Formal definition of I6's 'or' operator (not '||')

Authoring Inform 6
3

Yes, but that’s not much, unfortunately (only the brief description on pages 19 and 20), and DM4 is sometimes not the whole story. I haven’t read every scrap of every bit of documentation yet, so I hoped that someone might come forward with something from a source I don’t know. (Those seem to show up regularly!)

For the benefit of those looking for such a definition, it appears from experimentation that the or operator has two distinct modes, depending on the type of comparison operator used:

  • one for “positive-sense” comparison operators like ==, in, >, and <
  • one for “negative-sense” comparison operators like ~= and notin

It appears that the compiler treats a positive-sense comparison such as

x == 5 or 6

as the logical equivalent of

(x == 5) || (x == 6)

On the other hand, it also appears that the compiler treats a negative-sense comparison such as

x ~= 5 or 6

as the logical equivalent of

(~~(x == 5)) && (~~(x == 6))

So a possible formal definition might be:

Given a condition with structure

[expression E] [operator O] [value V1] or [value V2] (... or [value Vn])

When used in conjunction with positive-sense comparison operators, the or operator creates the equivalent of a logical OR between a series of conditions, each of which has a lefthand side of expression E, operator O, and a righthand side defined as one of the elements Vn in the or-ed list.

(E O V1) || (E O V2) || ... || (E O Vn)

When used in conjunction with negative-sense operators, the or operator creates the equivalent of a logical AND between a series of negated conditions, each of which has a lefthand side of expression E, the positive-sense complement of operator O (designated ~O), and a righthand side defined as one of the elements Vn in the or-ed list.

(~~(E ~O V1)) && (~~(E ~O V2)) && ... && (~~(E ~O Vn))

As noted elsewhere (see Next steps for Inform 6 compiler - #84 by zarf), this may cause confusion when using the >= or <= operators in conjunction with or, which are treated by the compiler (up to 6.35) as being negative-sense operators akin to ~< and ~>, respectively.

Please do feel free to point out any errors in the above and/or cite additional relevant sources.

2 Likes