On Wed, Mar 7, 2012 at 5:40 PM, Brandon Allbery
Data F = F Int
would give you something that could produce U(L), the U for the F constructor, the L for the contained Int.
Some experimentation suggests U is for unboxed. For example, module Test where f :: Int -> Int f x = x g :: Int -> Int g x = x + 1 gives this core Test.f :: Int -> Int Test.f = \ (x :: Int) -> x Test.g :: Int -> Int Test.g = \ (x :: Int) -> case x of _ { I# x# -> I# (+# x# 1) } and these strictness annotations f :: GHC.Types.Int -> GHC.Types.Int {- Arity: 1, HasNoCafRefs, Strictness: S, Unfolding: (\ x :: GHC.Types.Int -> x) -} g :: GHC.Types.Int -> GHC.Types.Int {- Arity: 1, HasNoCafRefs, Strictness: U(L)m, Unfolding: InlineRule (1, True, False) (\ x :: GHC.Types.Int -> case x of wild { GHC.Types.I# x1 -> GHC.Types.I# (GHC.Prim.+# x1 1) }) -} f is strict in its argument and so is g (U implies S.) The unboxed field is "lazy", but that's the annotation kind # things always get. I'm not sure but the trailing "m" in g's signature. Cheers, Johan