# composite-dhall

`ToDhall` and `FromDhall` instances for
[composite](https://hackage.haskell.org/package/composite-base) records.

You lets you use extensible records over any base functor that is representable
in dhall.

You can parse normal records:

```{.dhall}
{ a = "foo"
, b = +5 }
```

using a `Record` like:

```{.haskell}
withLensesAndProxies
  [d|
    type A = "a" :-> Text

    type B = "b" :-> Int
    |]

type MyRec = Record '[A, B]
```

This also works for `Rec Maybe` and `Rec []`.

```{.dhall}
{ a = Just "foo"
, b = Just +5 }
```

```{.haskell}
type MyMaybeRec = Rec Maybe '[A, B]
```

Similarly you can use a contravariant functor as the base functor
to parse a collection of templates for different types.

```{.dhall}
{ a = \(x : Text) -> "Hello ${x}."
, b = \(y : Integer) -> "You are ${Integer/show y} years old."
}
```

```{.haskell}
newtype TextTemplate a = TextTemplate (Op Text a)
  deriving newtype (FromDhall)

newtype TextTemplates = TextTemplates {unTemplates :: Rec TextTemplate '[A, B]}
  deriving (FromDhall) via (Rec (Op Text) '[A, B])
```