Re: Adding logical implication to boost

From: Alexander Grund (alexander.grund_at_[hidden])
Date: 2025-01-07 16:20:46

Am 07.01.25 um 16:16 schrieb Rainer Deyke via Boost:
> On 06.01.25 19:43, Ivan Matek via Boost wrote: 
>> boost::implies(student.graduaded(), student.num_active_courses() == 0)
>> reads much much nicer than
>> !student.graduaded() || student.num_active_courses() == 0
>
> The problem here is that your example is closer to the English usage 
> of the word "implies" than to the usage in boolean logic that we are 
> talking about.  "A implies B" in English usually means that there is a 
> logical connection such that B is necessarily true if A is true.  For 
> example, a student who has graduated necessarily has no active courses.
>
> "A => B" merely means that either A is false or B is true.  For 
> example, "I am the president of the United States => there is world 
> peace" is trivially true because I am not the president of the United 
> States, even though if I would almost certainly be unable to achieve 
> world peace even if I were the president of the United States.  The 
> C++ expression "!A || B" is actually closer to the correct English 
> than "A => B" for expressing this condition.
Isn't that is exactly what was written above?
I.e. `A => B` == `boost::implies(A, B)` == `!A || B`

Or with the example: if( boost::implies(student.graduaded(), 
student.num_active_courses() == 0) ) /*...*/;`
In plain English: "If a graduated student  implies he has no active courses"

So I don't see how "!A || B" is closer to anything than "A => B" if they 
are the same thing.
What am I missing here? Or did you mistype anything?


Aside from that I don't actually see how that could be useful outside of 
asserts. I.e. `assert ( implies(this, that) )` makes sense and is readable.
But `if implies(this, that)` not so much. Because it sounds much more 
general than it is it might be rather confusing.
With your example `if( i_am_president() "implies" is_world_peace() )` is 
likely true right now, but if I became president at some point someone 
trusting this statement would likely be disappointed. Not that such 
things ever stopped any politician ;-)

It still boils down to the English language which doesn't match the 
logical interpretation which is likely what you intended to say because 
also when reading the code `student.graduaded()` might not magically 
ensure `num_active_courses() == 0` although it reads (in English terms) 
that way.

TLDR: I wouldn't want such a function or even operator as I don't see 
any reasonable uses outside of assertions.