[commit: ghc] master: Refactoring of mkNewTypeEqn (f05d685)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/f05d685ae05ec293083f2fa7ec7ba057fbe64869/ghc

>---------------------------------------------------------------

commit f05d685ae05ec293083f2fa7ec7ba057fbe64869
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Thu Nov 10 15:52:31 2016 +0000

    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


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f05d685ae05ec293083f2fa7ec7ba057fbe64869
 compiler/typecheck/TcDeriv.hs | 37 +++++++++++++++++++------------------
 1 file changed, 19 insertions(+), 18 deletions(-)

diff --git a/compiler/typecheck/TcDeriv.hs b/compiler/typecheck/TcDeriv.hs
index 4722f16..0ff70fd 100644
--- a/compiler/typecheck/TcDeriv.hs
+++ b/compiler/typecheck/TcDeriv.hs
@@ -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
+        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