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

[Timotej Tomandl]

Hi,

after thinking about it a bit, I think it might be necessary to provide
more context, so I opened an issue in my toy project this relates to:

https://github.com/formrre/soft-heap-haskell/issues/5

--
 Timo

On Mon, Aug 21, 2017 at 5:52 AM, Timotej Tomandl <tomandltimotej at gmail.com>
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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