Brent Yorgey wrote:

This email is literate Haskell. I'm struggling to come up with the right way to add a fixpoint constructor to an expression language described by a type-indexed GADT (details below).

but since Haskell doesn't have type-level lambdas, I don't see how to make that work.

John Reynolds showed long ago that any higher-order language can be encoded in first-order. We witness this every day: higher-order language like Haskell is encoded in first-order language (machine code). The trick is just to add a layer of interpretive overhead -- I mean, a layer of interpretation. The closure conversion on type level was shown in http://okmij.org/ftp/Computation/lambda-calc.html#haskell-type-level Here's how a similar technique applies to the problem at hand. There is an alternative solution: encode higher-order functors by means of SKI combinators. Somehow I prefer the pointed solution. The complete code follows. {-# LANGUAGE TypeFamilies, KindSignatures, GADTs, FlexibleInstances #-} data U :: (* -> *) -> * where Unit :: U None Var :: U Id (:+:) :: U f -> U g -> U (Sum f g) (:*:) :: U f -> U g -> U (Prod f g) Mu :: HOFunctor f => f -> U (MU f) data None a = None deriving Show data Id a = Id a deriving Show data Sum f g a = Inl (f a) | Inr (g a) deriving Show data Prod f g a = Prod (f a) (g a) deriving Show newtype MU f a = MU (Res f (MU f) a) type family Res f self :: * -> * type instance Res List self = Sum None (Prod Id self) data List = List -- the code for the HO functor class HOFunctor f where fn :: f -> U g -> U (Res f g) instance HOFunctor List where fn _ self = Unit :+: (Var :*: self) enumShapes :: Int -> U f -> [f ()] enumShapes 0 Unit = [None] enumShapes _ Unit = [] enumShapes 1 Var = [Id ()] enumShapes _ Var = [] enumShapes n (f :+: g) = map Inl (enumShapes n f) ++ map Inr (enumShapes n g) enumShapes n (f :*: g) = [ Prod x y | x <- enumShapes n f, y <- enumShapes n g] enumShapes n self@(Mu f) = map MU $ enumShapes (n-1) (fn f self) test0 = enumShapes 0 (Unit :+: (Var :*: Var)) -- [Inl None] test1 = enumShapes 1 (Unit :+: (Var :*: Var)) -- [Inr (Prod (Id ()) (Id ()))] test2 = enumShapes 1 (Mu List) -- [Inl None] test3 = enumShapes 2 (Mu List) -- [Inr (Prod (Id ()) Inl None)] instance Show (MU List ()) where show (MU x) = show x