Ah, right. My new understanding, still somewhat shaky (partly because I don't have a recent enough version of GHC to play with these): Something passed into a

Ah, right. My new understanding, still somewhat shaky (partly because I don't have a recent enough version of GHC to play with these): Something passed into a function as a linear argument is considered to be "used" as many times as the result is consumed. So if we try to use either resulting `DArray` more than once (or forget to use one of them) we will have used the original `DArray` more than once (or forgotten to use it, ... or both). As you point out, that can't be done because we're inside the callback (although I'd followed that bit originally). And that's why we have `Unrestricted` - for returning results we can reuse (or needn't use) while still consuming the arguments exactly once.