Infix: now works on Ranges in Rakudo

Originally published on 28 November 2008 by Patrick Michaud.

Moritz found a bug in the infix: operator -- it wouldn't properly zip over a range:

09:04 <moritz_> rakudo: say (<a b c> Z 1..10).raku
09:05 <p6eval> rakudo 33300: OUTPUT[`elements` not implemented in class 'Range'␤current
               instr.: 'infix:Z' pc 4039 (src/gen_builtins.pir:2555)␤]

Normally we would expect this to return a list of lists:

(("a", 1), ("b", 2), ("c", 3))

The problem here was that infix: was trying to determine the size of each of its elements by using the 'elements' opcode and then get the individual elements by using a subscript. This failed for two reasons: (1) Ranges don't support postcircumfix:<[ ]>, and (2) Rakudo's implementation of Range didn't work for the 'elements' opcode.

The real approach, as with most things dealing with lists, is to use iterators. I changed the algorithm that it creates an iterator for each argument to infix:<Z>, and then walks those iterators in parallel to build the elements of the final result list. When any of the argument iterators is empty, we’re done building the list for infix:<Z>.

As a general rule, I suspect most list operators and builtin functions should not be counting the number of elements or using subscripts to access individual elements of a List. There are likely a few more functions like this in Rakudo that we should be replacing.