Anything involving existentials is going to be hard to implement using newtype directly. But as 'mokus' says, it might be possible to make a guarantee, right in the code generator, that certain sorts of data types do not allocate a box. The conditions are, I think, very nearly as 'mokus' says: 1. Only one constructor 2. Only one field with nonzero width in that constructor (counting constraints as fields) 3. That field is marked strict 4. That field has a boxed (or polymorphic) type I think this'd be do-able. The question is how important it is in
#1965: Allow unconstrained existential contexts in newtypes -------------------------------------+------------------------------------- Reporter: guest | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 6.8.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by dfeuer): Replying to [comment:9 simonpj]: practice; it's one more thing to maintain. I would like to have something like this very much! Among other things, it's one possible way to make `IntMap` nicer. One potential extension: I think constraints only need to count as fields if any of them are classes that have methods, or whose superclasses have methods. In particular, it could be very useful to have equality constraints involving type families. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/1965#comment:28 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler