[boost] [assign v2] thoughts

Subject: [boost] [assign v2] thoughts
From: Gordon Woodhull (gordon_at_[hidden])
Date: 2011-07-08 22:13:01

Hi Erwann, all,

I had a thought about this library which I wasn't able to articulate during the review, and I don't think anyone else raised it.

I think this library would be a useful utility if it had the right syntax, but I found issue especially with the use of operator| which is IMO not consistent with the way Boost.Range uses it.

I would expect the pipe operator to just stream data and not affect the object on the LHS.  I know everything is relative and in some EDSLs this might be appropriate, but it's a strain for me to think of pipe as anything other than unidirectional data flow.

The syntax that you have now

cal | do_csv_put<2>("jan", 31, "feb", 28, "mar", 31)

looks kind of like Smalltalk or Objective C to me, like you are trying to send some arbitrary message to the container on the LHS.  

Did you consider keeping the overloaded operator+= from Assign v1, or another assignment operator that always modifies the LHS?  

cal += values<2>("jan", 31, "feb", 28, "mar", 31)

is much clearer to me.  And the assignment operators are expected to return a reference to the LHS.  

Yes, there is an ambiguity with other libraries which have meant += to mean "add to each element" but with the rare exception of containers containing elements of their own type, this should always be resolvable.  If this is unacceptable, I think operator|=, meaning union-assign, also makes some sense.

Or am I missing something?

I always thought of Assign v1 as "the funny operator, library" and on at least one occasion didn't use it because I knew I'd have to explain what it's doing to my colleagues.  

So Assign v2 looks very powerful and useful to me, but in order to become a workhorse for everyone, it has to have the most blunt obvious syntax so that no one has to guess or explain anything.

Cheers,
Gordon