Re: [boost] [variant] Maintainer

Subject: Re: [boost] [variant] Maintainer
From: David Sankel (camior_at_[hidden])
Date: 2015-07-14 22:58:54

On Tue, Jul 14, 2015 at 7:01 AM, Larry Evans <cppljevans_at_[hidden]>
wrote:

>
> IOW, maybe:
>   variant<> is somewhat like the Zero
> and
>   tuple<> is somewhat like the One
> in that haskellforall page.
>
> Just thought it interesting, but I've no idea
> if Zero or One would be useful in c++.
> Anyone have any ideas about how they would be useful?
>

Heh, thanks for the fun observation Larry.

I think the extension of this principle is:

- given a binary function (say one of these is called 'foo')
- if that function forms a monoid with some empty element 'bar'
- then construct a syntax where
    - meaning( foo() ) = bar
    - meaning( foo( a, b ) ) = foo( meaning(a), meaning(b) )
    - meaning( foo( a₁, a₂, …, aₙ ) ) = foo( meaning(a₁), foo( meaning( a₂
), … meaning( aₙ ) ) … )

so if + and * have the normal arithmetic meanings and are prefix functions
(not operators), we would have

meaning( +( +(), 3 ) ) = 0 + 3
meaning( *( *(), 3 ) ) = 1 * 3

Of course we get the extension to binary type functions which are monoids.
These include the sum type (variant) and the product type (tuple). I wonder
which other common binary type functions are monoids.

Another interesting thing would be a free function 'append' which appends
strings together. That would imply that 'append()' is the empty string.

-- David