Re: [Haskell-beginners] Recursion with a self-defined type

So because Onion Shishkebab in the type definition is technically a data constructor *function, *the (shkb == Onion) in the code is comparing function-to-function, i.e., won't work. Thanks. The light finally went on. On Fri, Mar 12, 2021 at 6:28 PM Matthew Low wrote:

Pattern matches in Haskell are based on matching specific data constructors, with underscores `_` as a "match anything" mechanism. So one way to achieve something like what you want is

veggieKebab :: Shishkebab -> Bool veggieKebab Skewer = True veggieKebab (Onion (shk)) = veggieKebab shk veggieKebab (Tomato (shk)) = veggieKebab shk veggieKebab _ = False

This works because the matches are considered in top-to-bottom order, so the last case only matches if all the others fail to.

I'm not sure if it helps to build insight or not, but if you look at the the types of your data constructors in GHCI, you get, for example:

λ> :t Onion Onion :: Shishkebab -> Shishkebab

So even if you could pattern match as you wanted (veggieKebab (shkb (sk)) | (shkb == Onion)), you'd still be stuck with the problem of trying to compare two functions for equality, which isn't easy (and not something Haskell lets you do for arbitrary functions). You could get close to what you originally wrote by using a few more helper functions:

startsWithOnion :: Shishkebab -> Bool startsWithOnion (Onion _) = True startsWithOnion _ = False

startsWithTomato :: Shishkebab -> Bool startsWithTomato (Tomato _) = True startsWithTomato _ = False

restOfKebab :: Shishkebab -> Shishkebab restOfKebab Skewer = Skewer restOfKebab (Onion rst) = rst restOfKebab (Tomato rst) = rst restOfKebab (Lamb rst) = rst

veggieKebab :: Shishkebab -> Bool veggieKebab Skewer = True veggieKebab kebab | startsWithOnion kebab || startsWithTomato kebab = veggieKebab (restOfKebab kebab) | otherwise = False

On Fri, Mar 12, 2021 at 9:19 AM Galaxy Being wrote:

...

I'm trying to translate *The Little MLer *into Haskell. I've got this

data Shishkebab = Skewer | Onion Shishkebab | Lamb Shishkebab | Tomato Shishkebab deriving Show

Then I have this which works

veggieKebab :: Shishkebab -> Bool veggieKebab Skewer = True veggieKebab (Onion (shk)) = veggieKebab shk veggieKebab (Tomato (shk)) = veggieKebab shk veggieKebab (Lamb (shk)) = False

...

veggieKebab (Tomato (Onion (Tomato (Onion Skewer)))) True

but I'm wondering if I could do something like this

veggieKebab :: Shishkebab -> Bool veggieKebab Skewer = True veggieKebab (shkb (sk)) | (shkb == Onion) || (shkb == Tomato) = veggieKebab sk | otherwise = False

This doesn't work, giving a "Parse error in pattern: shkb". I've been advised that I'm trying to treat what is a data constructor like a variable, but I can't fathom what that means in this case. What I'm trying to leverage is what I've learned from dealing with lists and recursion through the consed list. So if effect I'm trying to recurse through a consed Shishkebab object. It works in the first case, but hyow could I do this in this more generic way like the second try does?

LB

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