Re: Unit unboxed tuples

11 Jan 2012

      On 10/01/2012 16:18, Dan Doel wrote:
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Copying the list, sorry. I have a lot of trouble replying correctly
with gmail's interface for some reason. :)
On Tue, Jan 10, 2012 at 11:14 AM, Dan Doel  wrote:
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On Tue, Jan 10, 2012 at 5:01 AM, Simon Marlow  wrote:
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On 09/01/2012 04:46, wren ng thornton wrote:
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Shouldn't (# T #) be identical to T?
No, because (# T #) is unlifted, whereas T is lifted.  In operational terms,
a function that returns (# T #) does not evaluate the T before returning it,
but a function returning T does.  This is used in GHC for example to fetch a
value from an array without evaluating it, for example:
indexArray :: Array# e ->  Int# ->  (# e #)
I don't really understand this explanation. (# T #) being unlifted
would mean it's isomorphic to T under the correspondence e<->  (# e
#). _|_ = (# _|_ #) : (# T #), so this works.
Does the difference have to do with unboxed types? For instance:
foo :: () ->  Int#
    foo _ = foo ()
    bar :: () ->  (# Int# #)
    bar _ = (# foo () #)
baz = case bar () of _ ->  5  -- 5
    quux = case foo () of _ ->  5 -- non-termination
Because in that case, either (# Int# #) is lifted, or the Int# is
effectively lifted when inside the unboxed tuple. The latter is a bit
of an oddity.
Unboxed types cannot be lifted, so in fact bar compiles to this:

   bar = \_ -> case foo () of x -> (# x #)

and both baz and quux diverge.

It might help to understand (# T #) by translating it to (# T, () #). 
There's really no difference.

Cheers,
	Simon