ES4 draft: assert expression

Lars Hansen lhansen at
Tue Apr 15 05:40:40 PDT 2008

> -----Original Message-----
> From: Mark S. Miller [mailto:erights at] 
> Sent: 14. april 2008 18:11
> To: Lars Hansen
> Cc: es4-discuss at
> Subject: Re: ES4 draft: assert expression
> 2008/4/11 Lars Hansen <lhansen at>:
> > We talked about debugging information and an assertion form on  
> > es4-discuss a while ago, in a thread about the Error object.  As a  
> > result of that discussion, I'm sending out a draft for an "assert"
> >  expression form.
> Hi Lars,
> I missed the earlier discussion. From the draft spec you 
> attached, it seems like the first argument to assert is 
> always evaluated, its value is always checked, and if false, 
> an error always thrown. While I like this better than the 
> behavior normally associated with the name "assert", it is 
> different, so I'm wondering if a different name is called for?
> In C, C++, Java, and IIRC Eiffel, the assert expressions (and 
> in Eiffel, the pre and post conditions as well) are not 
> supposed to be part of the meaning of the program. If P is a 
> correct program in any of these languages, then erasing P's 
> assert statements (or turning them off through other means) 
> should be correctness preserving. E and Caja have similar but 
> mandatory checking functions, named "require"
> and "enforce" respectively. A correct program can validly 
> depend on their arguments being executed, and can depend on 
> these function throwing when the argument evaluates to false.
> Can a correct ES4 program depend of the assert expression to 
> do its thing? If so, then we should consider these 
> expressions to be part of the meaning of the program, not 
> just descriptions of what the program's meaning should be. If 
> so, "assert" is a confusing name.

Since there is no way to turn off asserts the program can
(correctly) come to depend on the evaluation of the assertion.
And I think your point is therefore a good one, though
I don't know if it outweighs the benefits of (re)using a
common name for this functionality.

Of course, in C and C++ programs and I assume Java there is no
guarantee that the program does not depend on the evaluation
of the assert expression; I see bugs like that from time
to time.

(The inability to turn off asserts may in itself be a weakness,
so maybe that's where the problem lies.  But it is unclear
how we would handle that problem in practice.)


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