[Haskell-cafe] Modifying a product type generically

[Aleksey Khudyakov]

On 2 January 2015 at 07:02, Michael Orlitzky <michael at orlitzky.com> wrote:
> I've been playing with generic conversions between simple product types.
> For example, converting (Bar Int Int) back and forth between (Int,Int). See,
>
> https://www.haskell.org/pipermail/haskell-cafe/2014-December/117489.html
>
> These save me a *lot* of boilerplate. But naturally, I've gotten greedy.
> Another transformation I need to make takes one data type, and sticks a
> new field on the beginning of it. So I'd like to be able to take a,
>
>   Foo Int Int
>
> and construct a,
>
>   BigFoo String Int Int
>
> with as little code as possible. In my real problem, Foo may have 30
> fields, and BigFoo 31 -- if you know of a better way, stop me now.
>
Please stop. Some time ago I wrote library exactly for working with
product types: fixed-vector-hetero. Example code below. I think

> {-# LANGUAGE DeriveGeneric #-}
> import qualified Data.Vector.HFixed as H
> import GHC.Generics (Generic)
>
> data Foo = Foo Int Int
>            deriving (Show,Generic)
> data BigFoo = BigFoo String Int Int
>            deriving (Show,Generic)
>
> -- Instances are generated automatically using Generic. It's of course
> -- possible to write them manually
> instance H.HVector Foo
> instance H.HVector BigFoo

Now you can convert tuple to bar (or any other product types
which have same elemets)

*Main> H.convert (12,12) :: Foo
Foo 12 12
*Main> H.cons "A" (Foo 1 2) :: BigFoo
BigFoo "A" 1 2

Note. I've never tuned it for performance.