedence for people who know more about the subject than I do. It is fine with me so long as I can explicitly set the order using parenthesis.

[davidfor]

Yes please! Those will be great additions.

The precedence looks good to me. It matches most languages I have used and I can't see anything missing.

[chaley]

The changes are in calibre master source. It would be very useful if people who run from source try it and let me know about problems, and in particular any backwards compatibility issues.