topic Re: HP Pascal SELECT and SELECTONE expressions in Operating System - OpenVMS

HP Pascal SELECT and SELECTONE expressions

Jeremy Begg — Tue, 06 Feb 2007 19:40:39 GMT

Hi,

I'm writing a Pascal program which uses the SELECT and SELECTONE statements introduced in HP Pascal V6. The only documentation I can find is the description in the Release Notes and there's something I need clarified.

I'm assuming that the data types of the 'select-selector' expression and the 'select-label-list' expressions have to be the same, or at least type-compatible. But do they have to be ordinal types, or can they be more complex? For example, can these expressions be character strings?

(Yes, I know I can do this by making the 'select-selector' the boolean constant TRUE and then having each 'select-label-list' consist of string comparisons producing TRUE or FALSE results.)

Thanks,

Jeremy Begg

Re: HP Pascal SELECT and SELECTONE expressions

Heinz W Genhart — Wed, 07 Feb 2007 03:22:56 GMT

Hi Jeremy

you are right. There is no documentation online (yet).
I would expect to find the documentation for Pascal V6 on the following page:
http://h71000.www7.hp.com/doc/pascal.html

regards

Heinz

Re: HP Pascal SELECT and SELECTONE expressions

John Gillings — Wed, 07 Feb 2007 20:20:45 GMT

Hi Jeremy,

I'd expect ordinal types, but then HP Pascal frequently surprises me in the way it pushes boundaries ;-)

I expect you'll get an authoritative answer from John Reagan soon...

Re: HP Pascal SELECT and SELECTONE expressions

John Reagan — Thu, 08 Feb 2007 09:56:39 GMT

Here's some background on how I arrived on SELECT/SELECTONE.

An outstanding wishlist item was to CASE on character strings. Something like:

CASE string-expression OF

'ABC': stmt1;
'DEF': stmt2;
END;

However, I would have to start dealing with errors like:

CASE string-expression OF

'ABC': stmt1;
'DEF': stmt2;
'ABC ': stmt3;
END:

While trying to figure out the best place to do that (the internal structures were not prepared for string values), I mumbled to myself "Gee if only CASE could just do the FIRST one it found, then I wouldn't have to do any checking at all. Just dump the 'problem' back on the user." (I'm such a nice guy, eh?)

While in the compiler looking at CASE, I saw that I use the BLISS SELECT statement (which only takes ordinal values) as:

SELECTONE True OF
SET
[CH$EQL(.Expr,'ABC')]: ...
[CH$EQL(.Expr,'DEF')]: ...
TES;

I said, "Ah, if I allow full run-time expressions and define an 'order of evaluation' for the SELECT parts, that will let me provide a 'CASE over strings' plus with SELECT vs SELECTONE, you can get even more power.

So off I went to re-implement BLISS' SELECT and SELECTONE statements into Pascal.

With that background, the SELECT selector must be an ordinal type. The way you 'CASE on strings' is:

SELECTONE True OF
String-expression = 'ABC': ...
String-expression = 'DEF': ...
END;

John

PS As for adding it to the documenation, we didn't update the docset for V6.0. The release notes is the only place for the information right now. I hope to update it next time as I have a few more customer requests I'd like to add. (spoiler alert: I've had several requests for the compiler to understand/generate 64bit descriptors for conformant array parameters.)

Re: HP Pascal SELECT and SELECTONE expressions

Jeremy Begg — Thu, 08 Feb 2007 20:58:01 GMT

Great explanation, John.

Thanks!