On 09/01/2012 04:46, wren ng thornton wrote:
On 12/23/11 8:34 AM, Simon Peyton-Jones wrote:
More uniform! If you the singleton-unboxed-tuple data constructor in source code, as a function, you'd write (\x -> (# x #)). In a pattern, or applied, you'd write (# x #).
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 #) Cheers, Simon
I know that a putative (T) would be different from T because it would introduce an additional bottom, but I don't think that would apply to the unboxed case. Or is there something in the semantics of unboxed tuples that I'm missing?