Re: [Haskell-cafe] Why can't I bind unlifted values at the top level?

Unboxed things that don't contain pointers probably don't need to be garbage collected, right? On Sat, Jan 21, 2023, 2:01 PM Ben Gamari wrote:

Tom Ellis writes:

...

I can't bind unlifted values at the top level:

foo = (##)

Top-level bindings for unlifted types aren't allowed: foo = (##)

I can understand why I shouldn't be able to bind unlifted *expressions* at the top level

foo = complicatedExpression

Perhaps complicatedExpression doesn't terminate! But why can't I bind _values_, i.e. things that are already evaluated? Given that (##) and proxy# already exist at the top-level it seems reasonable that I should be allowed to define my own!

There are a few things going on here. The case you give here is not just unlifted but also unboxed (namely, being an unboxed tuple). It's hard to see how top-level binding of unboxed values would work operationally (e.g. how is the garbage collector to know how to trace this object, given that unboxed objects have no object header?).

However, I suspect you do have a point when it comes to unlifted data constructors. I think it would be fine to allow an application of a data constructor of an unlifted type on the top-level:

type UMaybe :: Type -> UnliftedType data UMaybe a = UNothing | UJust a

x :: UMaybe Int x = UJust 42

Perhaps you could open a ticket for this?

Cheers,

- Ben _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.