[Haskell-cafe] Prevent a type parameter from sharing some of its parameters with another parameter

[Li-yao Xia]

Hi Timotej,

What should happen if you capture the variable with an existential type:

data E a = forall s. E (STRef s a)

I guess there's not much we can do with this type anyway...

Taking a complementary approach, you can write a typeclass of types that 
are allowed as keys. Since s is used like in ST, it must not unify with 
any concrete type, and there will be no instance of Allowed to be found.

class Allowed a
instance Allowed a => Allowed [a]
instance (Allowed a, Allowed b) => Allowed (Either a b)

If you can use PolyKinds and UndecidableInstances, some boilerplate can 
be avoided.

-- Instance to traverse composite types.
instance (Allowed f, Allowed a) => Allowed (f a)

-- Every type constructor you can think of.
instance Allowed Int
instance Allowed []
instance Allowed Maybe
instance Allowed Either
-- etc

Li-yao

On 08/21/2017 06:52 AM, Timotej Tomandl wrote:
> As the title says.
>
> Motivation:
> in a mutable code I want to keep some parameters strictly immutable, i.e.
> not sharing s with the whole mutable type
>
> I think this is best illustrated by 2 examples:
> 1. abstract example:
> imagine I have a type
> X a b c
> and another type
> Y a b
> and I want to prevent any case where the first parameter of X "a" is shared
> with it's third parameter "c" i.e. X a b (Y a a), X a b (Y a b), X a b (Y b
> a), X a b (Y a d) etc., any nestings in Y referencing a etc.
> but allow
> X a b (Y b b), X a b (Y b c)
> and any other combination not ruled out above
> 2. concrete example:
> imagine I have a type:
> Item s element key
> where s is used in the same way as in STRef s a, i.e. it cannot leak
> outside of a certain context, but I want the key to be immutable i.e.
> independent of s under any circumstences, so I can't write a type like:
> Item s element (STRef s refType)
> this is equivalent to disallowing X a b (Y a d) in 1.
>
> Is there any way to write a restriction like this in Haskell?
>
> --
>    Timo
>
>
>
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