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

Ben Gamari ben at smart-cactus.org
Sat Jan 21 19:00:47 UTC 2023

Tom Ellis <tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> 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?


- Ben
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