Re: [Haskell-cafe] How to match on such type

Done it by implementing own `Eqn` class and all instances: class Eqn a where eqn :: a -> a -> Int -- ^ returns weight auto-deriving `Eq` instances help to make exact comparison, `eqn` - with weight. === Best regards, Paul

On 5 December 2017 at 21:47, Baa wrote:

...

Hello, All!

I have type:

infixr 9 ||| data a ||| b = A a|B b deriving (Eq, Data, Show)

and usually I wraps into it values which are instances of my class IsTag:

class IsTag a where anyTag :: a

So, I need to check if some value wrapping by `a|||b` is equal to `anyTag`, i.e.:

A (B (A x)) == (anyTag::TypeOf_x) ==> True

Do you:

a) know whether it should be wrapped as A or B?

b) how many layers down it is?

One solution is to use something like (untested):

-- | Is type @a@ equivalent to @anyTag :: x@ ? class EquivToTag a x where tagEquiv :: a -> Proxy x -> Bool

instance {-# OVERLAPPABLE #-} EquivToTag x x where tagEquiv _ _ = True

instance {-# OVERLAPPABLE #-} (EquivToTag a x) => EquivToTag (a ||| b) x where tagEquiv _ _ = True

instance {-# OVERLAPPABLE #-} (EquivToTag b x) => EquivToTag (a ||| b) x where tagEquiv _ _ = True

... Except there's no way of having False here, and the two ||| instances can't really both be there (which to pick?).

I'm not sure what the intent of this is, but would it make more sense to use a type family which resolves to a Constraint?

...

This function must be generic, ie, it can not know anything about concreate TypeOf_x, only: `a|||b` and `IsTag`. How to do it???

I added `Data` to `a|||b` and even to values which I "wrap" with `a|||b` (I assumed to use `gmap*` and Co), but this does not help me. Is it even possible??

== Best regadrs, Paul

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