The main complication with RecordUpd is that we need to explicitly
handle the *non-updated* fields. Consider:
- data T a b = MkT1 { fa :: a, fb :: b }
- | MkT2 { fa :: a, fc :: Int -> Int }
- | MkT3 { fd :: a }
+ data T a b c = MkT1 { fa :: a, fb :: (b,c) }
+ | MkT2 { fa :: a, fb :: (b,c), fc :: c -> c }
+ | MkT3 { fd :: a }
- upd :: T a b -> c -> T a c
+ upd :: T a b c -> (b',c) -> T a b' c
upd t x = t { fb = x}
-The type signature on upd is correct (i.e. the result should not be (T a b))
-because upd should be equivalent to:
+The result type should be (T a b' c)
+not (T a b c), because 'b' *is not* mentioned in a non-updated field
+not (T a b' c'), becuase 'c' *is* mentioned in a non-updated field
+NB that it's not good enough to look at just one constructor; we must
+look at them all; cf Trac #3219
+After all, upd should be equivalent to:
upd t x = case t of
MkT1 p q -> MkT1 p x
MkT2 a b -> MkT2 p b
So we need to give a completely fresh type to the result record,
and then constrain it by the fields that are *not* updated ("p" above).
+We call these the "fixed" type variables, and compute them in getFixedTyVars.
Note that because MkT3 doesn't contain all the fields being updated,
-its RHS is simply an error, so it doesn't impose any type constraints
+its RHS is simply an error, so it doesn't impose any type constraints.
+Hence the use of 'relevant_cont'.
Note [Implict type sharing]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
\begin{code}
tcExpr expr@(RecordUpd record_expr rbinds _ _ _) res_ty
- = do {
+ = ASSERT( notNull upd_fld_names )
+ do {
-- STEP 0
-- Check that the field names are really field names
- let upd_fld_names = hsRecFields rbinds
- ; MASSERT( notNull upd_fld_names )
; sel_ids <- mapM tcLookupField upd_fld_names
-- The renamer has already checked that
-- selectors are all in scope
-- STEP 3 Note [Criteria for update]
-- Check that each updated field is polymorphic; that is, its type
-- mentions only the universally-quantified variables of the data con
- ; let flds_w_tys = zipEqual "tcExpr:RecConUpd" con1_flds con1_arg_tys
- (upd_flds_w_tys, fixed_flds_w_tys) = partition is_updated flds_w_tys
+ ; let flds1_w_tys = zipEqual "tcExpr:RecConUpd" con1_flds con1_arg_tys
+ (upd_flds1_w_tys, fixed_flds1_w_tys) = partition is_updated flds1_w_tys
is_updated (fld,ty) = fld `elem` upd_fld_names
- bad_upd_flds = filter bad_fld upd_flds_w_tys
+ bad_upd_flds = filter bad_fld upd_flds1_w_tys
con1_tv_set = mkVarSet con1_tvs
bad_fld (fld, ty) = fld `elem` upd_fld_names &&
not (tyVarsOfType ty `subVarSet` con1_tv_set)
-- Figure out types for the scrutinee and result
-- Both are of form (T a b c), with fresh type variables, but with
-- common variables where the scrutinee and result must have the same type
- -- These are variables that appear anywhere *except* in the updated fields
- ; let common_tvs = exactTyVarsOfTypes (map snd fixed_flds_w_tys)
- `unionVarSet` constrainedTyVars con1_tvs relevant_cons
- is_common_tv tv = tv `elemVarSet` common_tvs
-
+ -- These are variables that appear in *any* arg of *any* of the relevant constructors
+ -- *except* in the updated fields
+ --
+ ; let fixed_tvs = getFixedTyVars con1_tvs relevant_cons
+ is_fixed_tv tv = tv `elemVarSet` fixed_tvs
mk_inst_ty tv result_inst_ty
- | is_common_tv tv = return result_inst_ty -- Same as result type
- | otherwise = newFlexiTyVarTy (tyVarKind tv) -- Fresh type, of correct kind
+ | is_fixed_tv tv = return result_inst_ty -- Same as result type
+ | otherwise = newFlexiTyVarTy (tyVarKind tv) -- Fresh type, of correct kind
; (_, result_inst_tys, result_inst_env) <- tcInstTyVars con1_tvs
; scrut_inst_tys <- zipWithM mk_inst_ty con1_tvs result_inst_tys
; return (mkHsWrap co_fn (RecordUpd (mkLHsWrap scrut_co record_expr') rbinds'
relevant_cons scrut_inst_tys result_inst_tys)) }
where
- constrainedTyVars :: [TyVar] -> [DataCon] -> TyVarSet
- -- Universally-quantified tyvars that appear in any of the
- -- *implicit* arguments to the constructor
+ upd_fld_names = hsRecFields rbinds
+
+ getFixedTyVars :: [TyVar] -> [DataCon] -> TyVarSet
-- These tyvars must not change across the updates
- -- See Note [Implict type sharing]
- constrainedTyVars tvs1 cons
+ getFixedTyVars tvs1 cons
= mkVarSet [tv1 | con <- cons
- , let (tvs, theta, _, _) = dataConSig con
- bad_tvs = tyVarsOfTheta theta
+ , let (tvs, theta, arg_tys, _) = dataConSig con
+ flds = dataConFieldLabels con
+ fixed_tvs = exactTyVarsOfTypes fixed_tys
+ -- fixed_tys: See Note [Type of a record update]
+ `unionVarSet` tyVarsOfTheta theta
+ -- Universally-quantified tyvars that appear in any of the
+ -- *implicit* arguments to the constructor are fixed
+ -- See Note [Implict type sharing]
+
+ fixed_tys = [ty | (fld,ty) <- zip flds arg_tys
+ , not (fld `elem` upd_fld_names)]
, (tv1,tv) <- tvs1 `zip` tvs -- Discards existentials in tvs
- , tv `elemVarSet` bad_tvs ]
+ , tv `elemVarSet` fixed_tvs ]
\end{code}
%************************************************************************
projects / ghc-hetmet.git / blobdiff