[Haskell-cafe] Massaging data from the `extensible` package

[Branimir Maksimovic]

write generic function then map that function.
Greets, branimir

> On 04.10.2021., at 15:53, Ruben Astudillo <ruben.astud at gmail.com> wrote:
> 
> Hello cafe
> 
> I have been enjoying the `extensible` package [1] for extensible records and
> variants, kudos to the author. I have had a problem of not understanding how
> to "massage" data from one representation to another. To exemplify, suppose
> I have the following two extensible records
> 
>    {-# language OverloadedLabels #-}
> 
>    import Data.Extensible
>    import Control.Lens
> 
>    type Ext1 = Record '[ "field1" >: String, "field2" >: Integer ]
>    type Ext2 = Record '[ "field1" >: Text, "field2" >: Double ]
> 
> and I want to map some piece of data between these two types. Currently I am
> projecting each field individually like this
> 
>    ex1 :: Ext1
>    ex1 = #field1 @= "hello" <: #field2 @= 2.71 <: nil
> 
>    currentMap :: Ext1 -> Ext2
>    currentMap s =
>      #field1 @= T.pack (view #field1 s)
>        <: #field2 @= fromInteger (view #field2 s)
>        <: nil
> 
> For data types with few fields it is not so bad. But I have to do this
> transformation of data types generated from TH, I am dealing with a few
> dozens. The transformations are always from `String -> Text` and `Integer ->
> Double`. I would like to write a high lever combinator that grabs an
> extensible record, transforms each `String` field on a `Text` one and does
> the same with `Integer -> Double`.
> 
> Most of the combinators for transforming the data deal with natural
> transformations, such as hmap, hsequence and the like. The closest one to
> what I want is `hfoldMapFor` where I have to define a class with instances
> for all the types the extensible record has, but it doesn't let me change
> the type of the result. Does anyone have an idea on how to solve this?
> 
> Thanks for your time.
> 
> [1]: https://hackage.haskell.org/package/extensible
> 
> -- 
> Rubén. (pgp: 4EE9 28F7 932E F4AD)
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