Unit unboxed tuples

[Roman Leshchinskiy]

On 11/01/2012, at 19:28, Dan Doel wrote:

> Then I'm afraid I still don't understand the difference. Is it that
> case in core always evaluates? So:
> 
>    case undefined of x -> ...
> 
> blows up, while
> 
>    case (# undefined #) of (# x #) -> ...
> 
> does not?

Yes.

> Also, if so, how is (core-wise):
> 
>    foo :: ... -> (# T #)
>    case foo <v> of (# x #) -> ...
> 
> different from:
> 
>    foo :: ... -> T
>    let x = foo <v> in ...
> 
> Stack vs. heap allocation?

The second version binds x to a thunk that, when evaluated, calls foo <v> (which yields an evaluated T). The first one calls foo <v> and then binds x to whatever T (possibly unevaluated) it returns.

It really is exactly the same as:

data Box a = Box a

foo :: ... -> Box T
case foo <v> of Box x -> ...

vs.

foo :: ... -> T
let x = foo <v> in ...

Except that Box T is lifted and (# T #) isn't.

Roman