#12768: 8.0.2 derives invalid code when class method is constrained by itself -------------------------------------+------------------------------------- Reporter: jophish | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.2 Component: Compiler | Version: 8.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): There is a real difficulty here, if I'm not mistaken. Consider {{{ class C a where op :: D a => a -> a instance C [a] where op = opList opList :: D [a] => [a] -> [a] opList = ... }}} Now suppose we try GND: {{{ newtype N a = MkN [a] deriving( C ) }}} The GND is expecting to get an implementation of `op` for `N` from that for `opList`, something like {{{ instance C [a] => C (N a) where op = opN opN :: D (N a) => N a -> N a opN = ...opList... -- Uses coerce }}} But the call to `opList` in the definition of `opN` needs `(D [a])` whereas what we are given is `D (N a)`. **And these are not inter-coercible**. For example we might have manually written instances {{{ instance D [a] where ... instance D (N a) where ... }}} and there is no relation between them. So in this case, the code for `opN` is **not** representationally the same as the code for `opList`, so GND (which is all about representational equivalence) doesn't really apply. In the actual example, `D` is just `C` again, but I don't think we should treat it any differently. The actual error is generated by an attempt to coerce {{{ coerce @(C m => m ()) @(C (N m) => N m ()) }}} which rightly fails. So I think GND just doesn't work here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12768#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler