ghc-9.12.1: GHC/Hs/Pat.hs
{-# LANGUAGE CPP #-}
{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE UndecidableInstances #-} -- Wrinkle in Note [Trees That Grow]
-- in module Language.Haskell.Syntax.Extension
{-# OPTIONS_GHC -Wno-orphans #-} -- Outputable
{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998
\section[PatSyntax]{Abstract Haskell syntax---patterns}
-}
module GHC.Hs.Pat (
Pat(..), LPat,
isInvisArgPat, isVisArgPat,
EpAnnSumPat(..),
ConPatTc (..),
ConLikeP,
HsPatExpansion(..),
XXPatGhcTc(..),
HsConPatDetails, hsConPatArgs, hsConPatTyArgs,
HsConPatTyArg(..),
HsRecFields(..), HsFieldBind(..), LHsFieldBind,
HsRecField, LHsRecField,
HsRecUpdField, LHsRecUpdField,
RecFieldsDotDot(..),
hsRecFields, hsRecFieldSel, hsRecFieldId, hsRecFieldsArgs,
mkPrefixConPat, mkCharLitPat, mkNilPat,
isSimplePat, isPatSyn,
looksLazyPatBind,
isBangedLPat,
gParPat, patNeedsParens, parenthesizePat,
isIrrefutableHsPat,
isBoringHsPat,
collectEvVarsPat, collectEvVarsPats,
pprParendLPat, pprConArgs,
pprLPat
) where
import GHC.Prelude
import Language.Haskell.Syntax.Pat
import Language.Haskell.Syntax.Expr ( HsExpr )
import {-# SOURCE #-} GHC.Hs.Expr (pprLExpr, pprUntypedSplice, HsUntypedSpliceResult(..))
-- friends:
import GHC.Hs.Binds
import GHC.Hs.Lit
import Language.Haskell.Syntax.Extension
import GHC.Parser.Annotation
import GHC.Hs.Extension
import GHC.Hs.Type
import GHC.Tc.Types.Evidence
import GHC.Types.Basic
import GHC.Types.SourceText
-- others:
import GHC.Core.Ppr ( {- instance OutputableBndr TyVar -} )
import GHC.Builtin.Types
import GHC.Types.Var
import GHC.Types.Name.Reader
import GHC.Core.ConLike
import GHC.Core.DataCon
import GHC.Utils.Outputable
import GHC.Core.Type
import GHC.Types.SrcLoc
import GHC.Data.Bag -- collect ev vars from pats
import GHC.Types.Name
import Data.Data
import qualified Data.List.NonEmpty as NE
type instance XWildPat GhcPs = NoExtField
type instance XWildPat GhcRn = NoExtField
type instance XWildPat GhcTc = Type
type instance XVarPat (GhcPass _) = NoExtField
type instance XLazyPat GhcPs = EpToken "~"
type instance XLazyPat GhcRn = NoExtField
type instance XLazyPat GhcTc = NoExtField
type instance XAsPat GhcPs = EpToken "@"
type instance XAsPat GhcRn = NoExtField
type instance XAsPat GhcTc = NoExtField
type instance XParPat GhcPs = (EpToken "(", EpToken ")")
type instance XParPat GhcRn = NoExtField
type instance XParPat GhcTc = NoExtField
type instance XBangPat GhcPs = EpToken "!"
type instance XBangPat GhcRn = NoExtField
type instance XBangPat GhcTc = NoExtField
type instance XListPat GhcPs = AnnList ()
-- After parsing, ListPat can refer to a built-in Haskell list pattern
-- or an overloaded list pattern.
type instance XListPat GhcRn = NoExtField
-- Built-in list patterns only.
-- After renaming, overloaded list patterns are expanded to view patterns.
-- See Note [Desugaring overloaded list patterns]
type instance XListPat GhcTc = Type
-- List element type, for use in hsPatType.
type instance XTuplePat GhcPs = (EpaLocation, EpaLocation)
type instance XTuplePat GhcRn = NoExtField
type instance XTuplePat GhcTc = [Type]
type instance XOrPat GhcPs = NoExtField
type instance XOrPat GhcRn = NoExtField
type instance XOrPat GhcTc = Type
type instance XSumPat GhcPs = EpAnnSumPat
type instance XSumPat GhcRn = NoExtField
type instance XSumPat GhcTc = [Type]
type instance XConPat GhcPs = (Maybe (EpToken "{"), Maybe (EpToken "}"))
type instance XConPat GhcRn = NoExtField
type instance XConPat GhcTc = ConPatTc
type instance XViewPat GhcPs = TokRarrow
type instance XViewPat GhcRn = Maybe (HsExpr GhcRn)
-- The @HsExpr GhcRn@ gives an inverse to the view function.
-- This is used for overloaded lists in particular.
-- See Note [Invertible view patterns] in GHC.Tc.TyCl.PatSyn.
type instance XViewPat GhcTc = Type
-- Overall type of the pattern
-- (= the argument type of the view function), for hsPatType.
type instance XSplicePat GhcPs = NoExtField
type instance XSplicePat GhcRn = HsUntypedSpliceResult (Pat GhcRn) -- See Note [Lifecycle of a splice] in GHC.Hs.Expr
type instance XSplicePat GhcTc = DataConCantHappen
type instance XLitPat (GhcPass _) = NoExtField
type instance XNPat GhcPs = EpToken "-"
type instance XNPat GhcRn = EpToken "-"
type instance XNPat GhcTc = Type
type instance XNPlusKPat GhcPs = EpToken "+"
type instance XNPlusKPat GhcRn = NoExtField
type instance XNPlusKPat GhcTc = Type
type instance XSigPat GhcPs = TokDcolon
type instance XSigPat GhcRn = NoExtField
type instance XSigPat GhcTc = Type
type instance XEmbTyPat GhcPs = EpToken "type"
type instance XEmbTyPat GhcRn = NoExtField
type instance XEmbTyPat GhcTc = Type
type instance XXPat GhcPs = DataConCantHappen
type instance XXPat GhcRn = HsPatExpansion (Pat GhcRn) (Pat GhcRn)
-- Original pattern and its desugaring/expansion.
-- See Note [Rebindable syntax and XXExprGhcRn].
type instance XXPat GhcTc = XXPatGhcTc
-- After typechecking, we add extra constructors: CoPat and XXExprGhcRn.
-- XXExprGhcRn allows us to handle RebindableSyntax in pattern position:
-- see "XXExpr GhcTc" for the counterpart in expressions.
type instance ConLikeP GhcPs = RdrName -- IdP GhcPs
type instance ConLikeP GhcRn = Name -- IdP GhcRn
type instance ConLikeP GhcTc = ConLike
type instance XConPatTyArg GhcPs = EpToken "@"
type instance XConPatTyArg GhcRn = NoExtField
type instance XConPatTyArg GhcTc = NoExtField
type instance XHsRecFields GhcPs = NoExtField
type instance XHsRecFields GhcRn = NoExtField
type instance XHsRecFields GhcTc = MultiplicityCheckCoercions
type instance XHsFieldBind _ = Maybe (EpToken "=")
-- The specificity of an invisible pattern from the parser is always
-- SpecifiedSpec. The specificity field supports code generated when deriving
-- newtype or via; see Note [Inferred invisible patterns].
type instance XInvisPat GhcPs = (EpToken "@", Specificity)
type instance XInvisPat GhcRn = Specificity
type instance XInvisPat GhcTc = Type
{- Note [Invisible binders in functions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
GHC Proposal #448 (section 1.5 Type arguments in lambda patterns) introduces
binders for invisible type arguments (@a-binders) in function equations and
lambdas, e.g.
1. {-# LANGUAGE TypeAbstractions #-}
id1 :: a -> a
id1 @t x = x :: t -- @t-binder on the LHS of a function equation
2. {-# LANGUAGE TypeAbstractions #-}
ex :: (Int8, Int16)
ex = higherRank (\ @a x -> maxBound @a - x )
-- @a-binder in a lambda pattern in an argument
-- to a higher-order function
higherRank :: (forall a. (Num a, Bounded a) => a -> a) -> (Int8, Int16)
higherRank f = (f 42, f 42)
In the AST, invisible patterns are represented as InvisPat constructor inside of Pat:
data Pat p
= ...
| InvisPat (LHsType p)
...
Just like `BangPat`, the `Pat` data type allows `InvisPat` to appear in
nested positions. But this is often not allowed; e.g.
f @a x = rhs -- YES
f (@a,x) = rhs -- NO
g = do { @a <- e1; e2 } -- NO
h x = case x of { @a -> rhs } -- NO
Rather than excluding these things syntactically, we reject them in the renamer
(see `rn_pats_general`). This actually gives a better error message than we
would get if they were rejected in the parser.
Each pattern is either visible (not prefixed with @) or invisible (prefixed with @):
f :: forall a. forall b -> forall c. Int -> ...
f @a b @c x = ...
In this example, the arg-patterns are
1. InvisPat @a -- in the type sig: forall a.
2. VarPat b -- in the type sig: forall b ->
3. InvisPat @c -- in the type sig: forall c.
4. VarPat x -- in the type sig: Int ->
Invisible patterns are always type patterns, i.e. they are matched with
forall-bound type variables in the signature. Consequently, those variables (and
their binders) are erased during compilation, having no effect on program
execution at runtime.
Visible patterns, on the other hand, may be matched with ordinary function
arguments (Int ->) as well as required type arguments (forall b ->). This means
that a visible pattern may either be erased or retained, and we only find out in
the type checker, namely in tcMatchPats, where we match up all arg-patterns with
quantifiers from the type signature.
In other words, invisible patterns are always /erased/, while visible patterns
are sometimes /erased/ and sometimes /retained/.
The desugarer has no use for erased patterns, as the type checker generates
HsWrappers to bind the corresponding type variables. Erased patterns are simply
discarded inside tcMatchPats, where we know if visible pattern retained or erased.
-}
-- ---------------------------------------------------------------------
-- API Annotations types
data EpAnnSumPat = EpAnnSumPat
{ sumPatParens :: (EpaLocation, EpaLocation)
, sumPatVbarsBefore :: [EpToken "|"]
, sumPatVbarsAfter :: [EpToken "|"]
} deriving Data
instance NoAnn EpAnnSumPat where
noAnn = EpAnnSumPat (noAnn, noAnn) [] []
-- ---------------------------------------------------------------------
-- | Extension constructor for Pat, added after typechecking.
data XXPatGhcTc
= -- | Coercion Pattern (translation only)
--
-- During desugaring a (CoPat co pat) turns into a cast with 'co' on the
-- scrutinee, followed by a match on 'pat'.
CoPat
{ -- | Coercion Pattern
-- If co :: t1 ~ t2, p :: t2,
-- then (CoPat co p) :: t1
co_cpt_wrap :: HsWrapper
, -- | Why not LPat? Ans: existing locn will do
co_pat_inner :: Pat GhcTc
, -- | Type of whole pattern, t1
co_pat_ty :: Type
}
-- | Pattern expansion: original pattern, and desugared pattern,
-- for RebindableSyntax and other overloaded syntax such as OverloadedLists.
-- See Note [Rebindable syntax and XXExprGhcRn].
| ExpansionPat (Pat GhcRn) (Pat GhcTc)
-- See Note [Rebindable syntax and XXExprGhcRn].
data HsPatExpansion a b
= HsPatExpanded a b
deriving Data
-- | This is the extension field for ConPat, added after typechecking
-- It adds quite a few extra fields, to support elaboration of pattern matching.
data ConPatTc
= ConPatTc
{ -- | The universal arg types 1-1 with the universal
-- tyvars of the constructor/pattern synonym
-- Use (conLikeResTy pat_con cpt_arg_tys) to get
-- the type of the pattern
cpt_arg_tys :: [Type]
, -- | Existentially bound type variables
-- in correctly-scoped order e.g. [k:* x:k]
cpt_tvs :: [TyVar]
, -- | Ditto *coercion variables* and *dictionaries*
-- One reason for putting coercion variable here I think
-- is to ensure their kinds are zonked
cpt_dicts :: [EvVar]
, -- | Bindings involving those dictionaries
cpt_binds :: TcEvBinds
, -- | Extra wrapper to pass to the matcher
-- Only relevant for pattern-synonyms;
-- ignored for data cons
cpt_wrap :: HsWrapper
}
hsRecFieldId :: HsRecField GhcTc arg -> Id
hsRecFieldId = hsRecFieldSel
{-
************************************************************************
* *
* Printing patterns
* *
************************************************************************
-}
instance Outputable (HsTyPat p) => Outputable (HsConPatTyArg p) where
ppr (HsConPatTyArg _ ty) = char '@' <> ppr ty
instance (Outputable arg, Outputable (XRec p (HsRecField p arg)), XRec p RecFieldsDotDot ~ LocatedE RecFieldsDotDot)
=> Outputable (HsRecFields p arg) where
ppr (HsRecFields { rec_flds = flds, rec_dotdot = Nothing })
= braces (fsep (punctuate comma (map ppr flds)))
ppr (HsRecFields { rec_flds = flds, rec_dotdot = Just (unLoc -> RecFieldsDotDot n) })
= braces (fsep (punctuate comma (map ppr (take n flds) ++ [dotdot])))
where
dotdot = text ".." <+> whenPprDebug (ppr (drop n flds))
instance (Outputable p, OutputableBndr p, Outputable arg)
=> Outputable (HsFieldBind p arg) where
ppr (HsFieldBind { hfbLHS = f, hfbRHS = arg,
hfbPun = pun })
= pprPrefixOcc f <+> (ppUnless pun $ equals <+> ppr arg)
instance OutputableBndrId p => Outputable (Pat (GhcPass p)) where
ppr = pprPat
-- See Note [Rebindable syntax and XXExprGhcRn].
instance (Outputable a, Outputable b) => Outputable (HsPatExpansion a b) where
ppr (HsPatExpanded a b) = ifPprDebug (vcat [ppr a, ppr b]) (ppr a)
pprLPat :: (OutputableBndrId p) => LPat (GhcPass p) -> SDoc
pprLPat (L _ e) = pprPat e
-- | Print with type info if -dppr-debug is on
pprPatBndr :: OutputableBndr name => name -> SDoc
pprPatBndr var
= getPprDebug $ \case
True -> parens (pprBndr LambdaBind var) -- Could pass the site to pprPat
-- but is it worth it?
False -> pprPrefixOcc var
pprParendLPat :: (OutputableBndrId p)
=> PprPrec -> LPat (GhcPass p) -> SDoc
pprParendLPat p = pprParendPat p . unLoc
pprParendPat :: forall p. OutputableBndrId p
=> PprPrec
-> Pat (GhcPass p)
-> SDoc
pprParendPat p pat = sdocOption sdocPrintTypecheckerElaboration $ \ print_tc_elab ->
if need_parens print_tc_elab pat
then parens (pprPat pat)
else pprPat pat
where
need_parens print_tc_elab pat
| GhcTc <- ghcPass @p
, XPat (CoPat {}) <- pat
= print_tc_elab
| otherwise
= patNeedsParens p pat
-- For a CoPat we need parens if we are going to show it, which
-- we do if -fprint-typechecker-elaboration is on (c.f. pprHsWrapper)
-- But otherwise the CoPat is discarded, so it
-- is the pattern inside that matters. Sigh.
pprPat :: forall p. (OutputableBndrId p) => Pat (GhcPass p) -> SDoc
pprPat (VarPat _ lvar) = pprPatBndr (unLoc lvar)
pprPat (WildPat _) = char '_'
pprPat (LazyPat _ pat) = char '~' <> pprParendLPat appPrec pat
pprPat (BangPat _ pat) = char '!' <> pprParendLPat appPrec pat
pprPat (AsPat _ name pat) = hcat [pprPrefixOcc (unLoc name), char '@',
pprParendLPat appPrec pat]
pprPat (ViewPat _ expr pat) = hcat [pprLExpr expr, text " -> ", ppr pat]
pprPat (ParPat _ pat) = parens (ppr pat)
pprPat (LitPat _ s) = ppr s
pprPat (NPat _ l Nothing _) = ppr l
pprPat (NPat _ l (Just _) _) = char '-' <> ppr l
pprPat (NPlusKPat _ n k _ _ _) = hcat [ppr_n, char '+', ppr k]
where ppr_n = case ghcPass @p of
GhcPs -> ppr n
GhcRn -> ppr n
GhcTc -> ppr n
pprPat (SplicePat ext splice) =
case ghcPass @p of
GhcPs -> pprUntypedSplice True Nothing splice
GhcRn | HsUntypedSpliceNested n <- ext -> pprUntypedSplice True (Just n) splice
GhcRn | HsUntypedSpliceTop _ p <- ext -> ppr p
GhcTc -> dataConCantHappen ext
pprPat (SigPat _ pat ty) = ppr pat <+> dcolon <+> ppr ty
pprPat (ListPat _ pats) = brackets (interpp'SP pats)
pprPat (OrPat _ pats) = pprWithSemis ppr (NE.toList pats)
pprPat (TuplePat _ pats bx)
-- Special-case unary boxed tuples so that they are pretty-printed as
-- `MkSolo x`, not `(x)`
| [pat] <- pats
, Boxed <- bx
= hcat [text (mkTupleStr Boxed dataName 1), pprParendLPat appPrec pat]
| otherwise
= tupleParens (boxityTupleSort bx) (pprWithCommas ppr pats)
pprPat (SumPat _ pat alt arity) = sumParens (pprAlternative ppr pat alt arity)
pprPat (ConPat { pat_con = con
, pat_args = details
, pat_con_ext = ext
}
)
= case ghcPass @p of
GhcPs -> pprUserCon (unLoc con) details
GhcRn -> pprUserCon (unLoc con) details
GhcTc -> sdocOption sdocPrintTypecheckerElaboration $ \case
False -> pprUserCon (unLoc con) details
True ->
-- Tiresome; in 'GHC.Tc.Gen.Bind.tcRhs' we print out a typechecked Pat in an
-- error message, and we want to make sure it prints nicely
ppr con
<> braces (sep [ hsep (map pprPatBndr (tvs ++ dicts))
, ppr binds ])
<+> pprConArgs details
where ConPatTc { cpt_tvs = tvs
, cpt_dicts = dicts
, cpt_binds = binds
} = ext
pprPat (EmbTyPat _ tp) = text "type" <+> ppr tp
pprPat (InvisPat x tp) = char '@' <> delimit (ppr tp)
where
delimit
| inferred = braces
| needs_parens = parens
| otherwise = id
inferred = case ghcPass @p of
GhcPs -> snd x == InferredSpec
GhcRn -> x == InferredSpec
GhcTc -> False
needs_parens = hsTypeNeedsParens appPrec $ unLoc $ hstp_body tp
pprPat (XPat ext) = case ghcPass @p of
GhcRn -> case ext of
HsPatExpanded orig _ -> pprPat orig
GhcTc -> case ext of
CoPat co pat _ ->
pprHsWrapper co $ \parens ->
if parens
then pprParendPat appPrec pat
else pprPat pat
ExpansionPat orig _ -> pprPat orig
pprUserCon :: (OutputableBndr con, OutputableBndrId p,
Outputable (Anno (IdGhcP p)))
=> con -> HsConPatDetails (GhcPass p) -> SDoc
pprUserCon c (InfixCon p1 p2) = ppr p1 <+> pprInfixOcc c <+> ppr p2
pprUserCon c details = pprPrefixOcc c <+> pprConArgs details
pprConArgs :: (OutputableBndrId p,
Outputable (Anno (IdGhcP p)))
=> HsConPatDetails (GhcPass p) -> SDoc
pprConArgs (PrefixCon ts pats) = fsep (pprTyArgs ts : map (pprParendLPat appPrec) pats)
where pprTyArgs tyargs = fsep (map ppr tyargs)
pprConArgs (InfixCon p1 p2) = sep [ pprParendLPat appPrec p1
, pprParendLPat appPrec p2 ]
pprConArgs (RecCon rpats) = ppr rpats
{-
************************************************************************
* *
* Building patterns
* *
************************************************************************
-}
mkPrefixConPat :: DataCon ->
[LPat GhcTc] -> [Type] -> LPat GhcTc
-- Make a vanilla Prefix constructor pattern
mkPrefixConPat dc pats tys
= noLocA $ ConPat { pat_con = noLocA (RealDataCon dc)
, pat_args = PrefixCon [] pats
, pat_con_ext = ConPatTc
{ cpt_tvs = []
, cpt_dicts = []
, cpt_binds = emptyTcEvBinds
, cpt_arg_tys = tys
, cpt_wrap = idHsWrapper
}
}
mkNilPat :: Type -> LPat GhcTc
mkNilPat ty = mkPrefixConPat nilDataCon [] [ty]
mkCharLitPat :: SourceText -> Char -> LPat GhcTc
mkCharLitPat src c = mkPrefixConPat charDataCon
[noLocA $ LitPat noExtField (HsCharPrim src c)] []
{-
************************************************************************
* *
* Predicates for checking things about pattern-lists in EquationInfo *
* *
************************************************************************
\subsection[Pat-list-predicates]{Look for interesting things in patterns}
Unlike in the Wadler chapter, where patterns are either ``variables''
or ``constructors,'' here we distinguish between:
\begin{description}
\item[unfailable:]
Patterns that cannot fail to match: variables, wildcards, and lazy
patterns.
These are the irrefutable patterns; the two other categories
are refutable patterns.
\item[constructor:]
A non-literal constructor pattern (see next category).
\item[literal patterns:]
At least the numeric ones may be overloaded.
\end{description}
A pattern is in {\em exactly one} of the above three categories; `as'
patterns are treated specially, of course.
The 1.3 report defines what ``irrefutable'' and ``failure-free'' patterns are.
-}
isBangedLPat :: LPat (GhcPass p) -> Bool
isBangedLPat = isBangedPat . unLoc
isBangedPat :: Pat (GhcPass p) -> Bool
isBangedPat (ParPat _ p) = isBangedLPat p
isBangedPat (BangPat {}) = True
isBangedPat _ = False
looksLazyPatBind :: HsBind GhcTc -> Bool
-- Returns True of anything *except*
-- a StrictHsBind (as above) or
-- a VarPat
-- In particular, returns True of a pattern binding with a compound pattern, like (I# x)
-- Looks through AbsBinds
looksLazyPatBind (PatBind { pat_lhs = p })
= looksLazyLPat p
looksLazyPatBind (XHsBindsLR (AbsBinds { abs_binds = binds }))
= any (looksLazyPatBind . unLoc) binds
looksLazyPatBind _
= False
looksLazyLPat :: LPat (GhcPass p) -> Bool
looksLazyLPat = looksLazyPat . unLoc
looksLazyPat :: Pat (GhcPass p) -> Bool
looksLazyPat (ParPat _ p) = looksLazyLPat p
looksLazyPat (AsPat _ _ p) = looksLazyLPat p
looksLazyPat (BangPat {}) = False
looksLazyPat (VarPat {}) = False
looksLazyPat (WildPat {}) = False
looksLazyPat _ = True
{-
Note [-XStrict and irrefutability]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When -XStrict is enabled the rules for irrefutability are slightly modified.
Specifically, the pattern in a program like
do ~(Just hi) <- expr
cannot be considered irrefutable. The ~ here merely disables the bang that
-XStrict would usually apply, rendering the program equivalent to the following
without -XStrict
do Just hi <- expr
To achieve make this pattern irrefutable with -XStrict the user would rather
need to write
do ~(~(Just hi)) <- expr
Failing to account for this resulted in #19027. To fix this isIrrefutableHsPat
takes care to check for two the irrefutability of the inner pattern when it
encounters a LazyPat and -XStrict is enabled.
See also Note [decideBangHood] in GHC.HsToCore.Utils.
-}
-- | @isIrrefutableHsPat p@ is true if matching against @p@ cannot fail
-- in the sense of falling through to the next pattern.
-- (NB: this is not quite the same as the (silly) defn
-- in 3.17.2 of the Haskell 98 report.)
--
-- If isIrrefutableHsPat returns 'True', the pattern is definitely irrefutable.
--
-- However, isIrrefutableHsPat returns 'False' if it's in doubt. It's a
-- best effort guess with the information we have available:
--
-- - we sometimes call 'isIrrefutableHsPat' from the renamer, in which case
-- we don't have type information to hand. This means we can't properly
-- handle GADTs, nor the result TyCon of COMPLETE pragmas.
-- - even when calling 'isIrrefutableHsPat' in the typechecker, we don't keep
-- track of any long distance information like the pattern-match checker does.
isIrrefutableHsPat
:: forall p
. IsPass p
=> Bool -- ^ Are we in a @-XStrict@ context?
-- See Note [-XStrict and irrefutability]
-> (ConLikeP (GhcPass p) -> Bool) -- ^ How to check whether the 'ConLike' in a
-- 'ConPat' pattern is irrefutable
-> LPat (GhcPass p) -- ^ The (located) pattern to check
-> Bool -- Is it irrefutable?
isIrrefutableHsPat is_strict irref_conLike pat = go (unLoc pat)
where
goL (L _ p) = go p
go :: Pat (GhcPass p) -> Bool
go (WildPat {}) = True
go (VarPat {}) = True
go (LazyPat _ p')
| is_strict
= isIrrefutableHsPat False irref_conLike p'
| otherwise = True
go (BangPat _ pat) = goL pat
go (ParPat _ pat) = goL pat
go (AsPat _ _ pat) = goL pat
go (ViewPat _ _ pat) = goL pat
go (SigPat _ pat _) = goL pat
go (TuplePat _ pats _) = all goL pats
go (OrPat _ pats) = any goL pats -- This is simplistic; see Note [Irrefutable or-patterns]
go (SumPat {}) = False -- See Note [Unboxed sum patterns aren't irrefutable]
go (ListPat {}) = False
-- See Note [Irrefutability of ConPat]
go (ConPat { pat_con = L _ con, pat_args = details })
= irref_conLike con
&& all goL (hsConPatArgs details)
go (LitPat {}) = False
go (NPat {}) = False
go (NPlusKPat {}) = False
-- We conservatively assume that no TH splices are irrefutable
-- since we cannot know until the splice is evaluated.
go (SplicePat {}) = False
-- The behavior of this case is unimportant, as GHC will throw an error shortly
-- after reaching this case for other reasons (see TcRnIllegalTypePattern).
go (EmbTyPat {}) = True
go (InvisPat {}) = True
go (XPat ext) = case ghcPass @p of
GhcRn -> case ext of
HsPatExpanded _ pat -> go pat
GhcTc -> case ext of
CoPat _ pat _ -> go pat
ExpansionPat _ pat -> go pat
-- | Is the pattern any of combination of:
--
-- - (pat)
-- - pat :: Type
-- - ~pat
-- - !pat
-- - x (variable)
isSimplePat :: LPat (GhcPass x) -> Maybe (IdP (GhcPass x))
isSimplePat p = case unLoc p of
ParPat _ x -> isSimplePat x
SigPat _ x _ -> isSimplePat x
LazyPat _ x -> isSimplePat x
BangPat _ x -> isSimplePat x
VarPat _ x -> Just (unLoc x)
_ -> Nothing
-- | Is this pattern boring from the perspective of pattern-match checking,
-- i.e. introduces no new pieces of long-distance information
-- which could influence pattern-match checking?
--
-- See Note [Boring patterns].
isBoringHsPat :: forall p. OutputableBndrId p => LPat (GhcPass p) -> Bool
-- NB: it's always safe to return 'False' in this function; that just means
-- performing potentially-redundant pattern-match checking.
isBoringHsPat = goL
where
goL :: forall p. OutputableBndrId p => LPat (GhcPass p) -> Bool
goL = go . unLoc
go :: forall p. OutputableBndrId p => Pat (GhcPass p) -> Bool
go = \case
WildPat {} -> True
VarPat {} -> True
LazyPat {} -> True
BangPat _ pat -> goL pat
ParPat _ pat -> goL pat
AsPat {} -> False -- the pattern x@y links x and y together,
-- which is a nontrivial piece of information
ViewPat _ _ pat -> goL pat
SigPat _ pat _ -> goL pat
TuplePat _ pats _ -> all goL pats
SumPat _ pat _ _ -> goL pat
ListPat _ pats -> all goL pats
ConPat { pat_con = con, pat_args = details }
-> case ghcPass @p of
GhcPs -> False -- conservative
GhcRn -> False -- conservative
GhcTc
| isVanillaConLike (unLoc con)
-> all goL (hsConPatArgs details)
| otherwise
-- A pattern match on a GADT constructor can introduce
-- type-level information (for example, T18572).
-> False
OrPat _ pats -> all goL pats
LitPat {} -> True
NPat {} -> True
NPlusKPat {} -> True
SplicePat {} -> False
EmbTyPat {} -> True
InvisPat {} -> True
XPat ext ->
case ghcPass @p of
GhcRn -> case ext of
HsPatExpanded _ pat -> go pat
GhcTc -> case ext of
CoPat _ pat _ -> go pat
ExpansionPat _ pat -> go pat
isPatSyn :: LPat GhcTc -> Bool
isPatSyn (L _ (ConPat {pat_con = L _ (PatSynCon{})})) = True
isPatSyn _ = False
{- Note [Irrefutability of ConPat]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A constructor pattern `ConPat { pat_con, pat_args }` is irrefutable under two
conditions:
Irref-ConLike: the constructor, pat_con, is itself irrefutable.
Irref-args : all of the argument patterns, pat_args, are irrefutable.
The (Irref-ConLike) condition can be stated as follows:
Irref-DataCon: a DataCon is irrefutable iff it is the only constructor of its
parent type constructor.
Irref-PatSyn: a PatSyn is irrefutable iff there is a COMPLETE pragma
containing this PatSyn as its sole member.
To understand this, let's consider some simple examples:
data A = MkA Int Bool
data BC = B Int | C
pattern P :: Maybe Int -> BC
pattern P mb_i <- ( ( \ case { B i -> Just i; C -> Nothing } ) -> mb_i )
{-# COMPLETE P #-}
In this case:
- the pattern 'A p1 p2' (for patterns 'p1 :: Int', 'p2 :: Bool') is irrefutable
precisely when both 'p1' and 'p2' are irrefutable (this is the same as
irrefutability of tuple patterns);
- neither of the patterns 'B p' (for any pattern 'p :: Int') or 'C' are irrefutable,
because the parent type constructor 'BC' contains more than one data constructor,
- the pattern 'P q', for a pattern 'q :: Maybe Int', is irrefutable precisely
when 'q' is irrefutable, due to the COMPLETE pragma on 'P'.
Wrinkle [Irrefutability and COMPLETE pragma result TyCons]
There is one subtlety in the Irref-PatSyn condition: COMPLETE pragmas may
optionally specify a result TyCon, as explained in Note [Implementation of COMPLETE pragmas]
in GHC.HsToCore.Pmc.Solver.
So, for a COMPLETE pragma with a result TyCon, we would need to compute
'completeMatchAppliesAtType' to ensure that the COMPLETE pragma is indeed
applicable. Doing so is not so straightforward in 'isIrrefutableHsPat', for
a couple of reasons:
1. 'isIrrefutableHsPat' is called from within the renamer, which means
we don't have the appropriate 'Type' to hand,
2. Even when 'isIrrefutableHsPat' is called from within the typechecker,
computing 'completeMatchAppliesAtType' for a 'ConPat' which might be
nested deep inside the top-level call, such as
( ( _ , P (x :: Int) ) :: ( Int, Int )
would require keeping track of types as we recur in 'isIrrefutableHsPat',
which would be much more involved and require duplicating code from
the pattern match checker (it performs this check using the notion
of "match variables", which we don't have in the typechecker).
Note [Irrefutable or-patterns]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When is an or-pattern ( p_1 ; ... ; p_n ) irrefutable? It certainly suffices
that individual pattern p_i is irrefutable, but it isn't necessary.
For example, with the datatype definition
data ABC = A | B | C
the or-pattern ( B ; C ; A ) is irrefutable. Similarly, one can take into
account COMPLETE pragmas, e.g. (P ; R ; Q) is irrefutable in the presence of
{-# COMPLETE P, Q, R #-}. This would extend Note [Irrefutability of ConPat] to
the case of disjunctions of constructor patterns.
For now, the function 'isIrrefutableHsPat' does not take into account these
additional complications, and considers an or-pattern irrefutable precisely when
any of the summands are irrefutable. This pessimistic behaviour is OK: the contract
of 'isIrrefutableHsPat' is that it can only return 'True' for definitely irrefutable
patterns, but may conservatively return 'False' in other cases.
The justification for this design choice is as follows:
1. Producing the correct answer in all cases would be rather difficult,
for example for a complex pattern such as ( P ; !( R ; S ; ( Q :: Ty ) ) ).
2. Irrefutable or-patterns aren't particularly common or useful, given that
(currently) or-patterns aren't allowed to bind variables.
Note [Unboxed sum patterns aren't irrefutable]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Unlike unboxed tuples, unboxed sums are *not* irrefutable when used as
patterns. A simple example that demonstrates this is from #14228:
pattern Just' x = (# x | #)
pattern Nothing' = (# | () #)
foo x = case x of
Nothing' -> putStrLn "nothing"
Just' -> putStrLn "just"
In foo, the pattern Nothing' (that is, (# x | #)) is certainly not irrefutable,
as does not match an unboxed sum value of the same arity—namely, (# | y #)
(covered by Just'). In fact, no unboxed sum pattern is irrefutable, since the
minimum unboxed sum arity is 2.
Failing to mark unboxed sum patterns as non-irrefutable would cause the Just'
case in foo to be unreachable, as GHC would mistakenly believe that Nothing'
is the only thing that could possibly be matched!
Note [Boring patterns]
~~~~~~~~~~~~~~~~~~~~~~
A pattern is called boring when no new information is gained upon successfully
matching on the pattern.
Some examples of boring patterns:
- x, for a variable x. We learn nothing about x upon matching this pattern.
- Just y. This pattern can fail, but if it matches, we don't learn anything
about y.
Some examples of non-boring patterns:
- x@(Just y). A match on this pattern introduces the fact that x is headed
by the constructor Just, which means that a subsequent pattern match such as
case x of { Just z -> ... }
should not be marked as incomplete.
- a@b. Matching on this pattern introduces a relation between 'a' and 'b',
which means that we shouldn't emit any warnings in code of the form
case a of
True -> case b of { True -> .. } -- no warning here!
False -> ...
- GADT patterns. For example, with the GADT
data G i where { MkGInt :: G Int }
a match on the pattern 'MkGInt' introduces type-level information:
foo :: G i -> i
foo MkGInt = 3
Here we learn that i ~ Int after matching on 'MkGInt', so this pattern
is not boring.
When a pattern is boring, and we are only interested in additional long-distance
information (not whether the pattern itself is fallible), we can skip pattern-match
checking entirely. Doing this saves about 10% allocations in test T11195.
This happens when we are checking pattern-matches in do-notation, for example:
do { x@(Just y) <- z
; ...
; return $ case x of { Just w -> ... } }
Here we *do not* want to emit a pattern-match warning on the first line for the
incomplete pattern-match, as incompleteness inside do-notation is handled
using MonadFail. However, we still want to propagate the fact that x is headed
by the 'Just' constructor, to avoid a pattern-match warning on the last line.
Note [Implementation of OrPatterns]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This Note describes the implementation of the extension -XOrPatterns.
* Proposal: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0522-or-patterns.rst
* Discussion: https://github.com/ghc-proposals/ghc-proposals/pull/522 and others
Parser
------
We parse an or-pattern `pat_1; ...; pat_k` into `OrPat [pat_1, ..., pat_k]`,
where `OrPat` is a constructor of `Pat` in Language.Haskell.Syntax.Pat.
We occasionally refer to any of the `pat_k` as "pattern alternatives" below.
The changes to the parser are as outlined in Section 8.1 of the proposal.
The main productions are
orpats -> exp | exp ';' orpats
aexp2 -> '(' orpats ')'
pat -> orpats
Renamer and typechecker
-----------------------
The typing rule for or-patterns in terms of pattern types is
Γ0, Σ0 ⊢ pat_i : τ ⤳ Γ0,Σi,Ψi
--------------------------------------------
Γ0, Σ0 ⊢ ( pat_1; ...; pat_n ) : τ ⤳ Γ0,Σ0,∅
(See the proposal for what a pattern type `Γ, Σ ⊢ pat : τ ⤳ Γ,Σ,Ψ` is.)
The main points
* None of the patterns may bind any variables, hence the same Γ0 in both input
and output.
* Any Given constraints bound by the pattern are discarded: the rule discards
the Σi returned by each pattern.
* Similarly any existentials Ψi bound by the pattern are discarded.
In GHC.Rename.Pat.rnPatAndThen, we reject visible term and type binders (i.e.
concerning Γ0).
Regarding the Givens Σi and existenials Ψi (i.e. invisible type binders)
introduced by the pattern alternatives `pat_i`, we discard them in
GHC.Tc.Gen.Pats.tc_pat in a manner similar to LazyPats;
see Note [Hopping the LIE in lazy patterns].
Why is it useful to allow Σi and Ψi only to discard them immediately after?
Consider
data T a where MkT :: forall a x. Num a => x -> T a
foo :: T a -> a
foo (MkT{}; MkT{}) = 3
We do want to allow matching on MkT{} in or-patterns, despite them invisibly
binding an existential type variable `x` and a new Given constraint `Num a`.
Clearly, `x` must be dead in the RHS of foo, because there is no field binder
that brings it to life, so no harm done.
But we must be careful not to solve the `Num a` Wanted constraint in the RHS of
foo with the Given constraint from the pattern alternatives, hence we are
Hopping the LIE.
Desugarer
---------
The desugaring of or-patterns is complicated by the fact that we have to avoid
exponential code blowup. Consider
f (LT; GT) (EQ; GT) = rhs1
f _ _ = rhs2
The naïve desugaring of or-patterns would explode every or-pattern, thus
f LT EQ = rhs1
f LT GT = rhs1
f GT EQ = rhs1
f GT GT = rhs1
f _ _ = rhs2
which leads to an exponential number of copies of `rhs1`.
Our current strategy, implemented in GHC.HsToCore.Match.tidy1, is to
desugar to LambdaCase and ViewPatterns,
f ((\case LT -> True; GT -> True; _ -> False) -> True)
((\case EQ -> True; GT -> True; _ -> False) -> True)
= rhs1
f _ _ = rhs2
The existing code for ViewPatterns makes sure that we do not duplicate `rhs1`
and the Simplifier will take care to turn this into efficient code.
Pattern-match checker
---------------------
The changes to the pattern-match checker are described in detail in Section 4.9
of the 2024 revision of the "Lower Your Guards" paper.
What follows is a brief summary of that change.
The pattern-match checker desugars patterns as well, into syntactic variants of
*guard trees* such as `PmMatch`, describing a single Match `f ps | grhss`.
It used to be that each such guard trees nicely captured the effects of pattern
matching `ps` in a conjunctive list of `PmGrd`s, each of which refines
the set of Nablas that reach the RHS of the clause.
`PmGrd` is the heart of the Lower Your Guards approach: it is compositional,
simple, and *non-recursive*, unlike or-patterns!
Conjunction is implemented with the `...Pmc.Check.leftToRight` combinator.
But to desugar or-patterns, we need to compose with `Pmc.Check.topToBottom`
to model first match semantics!
This was previously impossible in the pattern fragment, and indeed is
incompatible with the simple "list of `PmGrd`s" desugaring of patterns.
So our solution is to generalise "sequence of `PmGrd`" into a series-parallel
graph `GrdDag`, a special kind of DAG, where "series" corresponds to
left-to-right sequence and "parallel" corresponds to top-to-bottom or-pattern
alternatives. Example
f (LT; GT) True (EQ; GT) = rhs
desugars to
/- LT <- x -\ /- EQ <- z -\
. . True <- y . .-> rhs
\- GT <- x ./ \- GT <- z -/
Branching is GdAlt and models first-match semantics of or-patterns, and
sequencing is GdSeq.
We must take care of exponential explosion of Covered sets for long matches like
g (LT; GT) (LT; GT) ... True = 1
Fortunately, we can build on our existing throttling mechanism;
see Note [Countering exponential blowup] in GHC.HsToCore.Pmc.Check.
-}
-- | @'patNeedsParens' p pat@ returns 'True' if the pattern @pat@ needs
-- parentheses under precedence @p@.
patNeedsParens :: forall p. IsPass p => PprPrec -> Pat (GhcPass p) -> Bool
patNeedsParens p = go @p
where
-- Remark: go needs to be polymorphic, as we call it recursively
-- at a different GhcPass (see the case for GhcTc XPat below).
go :: forall q. IsPass q => Pat (GhcPass q) -> Bool
go (NPlusKPat {}) = p > opPrec
go (OrPat {}) = p > topPrec
go (SplicePat {}) = False
go (ConPat { pat_args = ds })
= conPatNeedsParens p ds
go (SigPat {}) = p >= sigPrec
go (ViewPat {}) = True
go (EmbTyPat {}) = True
go (InvisPat{}) = False
go (XPat ext) = case ghcPass @q of
GhcRn -> case ext of
HsPatExpanded orig _ -> go orig
GhcTc -> case ext of
CoPat _ inner _ -> go inner
ExpansionPat orig _ -> go orig
-- ^^^^^^^
-- NB: recursive call of go at a different GhcPass.
go (WildPat {}) = False
go (VarPat {}) = False
go (LazyPat {}) = False
go (BangPat {}) = False
go (ParPat {}) = False
go (AsPat {}) = False
-- Special-case unary boxed tuple applications so that they are
-- parenthesized as `Identity (Solo x)`, not `Identity Solo x` (#18612)
-- See Note [One-tuples] in GHC.Builtin.Types
go (TuplePat _ [_] Boxed)
= p >= appPrec
go (TuplePat{}) = False
go (SumPat {}) = False
go (ListPat {}) = False
go (LitPat _ l) = hsLitNeedsParens p l
go (NPat _ lol _ _) = hsOverLitNeedsParens p (unLoc lol)
-- | @'conPatNeedsParens' p cp@ returns 'True' if the constructor patterns @cp@
-- needs parentheses under precedence @p@.
conPatNeedsParens :: PprPrec -> HsConDetails t a b -> Bool
conPatNeedsParens p = go
where
go (PrefixCon ts args) = p >= appPrec && (not (null args) || not (null ts))
go (InfixCon {}) = p >= opPrec -- type args should be empty in this case
go (RecCon {}) = False
-- | Parenthesize a pattern without token information
gParPat :: forall p. IsPass p => LPat (GhcPass p) -> Pat (GhcPass p)
gParPat pat = ParPat x pat
where
x = case ghcPass @p of
GhcPs -> noAnn
GhcRn -> noExtField
GhcTc -> noExtField
-- | @'parenthesizePat' p pat@ checks if @'patNeedsParens' p pat@ is true, and
-- if so, surrounds @pat@ with a 'ParPat'. Otherwise, it simply returns @pat@.
parenthesizePat :: IsPass p
=> PprPrec
-> LPat (GhcPass p)
-> LPat (GhcPass p)
parenthesizePat p lpat@(L loc pat)
| patNeedsParens p pat = L loc (gParPat lpat)
| otherwise = lpat
{-
% Collect all EvVars from all constructor patterns
-}
-- May need to add more cases
collectEvVarsPats :: [Pat GhcTc] -> Bag EvVar
collectEvVarsPats = unionManyBags . map collectEvVarsPat
collectEvVarsLPat :: LPat GhcTc -> Bag EvVar
collectEvVarsLPat = collectEvVarsPat . unLoc
collectEvVarsPat :: Pat GhcTc -> Bag EvVar
collectEvVarsPat pat =
case pat of
LazyPat _ p -> collectEvVarsLPat p
AsPat _ _ p -> collectEvVarsLPat p
ParPat _ p -> collectEvVarsLPat p
BangPat _ p -> collectEvVarsLPat p
ListPat _ ps -> unionManyBags $ map collectEvVarsLPat ps
TuplePat _ ps _ -> unionManyBags $ map collectEvVarsLPat ps
OrPat _ ps -> unionManyBags $ map collectEvVarsLPat (NE.toList ps)
SumPat _ p _ _ -> collectEvVarsLPat p
ConPat
{ pat_args = args
, pat_con_ext = ConPatTc
{ cpt_dicts = dicts
}
}
-> unionBags (listToBag dicts)
$ unionManyBags
$ map collectEvVarsLPat
$ hsConPatArgs args
SigPat _ p _ -> collectEvVarsLPat p
XPat ext -> case ext of
CoPat _ p _ -> collectEvVarsPat p
ExpansionPat _ p -> collectEvVarsPat p
_other_pat -> emptyBag
{-
************************************************************************
* *
\subsection{Anno instances}
* *
************************************************************************
-}
type instance Anno (Pat (GhcPass p)) = SrcSpanAnnA
type instance Anno (HsOverLit (GhcPass p)) = EpAnnCO
type instance Anno ConLike = SrcSpanAnnN
type instance Anno (HsFieldBind lhs rhs) = SrcSpanAnnA
type instance Anno RecFieldsDotDot = EpaLocation