[erlang-questions] guard expression restriction

Robert Virding <>
Fri Dec 3 10:36:19 CET 2010


Yes, doing it that way was definitely a mistake. It seemed like a good idea at the time as you could use it to optimise the tests by converting some of them to guards. Unfortunately, we/I didn't think through the repurcussions enough before releasing it. And then it was too late.

I wonder what would happen if it was just silently changed? :-)

A general reflection is that one problem with trying to change something in Erlang/OTP is that not only does it have to work properly, it also has to fail properly. It is the failure which is the problem here.

Robert

----- "Richard O'Keefe" <> wrote:

> On 2/12/2010, at 7:22 AM, James Churchman wrote:
> 
> > Interestingly list comprehensions offer custom guards, if that it of
> use to you.
> 
> And what a disaster THAT is.
> Consider
> 
> 	1> 1+a > 2.
> 	** exception error: bad argument in an arithmetic expression
> 	      in operator +/2
>                  called as 1 + a
> 
> In a *guard*, this would be a silent failure.
> As an expression (which it should never have been allowed to be),
> exceptions are raised and propagated through > .
> 
> 	2> [X || X <- lists:seq(1,100), X+a > 2].
> 	[]
> 
> Here the exception became a silent failure.  So the guard is a guard.
> 
> 
> 	3> math:cos(foo) > 1.
> 	** exception error: bad argument
> 	     in function math:cos/1
> 	        called as math:cos(foo)
> 
> 	4> [X || X <- lists:seq(1,100), math:cos(foo) > X].
> 	** exception error: bad argument
> 	     in function math:cos/1
> 		called as math:cos(foo)
> 
> Here the exception did NOT become a silent failure!
> 
> If the guards in list comprehensions are GUARDS, then *any*
> exception inside one should turn into a silent failure, and
> query 4 should have the answer [] with no carping.
> 
> If the guards in list comprehensions are EXPRESSIONS, then
> every expression inside one should be propagated, and query
> 2 should have raised an exception.
> 
> It's even worse.  If guards in list comprehensions had been
> guards, we'd have been able to use Pattern = Expression to
> bind variables locally inside comprehensions, instead of
> having to use the kluge Pattern <- [Expression].
> 
> Inconsistent treatment of tests in comprehensions like this
> is a real menace.  I would value some kind of static check
> that the guards in comprehensions are proper guards.
> 
> 
> 
> 
> 
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