The special valuespan is quite important for effective array slicing in ArrayFire, but it was not previously exposed in the Haskell bindings. Indeed, as this is a constant rather than a function it cannot be imported (?) using the FFI, but the same value can simply be hardcoded as a Haskell constant.

Without this export, users could also hard-code it at each use site. But that would result in cryptic code, not to mention unsafe if the definition ofaf_span should ever change. It's certainly better to have it available in the library.