#8503: New GeneralizedNewtypeDeriving check still isn't permissive enough -------------------------------------+------------------------------------ Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by simonpj): Cool idea. I like it. For the most part, the 'deriving' mechanism generates an instance decl in `HsSyn RdrName` and feeds it through the renamer and type checker. We could do that for deriving decls too, like this: {{{ newtype Age = MkAge Int deriving( C ) }}} would generate {{{ instance C Int => C Age where meth = coerce (meth :: Int -> F Int) }}} Here we simply call `coerce`, which wakes up the `Coercible` stuff. It's a bit of a nuisance that we have to specify a type signature (in `HsType`) for `meth`, so that the from-type of the `coerce` is known, but not too bad. What you say about inferring `Coercible` constraints also makes sense. This would have another benefit: making GND much more uniform with the other instances. See `TcEnv.InstInfo`. We can probably get rid of `NewTypeDerived` altogether, by generating a `VanillaInst` instead! Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8503#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler