[Haskell-cafe] Pointwise Monoid with DerivingVia on n-tuples

[Matthew Pickering]

This question is answered in section 4.3 of the paper.

https://www.kosmikus.org/DerivingVia/deriving-via-paper.pdf

Cheers,

Matt

On Wed, Feb 6, 2019 at 1:40 PM Georgi Lyubenov <godzbanebane at gmail.com> wrote:
>
> Hi!
>
> I would like to use `DerivingVia` to do something like this:
> ```
> import Data.Semigroup
>
> data Four = Four Bool (Maybe Int) Bool Bool
>   deriving (Semigroup, Monoid) via (Any, Option (First Int), Any, Any)
> ```
>
> This, however, doesn't work, because `(Any, Option (First Int), Any, Any)` is not coercible to `Four`.
>
> Even if I do
> ```
> data Four a b c d = Four a b c d
>
> type Concrete = Four Bool (Maybe Int) Bool Bool
> ```
> `Concrete` is still not coercible to `(Any, Option (First Int), Any, Any)`.
>
> This seems like a very common thing you would want to do (for me at least it pops up often, to want to have a `Monoid` instance that is just a pointwise Monoid over it's arguments (exactly the way an n-tuple is a `Monoid`). But I don't seem to be finding an easy way to do this. I am aware of `generic-deriving` but `DerivingVia` seems like it should be able to handle something like this, with only `Coercible` holding it back (?).
>
> Thanks in advance,
>
> =======
> Georgi
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