Unit unboxed tuples

[wren ng thornton]

On 1/10/12 11:18 AM, Dan Doel wrote:
> On Tue, Jan 10, 2012 at 11:14 AM, Dan Doel<dan.doel at gmail.com>  wrote:
>> On Tue, Jan 10, 2012 at 5:01 AM, Simon Marlow<marlowsd at gmail.com>  wrote:
>>> On 09/01/2012 04:46, wren ng thornton wrote:
>>>> 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.

With my revised understanding we have the following family of types:

     T                 pointed,   lifted,   lazy
     (# T #)           pointed?,  unlifted, lazy
     !T                unpointed, lifted,   eager
     {-#UNPACK#-}!T    unpointed, unlifted, eager

where the two !T types are only specified in ADTs and through strictness 
analysis. Though this doesn't explain the difference in operational 
behavior for their use as return types.

-- 
Live well,
~wren