Re: [Haskell-cafe] SML vs Haskell types

So are you saying data Shish2 a = Holder2 a | Onion (Shish a) | Lamb (Shish a) | Tomato (Shish a) but then I'm having trouble with meal6 = (Onion (Tomato (Lamb (Holder2 (Fork))))) Couldn't match expected type `Shish a1' with actual type `Shish2 a0' * In the first argument of `Tomato', namely `(Lamb (Holder2 (Fork)))' In the first argument of `Onion', namely `(Tomato (Lamb (Holder2 (Fork))))' In the expression: (Onion (Tomato (Lamb (Holder2 (Fork))) On Fri, Mar 12, 2021 at 4:08 PM Tikhon Jelvis wrote:

Onion of 'a shish is equivalent to Onion (Shish a) in Haskell rather than Onion a (Shish a).

The latter version in Haskell creates a constructor with two arguments, something like Onion of ('a * 'a shish) would in SML. (Or, at least, OCaml—not 100% familiar with SML syntax myself!)

On Fri, Mar 12, 2021, 14:03 Galaxy Being wrote:

...

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

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