Commit f05d685a authored by Simon Peyton Jones's avatar Simon Peyton Jones

Refactoring of mkNewTypeEqn

The refactoring here is very small.  I did it while studying
Trac #12814.

To implement the change in #12814, we can just un-comment the
lines at line 1275.  It's ready to go but I didn't want to pull
the trigger in this commit
parent 7fe71636
......@@ -1256,6 +1256,7 @@ mkNewTypeEqn dflags overlap_mode tvs
-- Next we figure out what superclass dictionaries to use
-- See Note [Newtype deriving superclasses] above
sc_theta :: [PredOrigin]
cls_tyvars = classTyVars cls
dfun_tvs = tyCoVarsOfTypesWellScoped inst_tys
inst_ty = mkTyConApp tycon tc_args
......@@ -1264,24 +1265,24 @@ mkNewTypeEqn dflags overlap_mode tvs
substTheta (zipTvSubst cls_tyvars inst_tys) $
classSCTheta cls
-- Next we collect Coercible constraints between
-- the Class method types, instantiated with the representation and the
-- newtype type; precisely the constraints required for the
-- calls to coercible that we are going to generate.
coercible_constraints =
[ let (Pair t1 t2) = mkCoerceClassMethEqn cls dfun_tvs inst_tys rep_inst_ty meth
in mkPredOrigin (DerivOriginCoerce meth t1 t2) TypeLevel
(mkReprPrimEqPred t1 t2)
| meth <- classMethods cls ]
-- If there are no tyvars, there's no need
-- to abstract over the dictionaries we need
-- Example: newtype T = MkT Int deriving( C )
-- We get the derived instance
-- instance C T
-- rather than
-- instance C Int => C T
all_preds = rep_pred_o : coercible_constraints ++ sc_theta -- NB: rep_pred comes first
-- Next we collect constraints for the class methods
-- If there are no methods, we don't need any constraints
-- Otherwise we need (C rep_ty), for the representation methods,
-- and constraints to coerce each individual method
meth_theta :: [PredOrigin]
meths = classMethods cls
meth_theta = rep_pred_o : coercible_constraints
-- meth_theta | null meths = [] -- No methods => no constraints (Trac #12814)
-- | otherwise = rep_pred_o : coercible_constraints
= [ mkPredOrigin (DerivOriginCoerce meth t1 t2) TypeLevel
(mkReprPrimEqPred t1 t2)
| meth <- meths
, let (Pair t1 t2) = mkCoerceClassMethEqn cls dfun_tvs
inst_tys rep_inst_ty meth ]
all_preds :: [PredOrigin]
all_preds = meth_theta ++ sc_theta
-- Figuring out whether we can only do this newtype-deriving thing
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