Andre W B Furtado wrote:

I found the following text when visiting the Clean (a functional language) site:

"Clean is the only functional language in the world which has a special type system, uniqueness typing. It enables to update function arguments destructively retaining the purity of the language."

I believe Mercury borrowed their uniqueness type (mode) system from Clean. But since Mercury is functional/logical then I guess its still true that Clean is the only functional language with a uniqueness type system.

Then I have some questions: 1. Can anyone explain what does "update function arguments destructively" mean?

Uniquely typed values are guaranteed to be referenced at most once under program evaluation, these values can be modified in-place. As an example, suppose I have an array and I want to modify an element. In general I need to create a whole new copy of the array in order to make the modification since there may be other places in the program that require access to the old array. However, if I know that the old array has no other references (because the array has a unique type) then I do not need to make a copy, I can just modify the original array. This has obvious efficiency benefits.

2. If this is a desirable thing, why does Haskell not implement it?

- Uniqueness types, of course, require replacing the type system, this is a non-trivial task, - Destructive updates can already be accomplished with compiler supported libraries using monads, - Uniqueness types invalidate some program transformations. On the other hand I think uniqueness types are quite useful. Particularly as they allow much more flexible interaction between mutable data-structures than monadic approaches. I don't know of any concrete reasons a uniqueness type system couldn't be added to Haskell. This seems like a fine time to insert a plug for my Master's thesis, which describes a denotational semantics of uniqueness types: http://www.cpsc.ucalgary.ca/~danaha/uniqueness-types.ps Dana