Template if -- then -- else

[chaley]

I am considering implementing classic if-then-else clauses in general program mode of the template language. They would look something like this, borrowed from Algol 68.

if func_or_var then
    series of statements;
[else
    series of statements;]
fi

(The else part is optional.) The test would be an expression in the current template language that returns either an empty value or not. One can imagine using field(...), first_non_empty(...), and/or(), and strcmp, but all the functions will be available. Only one of the "then" or "else" parts will be evaluated. I wouldn't try hard to make it work in template program mode.

This isn't trivial to implement and might break something. For example, having variables named "if" will probably break things. The question is "Will people use it?" Thanks

[davidfor]

"If you build it, they will come."

I hate the current conditionals. There is something about "test" that means I just don't read it properly.

Of course, I'm expecting one of the first questions about using "if..then" to have code that looks like:

program:
if test(field('series'), field('series'), 'nonseries') then
   dir_name= field('series');
else
   dir_name= 'nonseries'
fi

And wonder why they never get "nonseries" as part of the result.

Of course it wouldn't be me. I'd never write something like that in other languages and then spend an embarrassing amount of time trying to work out what was wrong. No, not me, never.

[ownedbycats]

I suspect that this might be a little easier to figure out than the current conditionals. (I just can't seem to wrap my brain around them.)

[chaley]

In progress.

The template language will support if ... then ... else ... fi as an expression returning a value. This means that the following two fragments are equivalent.

program:
	if field('#mybool') then
		f = 'series'
	else
		f = 'authors'
	fi;
	field(f)

program:
	a = field(if field('#mybool') then 'series' else 'authors' fi);

Nested ifs will be supported. The else clause is optional.

In addition, I added 2 infix comparison operators, == and !=, so the following fragment will work, assigning the result of the 'if' to the variable 'q'. And yes, I know that the answer will be 'bar aa'.

program:
	q =
		if 'a' == 'b' then
			'foo'
		else
			'bar'
		fi;
	strcat(q, ' aa')

The operators compare strings. If you want to compare numbers you will still need to use the cmp() function. You will also need to use strcmp() if you want < or >.

I am not going to add infix booleans (and/or) because that would require adding precedence and parenthesized expressions. The functions are easy enough to use.

One problem is that the parser is rather stupid so error messages might not be helpful. For example, forgetting a 'fi' on an embedded if might indicate the error at the end of the outermost if. Fixing this would require building a real parser, which at the moment I don't want to do.

[chaley]

Originally Posted by chaley

The operators compare strings. If you want to compare numbers you will still need to use the cmp() function. You will also need to use strcmp() if you want < or >.

While improving performance I found I could add all the relational operators without a performance penalty. I also could add numeric relational ops, using slightly different operators. What will be there:

• String (case insensitive): ==, !=, <, <=, >, >=
  String comparisons of numbers will use lexical ordering. For example, 11 < 2 is True.
• Numeric: ==#, !=#, <#, <=#, >#, >=#
  Numeric comparison returns False if either left or right is non-numeric. Specifically, the empty string is not a number. For example, '' <# 1 is False

[chaley]

Known incompatibilities in the "improved" program mode language:

• The words if, then, else, and fi are now reserved. If you have variables or custom functions with these names then the template won't compile.
• In rare cases the old parser accepted expressions of the form
  

  'a' 'b'

  These are now always errors.

[chaley]

The improvements are now in calibre's source code and will be in the next calibre 5 beta.

Please let me know if something that used to work is broken, or if the new features do unexpected things.