Hello, This is my first post here, and it's an odd one, I'll admit. Basically, I'm trying to translate the material in *The Little MLer *to Haskell, the TLMLer being an excellent types workout. So in SML I have this datatype 'a shish = Bottom of 'a | Onion of 'a shish | Lamb of 'a shish | Tomato of 'a shish and this datatype rod = Dagger | Fork | Sword and then this SML function fun is_veggie (Bottom (x)) = true | is_veggie (Onion (x)) = is_veggie (x) | is_veggie (Lamb (x)) = false | is_veggie (Tomato (x)) = is_veggie (x) which has no problem handling tis is_veggie (Onion(Tomato(Bottom(Dagger)))) Now, in Haskell I've translated this (with minor alterations) to data Shish a = Holder a | Onion a (Shish a) | Lamb a (Shish a) | Tomato a (Shish a) data Rod = Dagger | Fork | Sword However, in Haskell these two expressions are different things entirely meal4 = Tomato Dagger (Onion Fork (Lamb Spear (Holder Fork))) meal5 = (Tomato (Onion (Lamb (Holder Fork)))) Here's my attempt at handling meal4 with a Haskell isVeggie isVeggie (Holder (sh)) = True isVeggie (Onion sh (sk)) = isVeggie sk isVeggie (Tomato sh (sk)) = isVeggie sk isVeggie (Lamb sh (sk)) = False This works for meal4, but not for meal5. And yet in the SML world their is_veggie handles (Onion(Tomato(Bottom(Dagger)))) just fine. TLMLer says Onion (Tomato (Bottom (Dagger))) belongs to the type rod shish, while in Haskell Onion (Tomato (Holder (Dagger))) is a bizarre nested beast due to the fact that the data constructor variable of Onion is Tomato (Holder (Dagger)) etc. etc. Can a single Haskell version of isVeggie handle both meal4 and meal5? No? I thought so. But then how would a separate Haskell version of isVeggie handle meal5 -- or is it just too weird? Also, but not critical, how could the Haskell isVeggie be done with guards, i.e., just like a consed list is matched on (x:xs) in the recursion case? I can see that 1:2:3:[] and Onion (Tomato (Bottom (Dagger))) are both conses, but the latter I don't know how to break out into head and tail for a guard case where the individual food items were not mentioned explicitly. IOW, this doesn't work isVeggieKebab :: Shish -> Bool isVeggieKebab Holder (sk) = True isVeggieKebab (shkb (sk)) | (shkb == Onion) || (shkb == Tomato) = isVeggieKebab sk | otherwise = False I'm feeling some doom and gloom about this project. Right at the start this seems to be an insurmountable difference between SML and Haskell type systems. Or I simply don't understand something fundamental here. LB