Re: [Groff] pic syntax blemishes

[Brian Kernighan]

The number of (g)pic users in the world must be pretty small
by this point, though I am among them, thanks to gpic et al.

Oddly the sequence
        d = 3
        r = 2d
does draw an error message from gpic.

On Wed, 26 Apr 2017, Doug McIlroy wrote:

> This pic fragment
>        d = 3
>        circle radius 2d
> draws a circle of radius 2, not 6 as one might suppose. It is parsed as
>        circle radius 2 d
> and the "irrelevant attribute" d is ignored.
>
> In another context
>        d = 3
>        move 2d
> the parsing results in a move of 2 then of d. Thus 2d is sometimes
> taken as 2 and sometimes as 2+d, but never as 2*d.
>
> Yet another confusion arises here:
>        nd = 3
>        move 2 nd
>        move 2nd
> The first move behaves like move 2+nd, but the second (pun not intended)
> is an error, because the pic tokenizer recognizes 2nd (also 2st and 2th)
> as an ordinal.
>
> The treatment of irrelevant attributes also allows crazy code like
>        box radius 3
>
> The convention of dropping irrelevant attributes may have been justified
> during the experimental days of pic's development, but seems cheesy now,
> nearly 40 years on.
>
> The fact that 2d gets tokenized differently from 2nd is also disconcerting.
>
> I suggest that irrelevant attributes and constructions like 2d should
> be errors. Any previously working code that such a tightening of syntax
> might reject will be easy to fix.
>
> What do folks think about this issue? Depending on response, I may try to
> do something about it.
>
> Doug