#14490: TTG Snags -------------------------------------+------------------------------------- Reporter: alanz | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #14482 | Differential Rev(s): Wiki Page: | ImplementingTreesThatGrow | -------------------------------------+------------------------------------- Comment (by alanz): Given the following module {{{#!hs {-# LANGUAGE TypeFamilies, GADTs, FlexibleInstances, StandaloneDeriving, DeriveDataTypeable #-} {-# LANGUAGE DataKinds #-} module Oddity where import Data.Data data GhcPass (p :: Pass) data Pass = Parsed | Renamed | Typechecked deriving (Data) deriving instance Typeable p => Data (GhcPass p) type family XOverLit x type instance XOverLit (GhcPass 'Parsed) = Int type instance XOverLit (GhcPass 'Renamed) = Int type instance XOverLit (GhcPass 'Typechecked) = Char data Foo p = Foo (XOverLit p) deriving instance (Typeable p) => Data (Foo (GhcPass p)) }}} I do not understand why deriving the `Data` instance for `Foo (GhcPass p)` fails {{{ Oddity.hs:21:1-56: error: ⢠Could not deduce (Data (XOverLit (GhcPass p))) arising from a use of âkâ }}} Is this a weakness in the deriving process? Because as I understand it, there is a concrete value for every `(GhcPass p)`, given the way `GhcPass` is defined as being indexed by `Pass`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14490#comment:41 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler