#9220: type roles for unboxed arrays -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: 7.10.1 Component: Core | Version: 7.8.1 Libraries | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by simonpj): Here's my summary * Plan A: * If an unboxed array of `Age` stores (unboxed) ages in a 16-bit field, but an unboxed array of `Int` stores (unboxed) ints in a 32-bit field, then it would be wrong to coerce from unboxed-array-of-`Age` to unboxed- array-of-`Int`. (The field width is described by `sizeOf` in the `Storable` class.) * This is not an unreasonable scenario. Perhaps you invented the newtype precisely because ages have a narrower range of values than ints. * In that case, unboxed arrays should have nominal role for their element type, and the GND example in comment:11 should rightfully fail. * Plan B: * An alternative is to insist that the `sizeOf` method should yield the same result for a newtype as for its underlying representation type. * Then a representational role for the element type would be justified. * Which would allow more expressive coercions; notably the GND in comment:11 I was originally voting for Plan B. But (a) it relies on an unenforced user convention about `Storable` instances, and (b) there are reasonable situations in which you might want a different width for a newtype. So on reflection I think Plan A is probably right. If we adopt it, we should add a digest of this thread (e.g. the above Plan A/B notes) as a Note with the role declaration. The core-libraries committee is already cc'd. Edward, we need a decision for 7.10. Thanks. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9220#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler