Re: [boost] [outcome] expected, etc. why are these assignable

Subject: Re: [boost] [outcome] expected, etc. why are these assignable
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2017-06-02 15:53:14

2017-06-02 17:17 GMT+02:00 Robert Ramey via Boost <boost_at_[hidden]>:

> It just occurred to me that for a similar component I created some time
> ago for my own purposes, I (intentionally?) support of the assignment
> operator.  Copy construction was permitted however.  So the only way
> to use the component is:
>
> checked_result<int> = function(...);
>
> something
>
> void function(checked_result & cr, ....)  will be the same
>
> as
>
> void function(const checked_result & cr, ...)
>
> because checked_result only contains const functions.  That is, once
> constructed, it's immutable.
>
> Personally I never use an interface which passes a mutable type to a
> function.  To me it violates the mathematical idea of what a function is.
>
> Doesn't this simplify things?  Doesn't this eliminate the whole questions
> about about empty state, etc.  Doesn't this better model the "mondad"
> concept we're trying to leverage on.  And why the hell are we modifying and
> an object designed to model a result in the first place? It doesn't make
> logical sense in terms of we expect a "result" to be.  I plunk down my
> money (a function) and ask for a pepperoni pizza.  Either they say they
> don't have pepperoni and give me my money back or give me a pizza.  I don't
> think of modifying this result - it's an artifact of history.  And wouldn't
> making these things immutable, eliminate a large part of the whole
> controversy?
>
> Just a thought.  I think sometimes something is hard because we haven't
> thought enough about what we're really trying to do - what problem we're
> trying to solve.
>

The consumer of the `outcome` could just observe the state. But the
producer may have legitimate reasons to change the value of `outcome`
objects when preparing its final value.

Also, some use cases require being able to store outcomes in containers.

I guess design would be easier if we could see all use cases listed.

Regards,
&rzej;