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