Request for opinion: a template while loop

[chaley]

Originally Posted by kovidgoyal

You could have both, in the for loop context make range special and everywhere else have it return a list. Reporting an error when limit is exceeded seems better to me.

If you do have range be a special function in the for context, you can statically determine when limit is excedded instead of checking on every iteration.

Done, using a context-sensitive range() function.

The grammar for for loops:

    for_expr        ::= for_list | for_range
    for_list        ::= 'for' identifier 'in' list_expr
                        [ 'separator' separator_expr ] ':' expression_list 'rof'
    for_range       ::= 'for' identifier 'in' range_expr ':' expression_list 'rof'
    range_expr      ::= 'range' '(' [ start_expr ',' ] stop_expr
                        [ ',' step_expr [ ',' limit_expr ] ] ')'
    list_expr       ::= top_expression
    separator_expr  ::= top_expression
    start_expr      ::= top_expression
    stop_expr       ::= top_expression
    step_expr       ::= top_expression
    limit_expr      ::= top_expression

In the context of a for loop the range() function uses the Python 3 range function, with the exception that the limit parameter is added to prevent (near-)infinite loops. Assuming that step is positive, generation stops when

current_value + step >= stop

The list is empty (no iteration) if start >= stop. No list is actually generated. In any other function call context it generates and returns the resulting list.

The example in post 10 is now expressed as

program:
	res = '';
	for i in range(strlen($title)):
	    c = substr($title, i, i+1);
	    res = strcat(res, if mod(i, 2) == 0 then uppercase(c) else c fi)
	rof

or using all the range() parameters:

program:
	res = '';
	for i in range(0, strlen($title), 1, 100):
	    c = substr($title, i, i+1);
	    res = strcat(res, if mod(i, 2) == 0 then uppercase(c) else c fi)
	rof

Range() documentation is:

range(start, stop, step, limit) -- returns a list of numbers generated by looping over the range specified by the parameters start, stop, and step, with a maximum length of limit. The first value produced is start. Subsequent values next_v = current_v + step. The loop continues while next_v < stop assuming step is positive, otherwise while next_v > stop. An empty list is produced if start fails the test: start >= stop if step is positive. The limit sets the maximum length of the list and has a default of 1000. The parameters start, step, and limit are optional. Calling range() with one argument specifies stop. Two arguments specify start and stop. Three arguments specify start, stop, and step. Four arguments specify start, stop, step and limit.

Examples:

range(5) -> '0, 1, 2, 3, 4'
range(0, 5) -> '0, 1, 2, 3, 4'
range(-1, 5) -> '-1, 0, 1, 2, 3, 4'
range(1, 5) -> '1, 2, 3, 4'
range(5, 0, -1) -> '5, 4, 3, 2, 1'
range(1, 5, 2) -> '1, 3'
range(1, 5, 2, 5) -> '1, 3'
range(1, 5, 2, 1) -> error(limit exceeded)