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
_______________________________________________
Glasgow-haskell-users mailing list
Glasgow-haskell-users@haskell.org
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
