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Re: Assertions in Mathematica?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg113468] Re: Assertions in Mathematica?
  • From: Charles Gillingham <cgillingham1 at me.com>
  • Date: Sat, 30 Oct 2010 04:36:30 -0400 (EDT)

Use:

Attributes[Assert] = {HoldAll};
Assert[exp_, msg_] := If[Not[exp], Message[Unevaluated[msg]]; Abort[];]

You also might want to consider adding arguments for some messages,  
e.g.:

Assert[exp_, msg_, args___] := If[Not[exp],  
Message[Unevaluated[msg],args]; Abort[];]

And you might also want to consider what happens if "exp" does not  
evaluate and remains in symbolic form, e.g.:

Assert[exp_, msg_,args___] := If[exp =!= True,  
Message[Unevaluated[msg],args]; Abort[];]


>> What's the best to implement assertions in Mathematica?  By  
>> assertions
>> I mean statements like assert(exp) in C, which generate an error
>> if exp evaluates to false.
>>
>> This *should* be trivial, but it's Mathematica, so...
>>
>> Naively, I tried defining:
>>
>> Assert[exp_, msg__] := If[!exp, Message[msg]; Abort[]]
>>
>> ...which, of course, failed to work (as I've learned to expect);
>> instead it produced a cryptic error "Message::name : Message name
>> ... is not of the form symbol::name or symbol::name::language."
>>
>> I tried many other things, but after wasting 1 hour on this
>> ridiculously trivial programming task, I'm reduced to begging for
>> help.  (This, by the way, is always the way it is with me and
>> Mathematica, and I've been using it on-and-off for almost 20 years.
>> The documentation is as useless to me today as it was 20 years ago.
>> I find it as horrible as the rest of Mathematica is brilliant.)
>>
>> I've posted desperate questions over programming mind-numbing
>> trivialities like this one in Mathematica before, i.e. questions
>> that seem so elementary and fundamental that no one who has access
>> to the documentation and who can read should *ever* have to ask
>> them.  I ask them less wanting to get the answer to the questions
>> themselves than hoping to learn how I could have answered such
>> questions by myself.  But I've never found how to do this.  Those
>> who know the answers *already* can give them to me if they feel so
>> inclined.  (And how they got to know the answer to begin with, I
>> don't know; I imagine it took years of sustained Mathematica
>> programming.  Or maybe they asked a similar question before to
>> someone who already knew the answer...)  But no one has been able
>> to tell me how someone who *doesn't* know the answers to such  
>> questions
>> already can figure it out without outside help.
>>
>> But hope springs eternal!  If someone is kind enough to tell me
>> how I could implement my Assert, I'd be most grateful.  If someone
>> can tell me how I could have arrived at this answer by myself by
>> consulting the documentation, I'd be ecstatic.
>>
>> TIA,
>>
>> kj


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