Failure to Decode Expression with extended Builtin Function

[julmue]

I'm trying to extend the Dhall language with an equality operation and then decode functions using the extended interface to haskell functions. Unfortunately that doesn't work.

For the implementation I follow the section Extending the language of the official tutorial.

naturalEqualType :: Dhall.Expr s a
naturalEqualType = Dhall.Pi Nothing "_" Dhall.Natural (Dhall.Pi Nothing "_" Dhall.Natural Dhall.Bool)

startingContext :: Dhall.Context.Context (Dhall.Expr s a)
startingContext = transform Dhall.Context.empty
  where
    transform :: Dhall.Context.Context (Dhall.Expr s a) -> Dhall.Context.Context (Dhall.Expr s a)
    transform =
      Dhall.Context.insert "Natural/equal" naturalEqualType

normalizer :: Dhall.Expr s1 a1 -> Maybe (Dhall.Expr s2 a2)
normalizer (Dhall.App (Dhall.App (Dhall.Var "Natural/equal") (Dhall.NaturalLit x)) (Dhall.NaturalLit y)) = Just (Dhall.BoolLit (x == y))
normalizer _ = Nothing

inputSettings :: Dhall.InputSettings
inputSettings = transform Dhall.defaultInputSettings
  where
    transform =
        set Dhall.normalizer      (Just (Dhall.ReifiedNormalizer (pure . normalizer)))
      . set Dhall.startingContext startingContext

input :: FromDhall a => Text -> IO a
input = Dhall.inputWithSettings inputSettings Dhall.auto

It works if the function is fully applied within the Dhall expression:

testNaturalEqSource1 :: Text
testNaturalEqSource1 = [NeatInterpolation.text|
let x = Natural/equal 1 1
in x : Bool

testNaturalEq1 :: Bool
testNaturalEq1 = unsafePerformIO $ input testNaturalEqSource1
|]

Executing testNaturalSource1 in ghci returns True as expected.

It fails if I try to decode the function without applying it to arguments:

testNaturalEqSource2 :: Text
testNaturalEqSource2 = [NeatInterpolation.text|
let x : Natural -> Natural -> Bool = \ (n1 : Natural) -> \ (n2 : Natural) -> Natural/equal n1 n2
in x
|]

testNaturalEq2 :: Natural -> Natural -> Bool
testNaturalEq2 = unsafePerformIO $ input testNaturalEqSource2

Running it in ghci results in an error

>>> testNaturalEq2 1 1

 *** Exception: FromDhall: You cannot decode a function if it does not have the correct type
CallStack (from HasCallStack):
  error, called at src/Dhall/Marshal/Decode.hs:1134:23 in dhall-1.41.2-DSv9eZfnsTVGwdH0GPi336:Dhall.Marshal.Decod

Decoding functions works for "real" builtins like Natural/subtract with no problem. Am I missing something?

Many thanks in advance

[julmue]

Works using the functionWith combinator.