{- I am experimenting with type-level naturals and with moving values known at runtime to type space. Is there a type signature that I can add to f1 below so that this will compile (maybe also requiring some additional LANGUAGE pragmas)? If I were to change the return value of f1 from "x" to "show x", then the program does compile, suggesting that there's nothing inherently wrong with "x". But I'd prefer the option to compose the output of f1 with other computations separately, e.g., "show . f1" . Thanks, Ken -} {-# LANGUAGE KindSignatures, ScopedTypeVariables, DataKinds, Rank2Types #-} import qualified GHC.TypeLits as TypeLits; import Data.Proxy(Proxy); main = undefined; data Foo (fn :: TypeLits.Nat) = Mkfoo deriving (Show); --f1 :: TypeLits.SomeNat -> ? ; f1 mynat = case mynat of { TypeLits.SomeNat (_ :: Proxy n) -> let { x :: Foo n; x = Mkfoo; } in x }; {- typelits.hs:27:8: error: * Couldn't match expected type `p' with actual type `Foo n' because type variable `n' would escape its scope This (rigid, skolem) type variable is bound by a pattern with constructor: TypeLits.SomeNat :: forall (n :: TypeLits.Nat). TypeLits.KnownNat n => Proxy n -> TypeLits.SomeNat, in a case alternative at smalltypelits2.hs:24:3-33 * In the expression: x In the expression: let x :: Foo n x = Mkfoo in x In a case alternative: TypeLits.SomeNat (_ :: Proxy n) -> let x :: Foo n x = Mkfoo in x * Relevant bindings include x :: Foo n (bound at smalltypelits2.hs:26:5) f1 :: TypeLits.SomeNat -> p (bound at smalltypelits2.hs:23:1) | 27 | } in x | ^ -}