On 1/10/12 11:18 AM, Dan Doel wrote:
On Tue, Jan 10, 2012 at 11:14 AM, Dan Doel<dan.d...@gmail.com> wrote:
On Tue, Jan 10, 2012 at 5:01 AM, Simon Marlow<marlo...@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
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