GHC.Tc.Deriv.Generate

A declarative description of an auxiliary binding that should be generated. See Note [Auxiliary binders] for a more detailed description of how these are used.

Take a Bag of AuxBindSpecs and generate the code for auxiliary bindings based on the declarative descriptions in the supplied AuxBindSpecs. See Note [Auxiliary binders].

Produces a function binding. When no equations are given, it generates a binding of the given arity and an empty case expression for the last argument that it passes to the given function to produce the right-hand side.

Produces a function binding. When there are no equations, it generates a binding with the given arity that produces an error based on the name of the type of the last argument.

getPossibleDataCons tycon tycon_args returns the constructors of tycon whose return types match when checked against tycon_args.

See Note [Filter out impossible GADT data constructors]

Information about the arguments to the class in a stock- or newtype-derived instance. For a deriving-generated instance declaration such as this one:

instance Ctx => Cls cls_ty_1 ... cls_ty_m (TC tc_arg_1 ... tc_arg_n) where ...

• dit_cls_tys corresponds to cls_ty_1 ... cls_ty_m.

• dit_tc corresponds to TC.

• dit_tc_args corresponds to tc_arg_1 ... tc_arg_n.

See Note [DerivEnv and DerivSpecMechanism] in GHC.Tc.Deriv.Utils for a more in-depth explanation, including the relationship between dit_tcdit_rep_tc and dit_tc_argsdit_rep_tc_args.

A DerivInstTys value can be seen as a more structured representation of the denv_inst_tys in a DerivEnv, as the denv_inst_tys is equal to dit_cls_tys ++ [mkTyConApp dit_tc dit_tc_args]. Other parts of the instance declaration can be found in the DerivEnv. For example, the Cls in the example above corresponds to the denv_cls field of DerivEnv.

Similarly, the type variables that appear in a DerivInstTys value are the same type variables as the denv_tvs in the parent DerivEnv. Accordingly, if we are inferring an instance context, the type variables will be TcTyVar skolems. Otherwise, they will be ordinary TyVars. See Note [Overlap and deriving] in GHC.Tc.Deriv.Infer.

buildDataConInstArgEnv tycon arg_tys constructs a cache that maps each of tycon's data constructors to their field types, with are to be instantiated with arg_tys. See Note [Instantiating field types in stock deriving].

Look up a data constructor's instantiated field types in a DerivInstTys. See Note [Instantiating field types in stock deriving].

Apply a substitution to all of the Types contained in a DerivInstTys. See Note [Instantiating field types in stock deriving] for why we need to substitute into a DerivInstTys in the first place.

Zonk the TcTyVars in a DerivInstTys value to TyVars. See Note [What is zonking?] in GHC.Tc.Zonk.Type.

This is only used in the final zonking step when inferring the context for a derived instance. See Note [Overlap and deriving] in GHC.Tc.Deriv.Infer.