Top-level bindings for unlifted types

[Simon Marlow]

Neil Mitchell wrote:

> The following program:
> 
> -------------------------------------------
> {-# OPTIONS_GHC -fglasgow-exts #-}
> module Test() where
> import GHC.Base
> test = realWorld#
> -----------------------------------------
> 
> gives the error message:
> 
>     Top-level bindings for unlifted types aren't allowed:
>         { test = realWorld# }
> 
> Changing to test _ = realWorld# works fine.
> 
> The question is why are these bindings disallowed? Reading the
> "Unboxed values as first class citizens" paper I can't see it listed
> as a restriction.

Let's consider unboxed values first.  They would have to be computed at 
compile-time, and that means the value of every top-level unlifted value 
needs to be visible in the interface file, for use in other modules. 
Cycles are disallowed, of course.  Top-level unboxed values would then 
behave just like #define constants, in fact.  This is certainly possible, 
it would just add complexity to the compiler in various places.

Alternatively you could compute them at load-time, but then you'd not only 
have to arrange to run the initialisers somehow, but also worry about 
ordering and cycles.  And then there's the issue that a top-level unboxed 
value would be represented by a pointer to the value rather than the value 
itself, as is the case with normal unboxed bindings.  This doesn't sound 
like a profitable direction.

Top-level unlifted/boxed values would be useful, for example

   x = case newMutVar# 0 realWorld# of (# s#, x# #) -> x#

eliminating a layer of indirection compared to the usual 
unsafePerformIO.newIORef.  These would also have to be computed at either 
compile-time or load-time, but there's no difficulty with the 
representation, because unlifted/boxed values are always represented by 
pointers anyway.  This is related to static arrays, which we don't have in 
GHC right now.  Conclusion: doable, but non-trivial.

realWorld# is a special case, but really falls into the unboxed category.

Cheers,
	Simon