#10048: {-# UNPACK #-} support for size-indexed types -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature request | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.11 Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by simonpj): A difficulty is that in GHC today, each data constructor (such as `Cons` above) has a single, fixed memory layout. So if we have {{{ data T = MkT Bool {-# UNPACK !(Int, Int) #-} }}} we can decide to lay out `MkT` with a `Bool` field and two `Int` fields. But if we don't know `l`, we don't know how `HList l` is represented, so we can't do that. It's not just an implementation matter. Suppose at an allocation site we were allocating a `Cons` whose first arg was `'[Int,Bool]`. Then you could imagine whizzing up a specialised `Cons` constructor. But then `hlength` above would have to be able to consume that constructor! I don't know any way to do this without monomorphising the code. Which is often, ''but not always'' possible, so you'd also need an escape hatch for the impossible case -- and it's not obvious how to build the hatch. PhD thesis anyone? Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10048#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler