Commit a7867c79 authored by Sebastian Graf's avatar Sebastian Graf
Browse files

Get rid of PmFake

The pattern match oracle can now cope with the abundance of information
that ViewPatterns, NPlusKPats, overloaded lists, etc. provide.

No need to have PmFake anymore!

Also got rid of a spurious call to `allCompleteMatches`, which we used to call
*for every constructor* match. Naturally this blows up quadratically for
programs like `ManyAlternatives`.

Metric Decrease:

Metric Increase:
parent ded96fb3
Pipeline #10426 passed with stages
in 407 minutes and 53 seconds
......@@ -33,7 +33,6 @@ import BasicTypes (Origin, isGenerated)
import CoreSyn (CoreExpr, Expr(Var))
import CoreUtils (exprType)
import FastString (unpackFS)
import Unify( tcMatchTy )
import DynFlags
import GHC.Hs
import TcHsSyn
......@@ -42,18 +41,15 @@ import ConLike
import Name
import FamInst
import TysWiredIn
import TyCon
import SrcLoc
import Util
import Outputable
import DataCon
import PatSyn
import HscTypes (CompleteMatch(..))
import BasicTypes (Boxity(..))
import Var (EvVar)
import Coercion
import TcEvidence
import {-# SOURCE #-} DsExpr (dsExpr, dsLExpr)
import {-# SOURCE #-} DsExpr (dsExpr, dsLExpr, dsSyntaxExpr)
import MatchLit (dsLit, dsOverLit)
import IOEnv
import DsMonad
......@@ -105,16 +101,12 @@ data PmPat where
, pm_grd_expr :: CoreExpr } -> PmPat
-- (PmGrd pat expr) matches expr against pat, binding the variables in pat
-- | A fake guard pattern (True <- _) used to represent cases we cannot handle.
PmFake :: PmPat
-- | Should not be user-facing.
instance Outputable PmPat where
ppr (PmCon alt _arg_tys _con_tvs con_args)
= cparen (notNull con_args) (hsep [ppr alt, hsep (map ppr con_args)])
ppr (PmVar vid) = ppr vid
ppr (PmGrd pv ge) = hsep (map ppr pv) <+> text "<-" <+> ppr ge
ppr PmFake = text "<PmFake>"
-- data T a where
-- MkT :: forall p q. (Eq p, Ord q) => p -> q -> T [p]
......@@ -435,12 +427,6 @@ truePattern :: PmPat
truePattern = nullaryConPattern (RealDataCon trueDataCon)
{-# INLINE truePattern #-}
-- | Generate a `canFail` pattern vector of a specific type
mkCanFailPmPat :: Type -> DsM PatVec
mkCanFailPmPat ty = do
var <- mkPmVar ty
return [var, PmFake]
vanillaConPattern :: ConLike -> [Type] -> PatVec -> PmPat
-- ADT constructor pattern => no existentials, no local constraints
vanillaConPattern con arg_tys args =
......@@ -518,19 +504,20 @@ translatePat fam_insts pat = case pat of
pure [xp,g]
-- (n + k) ===> x (True <- x >= k) (n <- x-k)
NPlusKPat ty (dL->L _ _n) _k1 _k2 _ge _minus -> mkCanFailPmPat ty
NPlusKPat pat_ty (dL->L _ n) k1 k2 ge minus -> do
(xp, xe) <- mkPmId2Forms pat_ty
let ke1 = HsOverLit noExtField (unLoc k1)
ke2 = HsOverLit noExtField k2
g1 <- mkGuardSyntaxExpr [truePattern] ge [unLoc xe, ke1]
g2 <- mkGuardSyntaxExpr [PmVar n] minus [ke2]
return [xp, g1, g2]
-- (fun -> pat) ===> x (pat <- fun x)
ViewPat arg_ty lexpr lpat -> do
ps <- translatePat fam_insts (unLoc lpat)
-- See Note [Guards and Approximation]
res <- allM cantFailPattern ps
case res of
True -> do
(xp,xe) <- mkPmId2Forms arg_ty
g <- mkGuard ps (HsApp noExtField lexpr xe)
return [xp, g]
False -> mkCanFailPmPat arg_ty
(xp,xe) <- mkPmId2Forms arg_ty
g <- mkGuard ps (HsApp noExtField lexpr xe)
return [xp, g]
-- list
ListPat (ListPatTc ty Nothing) ps -> do
......@@ -538,14 +525,20 @@ translatePat fam_insts pat = case pat of
return (foldr (mkListPatVec ty) [nilPattern ty] pv)
-- overloaded list
ListPat (ListPatTc _elem_ty (Just (pat_ty, _to_list))) lpats -> do
ListPat (ListPatTc elem_ty (Just (pat_ty, to_list))) lpats -> do
dflags <- getDynFlags
if xopt LangExt.RebindableSyntax dflags
then mkCanFailPmPat pat_ty
else case splitListTyConApp_maybe pat_ty of
Just e_ty -> translatePat fam_insts
(ListPat (ListPatTc e_ty Nothing) lpats)
Nothing -> mkCanFailPmPat pat_ty
case splitListTyConApp_maybe pat_ty of
Just e_ty
| not (xopt LangExt.RebindableSyntax dflags)
-- Just translate it as a regular ListPat
-> translatePat fam_insts (ListPat (ListPatTc e_ty Nothing) lpats)
_ -> do
ps <- translatePatVec fam_insts (map unLoc lpats)
(xp, xe) <- mkPmId2Forms pat_ty
let pats = foldr (mkListPatVec elem_ty) [nilPattern elem_ty] ps
g <- mkGuardSyntaxExpr pats to_list [unLoc xe]
return [xp,g]
-- (a) In the presence of RebindableSyntax, we don't know anything about
-- `toList`, we should treat `ListPat` as any other view pattern.
......@@ -568,16 +561,11 @@ translatePat fam_insts pat = case pat of
, pat_arg_tys = arg_tys
, pat_tvs = ex_tvs
, pat_args = ps } -> do
let ty = conLikeResTy con arg_tys
groups <- allCompleteMatches ty
case groups of
[] -> mkCanFailPmPat ty
_ -> do
args <- translateConPatVec fam_insts arg_tys ex_tvs con ps
return [PmCon { pm_con_con = PmAltConLike con
, pm_con_arg_tys = arg_tys
, pm_con_tvs = ex_tvs
, pm_con_args = args }]
args <- translateConPatVec fam_insts arg_tys ex_tvs con ps
return [PmCon { pm_con_con = PmAltConLike con
, pm_con_arg_tys = arg_tys
, pm_con_tvs = ex_tvs
, pm_con_args = args }]
NPat ty (dL->L _ olit) mb_neg _ -> do
-- See Note [Literal short cut] in MatchLit.hs
......@@ -713,14 +701,6 @@ translateGuards :: FamInstEnvs -> [GuardStmt GhcTc] -> DsM PatVec
translateGuards fam_insts guards =
concat <$> mapM (translateGuard fam_insts) guards
-- | Check whether a pattern can fail to match
cantFailPattern :: PmPat -> DsM Bool
cantFailPattern PmVar {} = pure True
cantFailPattern PmCon { pm_con_con = c, pm_con_arg_tys = tys, pm_con_args = ps}
= (&&) <$> singleMatchConstructor c tys <*> allM cantFailPattern ps
cantFailPattern (PmGrd pv _e) = allM cantFailPattern pv
cantFailPattern _ = pure False
-- | Translate a guard statement to Pattern
translateGuard :: FamInstEnvs -> GuardStmt GhcTc -> DsM PatVec
translateGuard fam_insts guard = case guard of
......@@ -756,90 +736,19 @@ translateBoolGuard e
{- Note [Guards and Approximation]
Even if the algorithm is really expressive, the term oracle we use is not.
Hence, several features are not translated *properly* but we approximate.
The list includes:
1. View Patterns
A view pattern @(f -> p)@ should be translated to @x (p <- f x)@. The term
oracle does not handle function applications so we know that the generated
constraints will not be handled at the end. Hence, we distinguish between two
a) Pattern @p@ cannot fail. Then this is just a binding and we do the *right
b) Pattern @p@ can fail. This means that when checking the guard, we will
generate several cases, with no useful information. E.g.:
h (f -> [a,b]) = ...
h x ([a,b] <- f x) = ...
uncovered set = { [x |> { False ~ (f x ~ []) }]
, [x |> { False ~ (f x ~ (t1:[])) }]
, [x |> { False ~ (f x ~ (t1:t2:t3:t4)) }] }
So we have two problems:
1) Since we do not print the constraints in the general case (they may
be too many), the warning will look like this:
Pattern match(es) are non-exhaustive
In an equation for `h':
Patterns not matched:
Which is not short and not more useful than a single underscore.
2) The size of the uncovered set increases a lot, without gaining more
expressivity in our warnings.
Hence, in this case, we replace the guard @([a,b] <- f x)@ with a *dummy*
@PmFake@: @True <- _@. That is, we record that there is a possibility
of failure but we minimize it to a True/False. This generates a single
warning and much smaller uncovered sets.
2. Overloaded Lists
An overloaded list @[...]@ should be translated to @x ([...] <- toList x)@. The
problem is exactly like above, as its solution. For future reference, the code
below is the *right thing to do*:
ListPat (ListPatTc elem_ty (Just (pat_ty, _to_list))) lpats
otherwise -> do
(xp, xe) <- mkPmId2Forms pat_ty
ps <- translatePatVec (map unLoc lpats)
let pats = foldr (mkListPatVec elem_ty) [nilPattern elem_ty] ps
g = mkGuard pats (HsApp (noLoc to_list) xe)
return [xp,g]
3. Overloaded Literals
The case with literals is a bit different. a literal @l@ should be translated
to @x (True <- x == from l)@. Since we want to have better warnings for
overloaded literals as it is a very common feature, we treat them differently.
They are mainly covered in Note [Undecidable Equality for PmAltCons] in PmTypes.
4. N+K Patterns & Pattern Synonyms
An n+k pattern (n+k) should be translated to @x (True <- x >= k) (n <- x-k)@.
Since the only pattern of the three that causes failure is guard @(n <- x-k)@,
and has two possible outcomes. Hence, there is no benefit in using a dummy and
we implement the proper thing. Pattern synonyms are simply not implemented yet.
Hence, to be conservative, we generate a dummy pattern, assuming that the
pattern can fail.
5. Actual Guards
During translation, boolean guards and pattern guards are translated properly.
Let bindings though are omitted by function @translateLet@. Since they are lazy
bindings, we do not actually want to generate a (strict) equality (like we do
in the pattern bind case). Hence, we safely drop them.
Additionally, top-level guard translation (performed by @translateGuards@)
replaces guards that cannot be reasoned about (like the ones we described in
1-4) with a single @PmFake@ to record the possibility of failure to match.
6. Combinatorial explosion
Precise pattern match exchaustiveness checking is necessarily exponential in
the size of some input programs. We implement a couple approximation and safe
guards to prevent exponential blow-up:
* Guards usually provide little information gain while quickly leading to
exponential blow-up. See Note [Combinatorial explosion in guards].
* Similar to the situation with guards, refining a variable to a pattern
synonym application provides little value while easily leading to
exponential blow-up due to lack of generativity compared to DataCons.
See Note [Limit the number of refinements].
Note [Combinatorial explosion in guards]
Function with many clauses and deeply nested guards like in #11195 tend to
overwhelm the checker because they lead to exponential splitting behavior.
See the comments on #11195 on refinement trees. Every guard refines the
......@@ -937,7 +846,6 @@ pmPatType (PmVar { pm_var_id = x }) = idType x
pmPatType (PmGrd { pm_grd_pv = pv })
= ASSERT(patVecArity pv == 1) (pmPatType p)
where Just p = find ((==1) . patternArity) pv
pmPatType PmFake = pmPatType truePattern
Note [Extensions to GADTs Meet Their Match]
......@@ -963,6 +871,11 @@ the paper. This Note serves as a reference for these new features.
mkGuard :: PatVec -> HsExpr GhcTc -> DsM PmPat
mkGuard pv e = PmGrd pv <$> dsExpr e
mkGuardSyntaxExpr :: PatVec -> SyntaxExpr GhcTc -> [HsExpr GhcTc] -> DsM PmPat
mkGuardSyntaxExpr pv f args = do
core_args <- traverse dsExpr args
PmGrd pv <$> dsSyntaxExpr f core_args
-- | Generate a variable pattern of a given type
mkPmVar :: Type -> DsM PmPat
mkPmVar ty = PmVar <$> mkPmId ty
......@@ -979,80 +892,6 @@ mkPmId2Forms ty = do
x <- mkPmId ty
return (PmVar x, noLoc (HsVar noExtField (noLoc x)))
-- | Check whether a 'PmAltCon' has the /single match/ property, i.e. whether
-- it is the only possible match in the given context. See also
-- 'allCompleteMatches' and Note [Single match constructors].
singleMatchConstructor :: PmAltCon -> [Type] -> DsM Bool
singleMatchConstructor PmAltLit{} _ = pure False
singleMatchConstructor (PmAltConLike cl) tys =
any isSingleton <$> allCompleteMatches (conLikeResTy cl tys)
Note [Single match constructors]
When translating pattern guards for consumption by the checker, we desugar
every pattern guard that might fail ('cantFailPattern') to 'PmFake'
(True <- _). Which patterns can't fail? Exactly those that only match on
Here are a few examples:
* @f a | (a, b) <- foo a = 42@: Product constructors are generally
single match. This extends to single constructors of GADTs like 'Refl'.
* If @f | Id <- id () = 42@, where @pattern Id = ()@ and 'Id' is part of a
singleton `COMPLETE` set, then 'Id' has the single match property.
In effect, we can just enumerate 'allCompleteMatches' and check if the conlike
occurs as a singleton set.
There's the chance that 'Id' is part of multiple `COMPLETE` sets. That's
irrelevant; If the user specified a singleton set, it is single-match.
Note that this doesn't really take into account incoming type constraints;
It might be obvious from type context that a particular GADT constructor has
the single-match property. We currently don't (can't) check this in the
translation step. See #15753 for why this yields surprising results.
-- | For a given type, finds all the COMPLETE sets of conlikes that inhabit it.
-- Note that for a data family instance, this must be the *representation* type.
-- e.g. data instance T (a,b) = T1 a b
-- leads to
-- data TPair a b = T1 a b -- The "representation" type
-- It is TPair a b, not T (a, b), that is given to allCompleteMatches
-- These come from two places.
-- 1. From data constructors defined with the result type constructor.
-- 2. From `COMPLETE` pragmas which have the same type as the result
-- type constructor. Note that we only use `COMPLETE` pragmas
-- *all* of whose pattern types match. See #14135
allCompleteMatches :: Type -> DsM [[ConLike]]
allCompleteMatches ty = case splitTyConApp_maybe ty of
Nothing -> pure [] -- NB: We don't know any COMPLETE set, as opposed to [[]]
Just (tc, tc_args) -> do
-- Look into the representation type of a data family instance, too.
env <- dsGetFamInstEnvs
let (tc', _tc_args', _co) = tcLookupDataFamInst env tc tc_args
let mb_rdcs = map RealDataCon <$> tyConDataCons_maybe tc'
let maybe_to_list = maybe [] (:[])
let rdcs = maybe_to_list mb_rdcs
-- NB: tc, because COMPLETE sets are associated with the parent data family
-- TyCon
pragmas <- dsGetCompleteMatches tc
let fams = mapM dsLookupConLike . completeMatchConLikes
pscs <- mapM fams pragmas
let candidates = rdcs ++ pscs
-- Check that all the pattern synonym return types in a `COMPLETE`
-- pragma subsume the type we're matching.
-- See Note [Filtering out non-matching COMPLETE sets]
pure (filter (isValidCompleteMatch ty) candidates)
isValidCompleteMatch :: Type -> [ConLike] -> Bool
isValidCompleteMatch ty = all p
p (RealDataCon _) = True
p (PatSynCon ps) = isJust (tcMatchTy (projResTy (patSynSig ps)) ty)
projResTy (_, _, _, _, _, res_ty) = res_ty
Note [Filtering out non-matching COMPLETE sets]
......@@ -1141,7 +980,6 @@ patVecArity = sum . map patternArity
-- | Compute the arity of a pattern
patternArity :: PmPat -> PmArity
patternArity (PmGrd {}) = 0
patternArity PmFake = 0
patternArity _other_pat = 1
......@@ -1204,7 +1042,7 @@ pmcheckGuards [] _ delta = return (usimple delta)
pmcheckGuards (gv:gvs) n delta = do
(PartialResult cs unc ds) <- pmcheckI gv [] [] n delta
let (n', unc')
-- See 6. in Note [Guards and Approximation]
-- See Note [Combinatorial explosion in guards]
| Just n' <- tryMultiplyDeltas (length unc) n = (n', unc)
| otherwise = (n, [delta])
(PartialResult css uncs dss) <- runMany (pmcheckGuardsI gvs n') unc'
......@@ -1228,16 +1066,6 @@ pmcheck [] guards [] n delta
| otherwise = pmcheckGuardsI guards n delta
-- Guard
pmcheck (PmFake : ps) guards vva n delta =
-- short-circuit if the guard pattern is useless.
-- we just have two possible outcomes: fail here or match and recurse
-- none of the two contains any useful information about the failure
-- though. So just have these two cases but do not do all the boilerplate
-- TODO: I don't think this should mkCons delta, rather than just replace the
-- presultUncovered by [delta] completely. Note that the uncovered set
-- returned from the recursive call can only be a refinement of the
-- original delta.
forces . mkCons delta <$> pmcheckI ps guards vva n delta
pmcheck (p@PmGrd { pm_grd_pv = pv, pm_grd_expr = e } : ps) guards vva n delta = do
tracePm "PmGrd: pmPatType" (vcat [ppr p, ppr (pmPatType p)])
x <- mkPmId (exprType e)
......@@ -1290,11 +1118,6 @@ pmcheck (_:_) _ [] _ _ = panic "pmcheck: cons-nil"
-- ----------------------------------------------------------------------------
-- * Utilities for main checking
updateUncovered :: (Uncovered -> Uncovered) -> (PartialResult -> PartialResult)
updateUncovered f p@(PartialResult { presultUncovered = old })
= p { presultUncovered = f old }
-- | Initialise with default values for covering and divergent information and
-- a singleton uncovered set.
usimple :: Delta -> PartialResult
......@@ -1308,10 +1131,6 @@ usimple delta = mempty { presultUncovered = [delta] }
mkUnion :: PartialResult -> PartialResult -> PartialResult
mkUnion = mappend
-- | Add a model to the uncovered set.
mkCons :: Delta -> PartialResult -> PartialResult
mkCons model = updateUncovered (model:)
-- | Set the divergent set to not empty
forces :: PartialResult -> PartialResult
forces pres = pres { presultDivergent = Diverged }
......@@ -3,7 +3,7 @@ T11822.hs:33:1: warning: [-Wincomplete-patterns (in -Wextra)]
Pattern match(es) are non-exhaustive
In an equation for ‘mkTreeNode’:
Patterns not matched:
_ (Data.Sequence.Internal.Seq _) _ _
_ (Data.Sequence.Internal.Seq _) _ p where p is not one of {0}
_ (Data.Sequence.Internal.Seq _) _ p where p is not one of {0}
_ (Data.Sequence.Internal.Seq _) _ p where p is not one of {0}
_ (Data.Sequence.Internal.Seq _) _ p where p is not one of {0}
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