- where
- (alts_wo_default, maybe_deflt) = findDefault alts
- imposs_cons = case scrut of
- Var v -> otherCons (idUnfolding v)
- other -> []
-
- -- "imposs_deflt_cons" are handled either by the context,
- -- OR by a branch in this case expression. (Don't include DEFAULT!!)
- imposs_deflt_cons = nub (imposs_cons ++ [con | (con,_,_) <- alts_wo_default])
-
- is_possible :: [Type] -> CoreAlt -> Bool
- is_possible tys (con, _, _) | con `elem` imposs_cons = False
- is_possible tys (DataAlt con, _, _) = dataConCanMatch tys con
- is_possible tys alt = True
-
-------------------------------------
-prepareDefault :: SimplEnv
- -> OutId -- Case binder; need just for its type. Note that as an
- -- OutId, it has maximum information; this is important.
- -- Test simpl013 is an example
- -> [AltCon] -- These cons can't happen when matching the default
- -> SimplCont
- -> Maybe InExpr
- -> SimplM [InAlt] -- One branch or none; still unsimplified
- -- We use a list because it's what mergeAlts expects
-
-prepareDefault env case_bndr' imposs_cons cont Nothing
- = return [] -- No default branch
-
-prepareDefault env case_bndr' imposs_cons cont (Just rhs)
- | -- This branch handles the case where we are
- -- scrutinisng an algebraic data type
- Just (tycon, inst_tys) <- splitTyConApp_maybe (idType case_bndr'),
- isAlgTyCon tycon, -- It's a data type, tuple, or unboxed tuples.
- not (isNewTyCon tycon), -- We can have a newtype, if we are just doing an eval:
- -- case x of { DEFAULT -> e }
- -- and we don't want to fill in a default for them!
- Just all_cons <- tyConDataCons_maybe tycon,
- not (null all_cons), -- This is a tricky corner case. If the data type has no constructors,
- -- which GHC allows, then the case expression will have at most a default
- -- alternative. We don't want to eliminate that alternative, because the
- -- invariant is that there's always one alternative. It's more convenient
- -- to leave
- -- case x of { DEFAULT -> e }
- -- as it is, rather than transform it to
- -- error "case cant match"
- -- which would be quite legitmate. But it's a really obscure corner, and
- -- not worth wasting code on.
-
- let imposs_data_cons = [con | DataAlt con <- imposs_cons] -- We now know it's a data type
- is_possible con = not (con `elem` imposs_data_cons)
- && dataConCanMatch inst_tys con
- = case filter is_possible all_cons of
- [] -> return [] -- Eliminate the default alternative
- -- altogether if it can't match
-
- [con] -> -- It matches exactly one constructor, so fill it in
- do { tick (FillInCaseDefault case_bndr')
- ; us <- getUniquesSmpl
- ; let (ex_tvs, co_tvs, arg_ids) =
- dataConRepInstPat us con inst_tys
- ; return [(DataAlt con, ex_tvs ++ co_tvs ++ arg_ids, rhs)] }
-
- two_or_more -> return [(DEFAULT, [], rhs)]
-
- | otherwise
- = return [(DEFAULT, [], rhs)]