#1830: Automatic derivation of Lift -------------------------------------+------------------------------------- Reporter: guest | Owner: | RyanGlScott Type: feature request | Status: new Priority: normal | Milestone: ⊥ Component: Template Haskell | 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 Revisions: Phab:D1168 -------------------------------------+------------------------------------- Comment (by RyanGlScott): Replying to [comment:10 goldfire]:
What's wrong with using `th-lift`?
Type inference. `th-lift` is fundamentally limited in that it cannot infer the correct instance context for complex data structures such as {{{#!hs data Nested f a = Nested (f a) deriving Lift -- instance Lift (f a) => Lift (Nested f a) where ... }}} or {{{#!hs data R a = R a instance (Show a, Lift a) => Lift (R a) where ... data Restricted a = Restricted (R a) deriving Lift -- instance (Show a, Lift a) => Lift (Restricted a) where ... }}} Granted, this could be improved if `th-lift` were changed to allow something like this: {{{#!hs instance (Show a, Lift a) => Lift (Restricted a) where lift = $(makeLift ''Restricted) }}} But a `deriving` statement is infinitely more aesthetically pleasing, in my opinion.
But that argument falls flat here, because users are clearly already using TH!
Well, they're using the `template-haskell` library, but not necessarily the `TemplateHaskell` extension. Users may want to define `Lift` instances for data types without actually using the `TemplateHaskell` extension (as doing so precludes a library from being built on a stage-1 compiler). Having access to `DeriveLift` allows a library to be built with `Lift` support on GHC stage 1 and 2, and then downstream packages that depend on the library can decide if they want to actually utilize the `Lift` instances by means of the `TemplateHaskell` extension. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/1830#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler