On 12/03/2012 14:22, Edward Kmett wrote:

On Mon, Mar 12, 2012 at 6:45 AM, Simon Marlow <marlowsd@gmail.com
<mailto:marlowsd@gmail.com>> wrote:
But I can only pass unboxed types to foreign prim.

Is this an intrinsic limitation or just an artifact of the use cases
that have presented themselves to date?

It's an intrinsic limitation - the I# box is handled entirely at the
Haskell level, primitives only deal with primitive types.

Ah. I was reasoning by comparison to atomicModifyMutVar#, which deals
with unboxed polymorphic types, and even lies with a too general return
type. Though the result there is returned in an unboxed tuple, the
argument is passed unboxed.

Is that implemented specially?

I'm a little bit confused.

atomicModifyMutVar#
:: MutVar# s a -> (a -> b) -> State# s -> (# State# s, c #)

Is the "unboxed polymorphic type" you're referring to the "MutVar# s a"? Perhaps the confusion is around the term "unboxed" - we normally say that MutVar# is "unlifted" (no _|_), but it is not "unboxed" because its representation is a pointer to a heap object.

I was talking about the (a -> b). I used it because the extraction of 'c' rather than a proper pair was closest to my situation. A less confused example might be newArray# which accepts a polymorphic 'a'.

The problem is, that can't be done reliably. For dataToTag# the compiler automatically inserts the seq just before code generation if it can't prove that the argument is already evaluated, I think we would want to do the same thing for unsafeField#.

Fair enough.

Thanks again.

-Edward