Re: .ie as target of .if

[John Gardner]

No, it isn't valid. What .if and .ie do is read one line, then decide
whether it gets processed or skipped, depending on the condition. The real
magic is in the \{ sequence, which is the only thing in the Roff language
resembling a "multi-line" statement.

Troff, in a manner befitting a text processor, processes its input one line
at a time. This is an important thing to wrap your head around, because it
demystifies a lot of the syntax's "weirdness". Here's a dummy program to
illustrate how \{ … \} get interpreted the same as everything else: once
they're reached:

.de BREAK
' \}
' ig }
..
.
.de IF
' if \\$1 \{'
..
.
.nf
.IF 1 {
1. Foo
2. Bar
.BREAK
3. Baz
4. Qux
.}
End


Unless you remove the .BREAK line, the output will be:

1. Foo
2. Bar
END



On Tue, 1 Sep 2020 at 21:37, Tadziu Hoffmann <hoffmann@usm.uni-muenchen.de>
wrote:

>
> > .if COND1 .ie COND2 xxx
> > .          el yyy
>
> > If this is considered working as designed, however, the
> > documentation ought to mention the restriction.  It currently
> > implies the opposite, by saying the part after the condition
> > in an .if request "is interpreted as though it was on a line
> > by itself."  Were the .ie in fact on a line by itself, groff
> > wouldn't grumble about the .el.
>
> I think it is working as intended.  "ie" pushes the result
> of the conditional test onto a stack ("if" does not), to be
> popped by a future "el".  So if COND1 is true, that stack
> contains the test result of COND2, which the "el" can use.
> If COND1 is false, the "ie" is never executed, and the stack
> remains empty, so "el" complains.
>
> Note that if there had been another "ie" before your program
> fragment, then your "el" would be consuming its result.
> So you can create some very interesting code this way.
>
>
> The documentation should say that it "is interpreted as
> though it was on a line by itself IF THE CONDITION IS TRUE".
> If not, it is never executed (except perhaps macro expansions
> that take place while the line is being read).
>
>
>
>