[Info-vax] puzzled by DCL symbol substitution

[Phillip Helbig---undress to reply]

In article
<7d997e21-b5be-488e-aa3d-e835519176b4 at z1g2000vbx.googlegroups.com>, AEF
<spamsink2001 at yahoo.com> writes: 

> It works because the apostrophes force ampersand substitution to take
> place.

That's obviously what is going on.  However, is it actually mentioned in 
the documentation?  I just fired up BOOKREADER and read the section on 
this in the 7.1 User Manual, but didn't see this mentioned.

> $  VERIFY_PROCEDURE = "''F$PARSE(F$ENVIRONMENT("PROCEDURE"),,,"NAME")'_VERIFY"
> 
> The second line is "magic", too. Quotation marks normally have to be
> doubled inside a quoted string, but this is the exception. The
> quotation marks around PROCEDURE and NAME must *not* be doubled. For
> some reason it's different when lexical functions are used in this
> manner.

Right.  This is another case, though, and is documented.

> Ampersand changes VERIFY_PROCEDURE to its value. Then the apostrophes
> change that result to *its* value. Then the resultant string is
> assigned to VERIFY_PROCEDURE.

Right.  Looking up ampersand, one reads that it is not evaluated inside 
quotation marks.  Presumably this should also say: unless it is inside 
quotes, in which case it is evaluated before the quotes are evaluated.

> As you can see in the second case, the symbol "A" gets its value
> before any subsequent code is executed.

Right.

> > Does anyone know who invented this?
> 
> The authors of DCL.

:-)

> No, this is not documented in the manuals, 

Why not?

> but that's how it works. On
> the other hand, isn't this just like the apostrophes forcing the
> lexical function to be evaluated in line 2 or your procedure? It is.
> It's the same thing. Lexical substitution is normally done during
> phase 3, but the apostrophes force it to be carried out in step 1.

The difference is that the latter is documented.