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To avoid having to manually plumb everything in DerivEnv throughout various functions in GHC.Tc.Deriv and GHC.Tc.Deriv.Infer, we use DerivM, which is a simple reader around TcRn.

Contains all of the information known about a derived instance when determining what its EarlyDerivSpec should be. See Note [DerivEnv and DerivSpecMechanism].

Information about the arguments to the class in a stock- or newtype-derived instance. See Note [DerivEnv and DerivSpecMechanism].

What action to take in order to derive a class instance. See Note [DerivEnv and DerivSpecMechanism], as well as Note [Deriving strategies] in GHC.Tc.Deriv.

Convert a DerivSpecMechanism to its corresponding DerivStrategy.

Whether GHC is processing a deriving clause or a standalone deriving declaration.

Records whether a particular class can be derived by way of an originative deriving strategy (i.e., stock or anyclass).

See Note [Deriving strategies] in GHC.Tc.Deriv.

Is GHC processing a standalone deriving declaration?

Is GHC processing a standalone deriving declaration with an extra-constraints wildcard as the context? (e.g., deriving instance _ => Eq (Foo a))

mkDerivOrigin wc returns StandAloneDerivOrigin if wc is True, and DerivClauseOrigin if wc is False. Useful for error-reporting.

A PredType annotated with the origin of the constraint CtOrigin, and whether or the constraint deals in types or kinds.

A list of wanted PredOrigin constraints (to_wanted_origins) to simplify when inferring a derived instance's context. These are used in all deriving strategies, but in the particular case of DeriveAnyClass, we need extra information. In particular, we need:

• to_anyclass_skols, the list of type variables bound by a class method's regular type signature, which should be rigid.
• to_anyclass_metas, the list of type variables bound by a class method's default type signature. These can be unified as necessary.
• to_anyclass_givens, the list of constraints from a class method's regular type signature, which can be used to help solve constraints in the to_wanted_origins.

(Note that to_wanted_origins will likely contain type variables from the derived type class or data type, neither of which will appear in to_anyclass_skols or to_anyclass_metas.)

For all other deriving strategies, it is always the case that to_anyclass_skols, to_anyclass_metas, and to_anyclass_givens are empty.

Here is an example to illustrate this:

class Foo a where
  bar :: forall b. Ix b => a -> b -> String
  default bar :: forall y. (Show a, Ix y) => a -> y -> String
  bar x y = show x ++ show (range (y, y))

  baz :: Eq a => a -> a -> Bool
  default baz :: Ord a => a -> a -> Bool
  baz x y = compare x y == EQ

data Quux q = Quux deriving anyclass Foo

Then it would generate two ThetaOrigins, one for each method:

[ ThetaOrigin { to_anyclass_skols  = [b]
              , to_anyclass_metas  = [y]
              , to_anyclass_givens = [Ix b]
              , to_wanted_origins  = [ Show (Quux q), Ix y
                                     , (Quux q -> b -> String) ~
                                       (Quux q -> y -> String)
                                     ] }
, ThetaOrigin { to_anyclass_skols  = []
              , to_anyclass_metas  = []
              , to_anyclass_givens = [Eq (Quux q)]
              , to_wanted_origins  = [ Ord (Quux q)
                                     , (Quux q -> Quux q -> Bool) ~
                                       (Quux q -> Quux q -> Bool)
                                     ] }
]

(Note that the type variable q is bound by the data type Quux, and thus it appears in neither to_anyclass_skols nor to_anyclass_metas.)

See Note [Gathering and simplifying constraints for DeriveAnyClass] in GHC.Tc.Deriv.Infer for an explanation of how to_wanted_origins are determined in DeriveAnyClass, as well as how to_anyclass_skols, to_anyclass_metas, and to_anyclass_givens are used.