The Haskell Report is vague about exactly when a deriving clause is legal. For example:
{{{#!hs data T0 f a = MkT0 a deriving( Eq ) data T1 f a = MkT1 (f a) deriving( Eq ) data T2 f a = MkT2 (f (f a)) deriving( Eq ) }}}
The natural generated Eq code would result in these instance declarations:
{{{#!hs instance Eq a => Eq (T0 f a) where ... instance Eq (f a) => Eq (T1 f a) where ... instance Eq (f (f a)) => Eq (T2 f a) where ... }}}
The first of these is obviously fine. The second is still fine, although less obviously. The third is not Haskell 98, and risks losing termination of instances.
GHC takes a conservative position: it accepts the first two, but not the
#16323: Cannot deduce X error with X provided in TypeFamilies -------------------------------------+------------------------------------- Reporter: pjljvdlaar | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: duplicate | Keywords: Deriving Operating System: Windows | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11008, #15868 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => closed * resolution: => duplicate * related: => #11008, #15868 Comment: This is by design. Per the [https://downloads.haskell.org/~ghc/8.6.3/docs/html/users_guide/glasgow_exts.... #inferred-context-for-deriving-clauses users' guide section] on `deriving`: third. The rule is this: each constraint in the inferred instance context must consist only of type variables, with no repetitions.
This rule is applied regardless of flags. If you want a more exotic
context, you can write it yourself, using the standalone deriving mechanism. A similar situation is happening in the code that you are trying to derive. For instance, this: {{{#!hs data RefMap a = RefMap (Map (Ref a) a) deriving Eq }}} Would require generating the following code: {{{#!hs instance (Eq a, Eq (Ref a)) => Eq (RefMap a) where ... }}} The constraint `Eq (Ref a)` has an occurrence of `Ref` underneath the class `Eq`, which isn't a type variable. Therefore, GHC conservatively backs out and refuses to infer it. In order to write this, GHC requires that you use `StandaloneDeriving`, like so: {{{#!hs deriving instance (Eq a, Eq (Ref a)) => Eq (RefMap a) }}} #11008 and #15868 are about this same issue, so I'll close this ticket as a duplicate. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16323#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler