Re: EXTERNAL: [vhdl-200x] VHDL ternary operation

From: Jim Lewis <>
Date: Thu Mar 28 2013 - 11:33:26 PDT
We talked about something for 2008, however, it evolved from:
("pass" when fault else "fail")

But this was problematic due to conditional assignment.
Then we considered:
("pass" if fault else "fail")

This was ok, but was rejected since some wanted "less"
typing, and hence, proposed:
("pass" if fault, "fail")

This turned out to have some ambiguity problems and was
rejected.  We never did go back to considering "else".

For the next revision, I am hoping to have functions that
allow usage of anonymous types.  And hence, we could write
something like:

function cond (
   sel : boolean ;
   A   : anonymous ;
   B   : A'subtype
) return A'subtype is
   if Sel then
     return A ;
     return B ;
end function cond ;

Anonymous types would own a broad space of the overloading.
Hence, we could not write a function with the signature:
cond[boolean, std_logic, std_logic return std_logic]

but we could write a function with the signature:
cond[std_logic, anonymous, anonymous return anonymous]

For this particular issue we could of course also
consider language syntax.  However, I like general features
and subprograms as a little gives us alot of capability.

Best Regards,

> Daniel,
> /_"pass" when fault else "fail"_/is NOT an expression. It is the RHS of a conditional assignment statement. Such a RHS may contain expressions, but expressions may not contain a conditional assignment
> RHS.
> Since conditional assignments can be open-ended, in effect meaning that the assignment does not occur if no “when” expression evaluates as true, its use as an expression is not possible.
> To solve your immediate problem, you could write a function that takes a boolean argument and returns “pass” or “fail” strings, or you could create a constant array of two strings, indexed by boolean,
> e.g. (true => “fail”, false => “pass”). Then you could include the function call or constant in your report expression.
> Andy
> *From:* [] *On Behalf Of *Daniel Kho
> *Sent:* Thursday, March 28, 2013 4:19 AM
> *To:*
> *Subject:* EXTERNAL: [vhdl-200x] VHDL ternary operation
> Hi all,
> I'm wondering if the current revision supports something like the following?
> report to_hstring(index) & ", " & ("pass" when fault else "fail") & lf & nul;
> where "index" is an unsigned vector, and "fault" is a boolean.
> The expression ("pass" when fault else "fail") can already be used directly in clocked processes, but I'm not too sure if the standard allows this to be used within report statements.
> If there's a better way of writing which could produce the same effect, do share with me as well.
> regards, daniel
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Jim Lewis
Director of Training   
SynthWorks Design Inc. 

Expert VHDL training with a focus on hardware design and test.

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Received on Thu Mar 28 11:34:11 2013

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