[Haskell-cafe] SML vs Haskell types

[Galaxy Being]

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 <tikhon at jelv.is> 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 <borgauf at gmail.com> 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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