-- Simplify 'wanted' as much as possibles
-- Fail if not possible
simplifyDeriv orig tvs theta
- = do { tvs_skols <- tcInstSuperSkolTyVars tvs -- Skolemize
- -- One reason is that the constraint solving machinery
- -- expects *TcTyVars* not TyVars. Another is that
- -- when looking up instances we don't want overlap
- -- of type variables
+ = do { tvs_skols <- tcInstSkolTyVars tvs -- Skolemize
+ -- The constraint solving machinery
+ -- expects *TcTyVars* not TyVars.
+ -- We use *non-overlappable* (vanilla) skolems
+ -- See Note [Overlap and deriving]
; let skol_subst = zipTopTvSubst tvs $ map mkTyVarTy tvs_skols
subst_skol = zipTopTvSubst tvs_skols $ map mkTyVarTy tvs
; return (substTheta subst_skol min_theta) }
\end{code}
+Note [Overlap and deriving]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider some overlapping instances:
+ data Show a => Show [a] where ..
+ data Show [Char] where ...
+
+Now a data type with deriving:
+ data T a = MkT [a] deriving( Show )
+
+We want to get the derived instance
+ instance Show [a] => Show (T a) where...
+and NOT
+ instance Show a => Show (T a) where...
+so that the (Show (T Char)) instance does the Right Thing
+
+It's very like the situation when we're inferring the type
+of a function
+ f x = show [x]
+and we want to infer
+ f :: Show [a] => a -> String
+
+BOTTOM LINE: use vanilla, non-overlappable skolems when inferring
+ the context for the derived instance.
+ Hence tcInstSkolTyVars not tcInstSuperSkolTyVars
+
Note [Exotic derived instance contexts]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In a 'derived' instance declaration, we *infer* the context. It's a
| otherwise
= do { -- See Note [Preparing inert set for implications]
-- Push the unsolved wanteds inwards, but as givens
- traceTcS "solveWanteds: preparing inerts for implications {" empty
-
- ; let pushed_givens = givensFromWanteds unsolved_cans
+ let pushed_givens = givensFromWanteds unsolved_cans
tcs_untouchables = filterVarSet isFlexiTcsTv $
tyVarsOfEvVarXs pushed_givens
-- See Note [Extra TcsTv untouchables]
+ ; traceTcS "solveWanteds: preparing inerts for implications {"
+ (vcat [ppr tcs_untouchables, ppr pushed_givens])
+
; (_, inert_for_implics) <- solveInteract just_given_inert pushed_givens
; traceTcS "solveWanteds: } now doing nested implications {" $
constraints. In effect, by floating an equality out of the
implication we are committing to have it solved in the outside.
-NB: A consequence is that every simplifier-generated TcsTv variable that gets floated out
- of an implication becomes now untouchable next time we go inside that implication to
- solve any residual constraints. In effect, by floating an equality out of the implication
- we are committing to have it solved in the outside.
+Note [Float Equalities out of Implications]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We want to float equalities out of vanilla existentials, but *not* out
+of GADT pattern matches.
\begin{code}