dhall-to-{json,yaml,yaml-ng}: support for recursive types

[WillSewell]

Support recursive types as described in https://docs.dhall-lang.org/howtos/How-to-translate-recursive-code-to-Dhall.html.

dhall-to-{json,yaml,yaml-ng} already has special case support for the recursive JSON type:

dhall-haskell/dhall-json/src/Dhall/JSON.hs

Lines 503 to 589 in 4d3bb47

	Core.Lam _ (Core.functionBindingAnnotation -> Core.Const Core.Type)
	(Core.Lam _ (Core.functionBindingAnnotation ->
	(Core.Record
	[ ("array" , Core.recordFieldValue -> Core.Pi _ _ (Core.App Core.List (V 0)) (V 1))
	, ("bool" , Core.recordFieldValue -> Core.Pi _ _ Core.Bool (V 1))
	, ("null" , Core.recordFieldValue -> V 0)
	, ("number", Core.recordFieldValue -> Core.Pi _ _ Core.Double (V 1))
	, ("object", Core.recordFieldValue ->
	Core.Pi _ _ (Core.App Core.List (Core.Record
	[ ("mapKey", Core.recordFieldValue -> Core.Text)
	, ("mapValue", Core.recordFieldValue -> V 0)])) (V 1))
	, ("string", Core.recordFieldValue -> Core.Pi _ _ Core.Text (V 1))
	]
	))
	value
	) -> do
	let outer (Core.Field (V 0) (FA "null")) = return Aeson.Null
	outer (Core.App (Core.Field (V 0) (FA "bool")) (Core.BoolLit b)) =
	return (Aeson.Bool b)
	outer (Core.App (Core.Field (V 0) (FA "array")) (Core.ListLit _ xs)) = do
	ys <- traverse outer (Foldable.toList xs)
	return (Aeson.Array (Vector.fromList ys))
	outer (Core.App (Core.Field (V 0) (FA "object")) (Core.ListLit _ xs)) = do
	let inner (Core.RecordLit
	[ ("mapKey", Core.recordFieldValue -> Core.TextLit (Core.Chunks [] mapKey))
	, ("mapValue", Core.recordFieldValue -> mapExpression)]) = do
	mapValue <- outer mapExpression
	return (mapKey, mapValue)
	inner _ = Left (Unsupported e)
	ys <- traverse inner (Foldable.toList xs)
	return (Aeson.Object (HashMap.fromList ys))
	outer (Core.App (Core.Field (V 0) (FA "number")) (Core.DoubleLit (DhallDouble n))) =
	return (Aeson.toJSON n)
	outer (Core.App (Core.Field (V 0) (FA "string")) (Core.TextLit (Core.Chunks [] text))) =
	return (toJSON text)
	outer _ = Left (Unsupported e)
	outer value
	Core.Lam _ (Core.functionBindingAnnotation -> Core.Const Core.Type)
	(Core.Lam _ (Core.functionBindingAnnotation ->
	(Core.Record
	[ ("array" , Core.recordFieldValue -> Core.Pi _ _ (Core.App Core.List (V 0)) (V 1))
	, ("bool" , Core.recordFieldValue -> Core.Pi _ _ Core.Bool (V 1))
	, ("double", Core.recordFieldValue -> Core.Pi _ _ Core.Double (V 1))
	, ("integer", Core.recordFieldValue -> Core.Pi _ _ Core.Integer (V 1))
	, ("null" , Core.recordFieldValue -> V 0)
	, ("object", Core.recordFieldValue ->
	Core.Pi _ _ (Core.App Core.List (Core.Record
	[ ("mapKey", Core.recordFieldValue -> Core.Text)
	, ("mapValue", Core.recordFieldValue -> V 0)
	])) (V 1))
	, ("string", Core.recordFieldValue -> Core.Pi _ _ Core.Text (V 1))
	]
	))
	value
	) -> do
	let outer (Core.Field (V 0) (FA "null")) =
	return Aeson.Null
	outer (Core.App (Core.Field (V 0) (FA "bool")) (Core.BoolLit b)) =
	return (Aeson.Bool b)
	outer (Core.App (Core.Field (V 0) (FA "array")) (Core.ListLit _ xs)) = do
	ys <- traverse outer (Foldable.toList xs)
	return (Aeson.Array (Vector.fromList ys))
	outer (Core.App (Core.Field (V 0) (FA "object")) (Core.ListLit _ xs)) = do
	let inner (Core.RecordLit
	[ ("mapKey", Core.recordFieldValue -> Core.TextLit (Core.Chunks [] mapKey))
	, ("mapValue", Core.recordFieldValue -> mapExpression)]) = do
	mapValue <- outer mapExpression
	return (mapKey, mapValue)
	inner _ = Left (Unsupported e)
	ys <- traverse inner (Foldable.toList xs)
	return (Aeson.Object (HashMap.fromList ys))
	outer (Core.App (Core.Field (V 0) (FA "double")) (Core.DoubleLit (DhallDouble n))) =
	return (Aeson.toJSON n)
	outer (Core.App (Core.Field (V 0) (FA "integer")) (Core.IntegerLit n)) =
	return (Aeson.toJSON n)
	outer (Core.App (Core.Field (V 0) (FA "string")) (Core.TextLit (Core.Chunks [] text))) =
	return (toJSON text)
	outer _ = Left (Unsupported e)

In dhall-lang/dhall-kubernetes#111 (comment), @Gabriel439 suggested adding support for arbitrary recursive types. I thought it was better to create a separate issue to track this because the limitation is more general than that particular instance of the issue.

I would like to help with this, but to be honest I'm not sure how successful I would be given my current Dhall experience (understanding how to model recursive types was tricky enough!). However if anyone could give any hints/pointers I could give it a go.