Extensible Records?

[rabipelais]

I think it would be nice to be able to do something like this:

type Person = [r| {name :: String, age :: Int} |]
type HappyPerson = [r| {name :: String, age :: Int, favouriteFood :: String} |]

saruman :: Person
saruman = [r| {name = "Saruman", age = 2019} |]

happy :: Person -> HappyPerson
happy p = p <+> f
  where
    f ::  [r| {favouriteFood :: String} |]
    f = [r| {favouriteFood = "Lemon Pie"} |]

I think it could be done with a type class

class Extensible a b c where
   (<+>) :: a -> b -> c

which could be instantiated with Template Haskell (and that would be an ungodly amount of instances...). But perhaps there is some hairy type extension that could help?

A possible problem would be the ordering of the fields.

Do you know how the Elm people do it?

[nikita-volkov]

A possible problem would be the ordering of the fields.

Yes, and unfortunately I just can't imagine a solution to this, which does not involve making this a special case in the compiler.

[aavogt]

With ghc >= 7.8 you have GHC.TypeLits.CmpSymbol to sort. https://gist.github.com/aavogt/035d8e6374e50e400035 seems to work

[nikita-volkov]

Seems very intriguing. I'll check it out later. Thanks.

[aavogt]

I've cleaned up/commented that file a bit and put it in https://github.com/aavogt/record/blob/append/library/Record/Append.hs