ghc-9.0.2: GHC/HsToCore/Utils.hs
{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998
Utilities for desugaring
This module exports some utility functions of no great interest.
-}
{-# LANGUAGE CPP #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE ViewPatterns #-}
-- | Utility functions for constructing Core syntax, principally for desugaring
module GHC.HsToCore.Utils (
EquationInfo(..),
firstPat, shiftEqns,
MatchResult (..), CaseAlt(..),
cantFailMatchResult, alwaysFailMatchResult,
extractMatchResult, combineMatchResults,
adjustMatchResultDs,
shareFailureHandler,
mkCoLetMatchResult, mkViewMatchResult, mkGuardedMatchResult,
matchCanFail, mkEvalMatchResult,
mkCoPrimCaseMatchResult, mkCoAlgCaseMatchResult, mkCoSynCaseMatchResult,
wrapBind, wrapBinds,
mkErrorAppDs, mkCoreAppDs, mkCoreAppsDs, mkCastDs,
seqVar,
-- LHs tuples
mkLHsPatTup, mkVanillaTuplePat,
mkBigLHsVarTupId, mkBigLHsTupId, mkBigLHsVarPatTupId, mkBigLHsPatTupId,
mkSelectorBinds,
selectSimpleMatchVarL, selectMatchVars, selectMatchVar,
mkOptTickBox, mkBinaryTickBox, decideBangHood,
isTrueLHsExpr
) where
#include "HsVersions.h"
import GHC.Prelude
import {-# SOURCE #-} GHC.HsToCore.Match ( matchSimply )
import {-# SOURCE #-} GHC.HsToCore.Expr ( dsLExpr )
import GHC.Hs
import GHC.Tc.Utils.Zonk
import GHC.Tc.Utils.TcType( tcSplitTyConApp )
import GHC.Core
import GHC.HsToCore.Monad
import GHC.Core.Utils
import GHC.Core.Make
import GHC.Types.Id.Make
import GHC.Types.Id
import GHC.Types.Literal
import GHC.Core.TyCon
import GHC.Core.DataCon
import GHC.Core.PatSyn
import GHC.Core.Type
import GHC.Core.Coercion
import GHC.Builtin.Types.Prim
import GHC.Builtin.Types
import GHC.Types.Basic
import GHC.Core.ConLike
import GHC.Types.Unique.Set
import GHC.Types.Unique.Supply
import GHC.Unit.Module
import GHC.Builtin.Names
import GHC.Types.Name( isInternalName )
import GHC.Utils.Outputable
import GHC.Types.SrcLoc
import GHC.Utils.Misc
import GHC.Driver.Session
import GHC.Data.FastString
import qualified GHC.LanguageExtensions as LangExt
import GHC.Tc.Types.Evidence
import Control.Monad ( zipWithM )
import Data.List.NonEmpty (NonEmpty(..))
import Data.Maybe (maybeToList)
import qualified Data.List.NonEmpty as NEL
{-
************************************************************************
* *
\subsection{ Selecting match variables}
* *
************************************************************************
We're about to match against some patterns. We want to make some
@Ids@ to use as match variables. If a pattern has an @Id@ readily at
hand, which should indeed be bound to the pattern as a whole, then use it;
otherwise, make one up. The multiplicity argument is chosen as the multiplicity
of the variable if it is made up.
-}
selectSimpleMatchVarL :: Mult -> LPat GhcTc -> DsM Id
-- Postcondition: the returned Id has an Internal Name
selectSimpleMatchVarL w pat = selectMatchVar w (unLoc pat)
-- (selectMatchVars ps tys) chooses variables of type tys
-- to use for matching ps against. If the pattern is a variable,
-- we try to use that, to save inventing lots of fresh variables.
--
-- OLD, but interesting note:
-- But even if it is a variable, its type might not match. Consider
-- data T a where
-- T1 :: Int -> T Int
-- T2 :: a -> T a
--
-- f :: T a -> a -> Int
-- f (T1 i) (x::Int) = x
-- f (T2 i) (y::a) = 0
-- Then we must not choose (x::Int) as the matching variable!
-- And nowadays we won't, because the (x::Int) will be wrapped in a CoPat
selectMatchVars :: [(Mult, Pat GhcTc)] -> DsM [Id]
-- Postcondition: the returned Ids have Internal Names
selectMatchVars ps = mapM (uncurry selectMatchVar) ps
selectMatchVar :: Mult -> Pat GhcTc -> DsM Id
-- Postcondition: the returned Id has an Internal Name
selectMatchVar w (BangPat _ pat) = selectMatchVar w (unLoc pat)
selectMatchVar w (LazyPat _ pat) = selectMatchVar w (unLoc pat)
selectMatchVar w (ParPat _ pat) = selectMatchVar w (unLoc pat)
selectMatchVar _w (VarPat _ var) = return (localiseId (unLoc var))
-- Note [Localise pattern binders]
--
-- Remark: when the pattern is a variable (or
-- an @-pattern), then w is the same as the
-- multiplicity stored within the variable
-- itself. It's easier to pull it from the
-- variable, so we ignore the multiplicity.
selectMatchVar _w (AsPat _ var _) = ASSERT( isManyDataConTy _w ) (return (unLoc var))
selectMatchVar w other_pat = newSysLocalDsNoLP w (hsPatType other_pat)
{- Note [Localise pattern binders]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider module M where
[Just a] = e
After renaming it looks like
module M where
[Just M.a] = e
We don't generalise, since it's a pattern binding, monomorphic, etc,
so after desugaring we may get something like
M.a = case e of (v:_) ->
case v of Just M.a -> M.a
Notice the "M.a" in the pattern; after all, it was in the original
pattern. However, after optimisation those pattern binders can become
let-binders, and then end up floated to top level. They have a
different *unique* by then (the simplifier is good about maintaining
proper scoping), but it's BAD to have two top-level bindings with the
External Name M.a, because that turns into two linker symbols for M.a.
It's quite rare for this to actually *happen* -- the only case I know
of is tc003 compiled with the 'hpc' way -- but that only makes it
all the more annoying.
To avoid this, we craftily call 'localiseId' in the desugarer, which
simply turns the External Name for the Id into an Internal one, but
doesn't change the unique. So the desugarer produces this:
M.a{r8} = case e of (v:_) ->
case v of Just a{r8} -> M.a{r8}
The unique is still 'r8', but the binding site in the pattern
is now an Internal Name. Now the simplifier's usual mechanisms
will propagate that Name to all the occurrence sites, as well as
un-shadowing it, so we'll get
M.a{r8} = case e of (v:_) ->
case v of Just a{s77} -> a{s77}
In fact, even GHC.Core.Subst.simplOptExpr will do this, and simpleOptExpr
runs on the output of the desugarer, so all is well by the end of
the desugaring pass.
See also Note [MatchIds] in GHC.HsToCore.Match
************************************************************************
* *
* type synonym EquationInfo and access functions for its pieces *
* *
************************************************************************
\subsection[EquationInfo-synonym]{@EquationInfo@: a useful synonym}
The ``equation info'' used by @match@ is relatively complicated and
worthy of a type synonym and a few handy functions.
-}
firstPat :: EquationInfo -> Pat GhcTc
firstPat eqn = ASSERT( notNull (eqn_pats eqn) ) head (eqn_pats eqn)
shiftEqns :: Functor f => f EquationInfo -> f EquationInfo
-- Drop the first pattern in each equation
shiftEqns = fmap $ \eqn -> eqn { eqn_pats = tail (eqn_pats eqn) }
-- Functions on MatchResult CoreExprs
matchCanFail :: MatchResult a -> Bool
matchCanFail (MR_Fallible {}) = True
matchCanFail (MR_Infallible {}) = False
alwaysFailMatchResult :: MatchResult CoreExpr
alwaysFailMatchResult = MR_Fallible $ \fail -> return fail
cantFailMatchResult :: CoreExpr -> MatchResult CoreExpr
cantFailMatchResult expr = MR_Infallible $ return expr
extractMatchResult :: MatchResult CoreExpr -> CoreExpr -> DsM CoreExpr
extractMatchResult match_result failure_expr =
runMatchResult
failure_expr
(shareFailureHandler match_result)
combineMatchResults :: MatchResult CoreExpr -> MatchResult CoreExpr -> MatchResult CoreExpr
combineMatchResults match_result1@(MR_Infallible _) _
= match_result1
combineMatchResults match_result1 match_result2 =
-- if the first pattern needs a failure handler (i.e. if it is fallible),
-- make it let-bind it bind it with `shareFailureHandler`.
case shareFailureHandler match_result1 of
MR_Infallible _ -> match_result1
MR_Fallible body_fn1 -> MR_Fallible $ \fail_expr ->
-- Before actually failing, try the next match arm.
body_fn1 =<< runMatchResult fail_expr match_result2
adjustMatchResultDs :: (a -> DsM b) -> MatchResult a -> MatchResult b
adjustMatchResultDs encl_fn = \case
MR_Infallible body_fn -> MR_Infallible $
encl_fn =<< body_fn
MR_Fallible body_fn -> MR_Fallible $ \fail ->
encl_fn =<< body_fn fail
wrapBinds :: [(Var,Var)] -> CoreExpr -> CoreExpr
wrapBinds [] e = e
wrapBinds ((new,old):prs) e = wrapBind new old (wrapBinds prs e)
wrapBind :: Var -> Var -> CoreExpr -> CoreExpr
wrapBind new old body -- NB: this function must deal with term
| new==old = body -- variables, type variables or coercion variables
| otherwise = Let (NonRec new (varToCoreExpr old)) body
seqVar :: Var -> CoreExpr -> CoreExpr
seqVar var body = mkDefaultCase (Var var) var body
mkCoLetMatchResult :: CoreBind -> MatchResult CoreExpr -> MatchResult CoreExpr
mkCoLetMatchResult bind = fmap (mkCoreLet bind)
-- (mkViewMatchResult var' viewExpr mr) makes the expression
-- let var' = viewExpr in mr
mkViewMatchResult :: Id -> CoreExpr -> MatchResult CoreExpr -> MatchResult CoreExpr
mkViewMatchResult var' viewExpr = fmap $ mkCoreLet $ NonRec var' viewExpr
mkEvalMatchResult :: Id -> Type -> MatchResult CoreExpr -> MatchResult CoreExpr
mkEvalMatchResult var ty = fmap $ \e ->
Case (Var var) var ty [(DEFAULT, [], e)]
mkGuardedMatchResult :: CoreExpr -> MatchResult CoreExpr -> MatchResult CoreExpr
mkGuardedMatchResult pred_expr mr = MR_Fallible $ \fail -> do
body <- runMatchResult fail mr
return (mkIfThenElse pred_expr body fail)
mkCoPrimCaseMatchResult :: Id -- Scrutinee
-> Type -- Type of the case
-> [(Literal, MatchResult CoreExpr)] -- Alternatives
-> MatchResult CoreExpr -- Literals are all unlifted
mkCoPrimCaseMatchResult var ty match_alts
= MR_Fallible mk_case
where
mk_case fail = do
alts <- mapM (mk_alt fail) sorted_alts
return (Case (Var var) var ty ((DEFAULT, [], fail) : alts))
sorted_alts = sortWith fst match_alts -- Right order for a Case
mk_alt fail (lit, mr)
= ASSERT( not (litIsLifted lit) )
do body <- runMatchResult fail mr
return (LitAlt lit, [], body)
data CaseAlt a = MkCaseAlt{ alt_pat :: a,
alt_bndrs :: [Var],
alt_wrapper :: HsWrapper,
alt_result :: MatchResult CoreExpr }
mkCoAlgCaseMatchResult
:: Id -- ^ Scrutinee
-> Type -- ^ Type of exp
-> NonEmpty (CaseAlt DataCon) -- ^ Alternatives (bndrs *include* tyvars, dicts)
-> MatchResult CoreExpr
mkCoAlgCaseMatchResult var ty match_alts
| isNewtype -- Newtype case; use a let
= ASSERT( null match_alts_tail && null (tail arg_ids1) )
mkCoLetMatchResult (NonRec arg_id1 newtype_rhs) match_result1
| otherwise
= mkDataConCase var ty match_alts
where
isNewtype = isNewTyCon (dataConTyCon (alt_pat alt1))
-- [Interesting: because of GADTs, we can't rely on the type of
-- the scrutinised Id to be sufficiently refined to have a TyCon in it]
alt1@MkCaseAlt{ alt_bndrs = arg_ids1, alt_result = match_result1 } :| match_alts_tail
= match_alts
-- Stuff for newtype
arg_id1 = ASSERT( notNull arg_ids1 ) head arg_ids1
var_ty = idType var
(tc, ty_args) = tcSplitTyConApp var_ty -- Don't look through newtypes
-- (not that splitTyConApp does, these days)
newtype_rhs = unwrapNewTypeBody tc ty_args (Var var)
mkCoSynCaseMatchResult :: Id -> Type -> CaseAlt PatSyn -> MatchResult CoreExpr
mkCoSynCaseMatchResult var ty alt = MR_Fallible $ mkPatSynCase var ty alt
mkPatSynCase :: Id -> Type -> CaseAlt PatSyn -> CoreExpr -> DsM CoreExpr
mkPatSynCase var ty alt fail = do
matcher <- dsLExpr $ mkLHsWrap wrapper $
nlHsTyApp matcher [getRuntimeRep ty, ty]
cont <- mkCoreLams bndrs <$> runMatchResult fail match_result
return $ mkCoreAppsDs (text "patsyn" <+> ppr var) matcher [Var var, ensure_unstrict cont, Lam voidArgId fail]
where
MkCaseAlt{ alt_pat = psyn,
alt_bndrs = bndrs,
alt_wrapper = wrapper,
alt_result = match_result} = alt
(matcher, needs_void_lam) = patSynMatcher psyn
-- See Note [Matchers and builders for pattern synonyms] in GHC.Core.PatSyn
-- on these extra Void# arguments
ensure_unstrict cont | needs_void_lam = Lam voidArgId cont
| otherwise = cont
mkDataConCase :: Id -> Type -> NonEmpty (CaseAlt DataCon) -> MatchResult CoreExpr
mkDataConCase var ty alts@(alt1 :| _)
= liftA2 mk_case mk_default mk_alts
-- The liftA2 combines the failability of all the alternatives and the default
where
con1 = alt_pat alt1
tycon = dataConTyCon con1
data_cons = tyConDataCons tycon
sorted_alts :: [ CaseAlt DataCon ]
sorted_alts = sortWith (dataConTag . alt_pat) $ NEL.toList alts
var_ty = idType var
(_, ty_args) = tcSplitTyConApp var_ty -- Don't look through newtypes
-- (not that splitTyConApp does, these days)
mk_case :: Maybe CoreAlt -> [CoreAlt] -> CoreExpr
mk_case def alts = mkWildCase (Var var) (idScaledType var) ty $
maybeToList def ++ alts
mk_alts :: MatchResult [CoreAlt]
mk_alts = traverse mk_alt sorted_alts
mk_alt :: CaseAlt DataCon -> MatchResult CoreAlt
mk_alt MkCaseAlt { alt_pat = con
, alt_bndrs = args
, alt_result = match_result } =
flip adjustMatchResultDs match_result $ \body -> do
case dataConBoxer con of
Nothing -> return (DataAlt con, args, body)
Just (DCB boxer) -> do
us <- newUniqueSupply
let (rep_ids, binds) = initUs_ us (boxer ty_args args)
let rep_ids' = map (scaleVarBy (idMult var)) rep_ids
-- Upholds the invariant that the binders of a case expression
-- must be scaled by the case multiplicity. See Note [Case
-- expression invariants] in CoreSyn.
return (DataAlt con, rep_ids', mkLets binds body)
mk_default :: MatchResult (Maybe CoreAlt)
mk_default
| exhaustive_case = MR_Infallible $ return Nothing
| otherwise = MR_Fallible $ \fail -> return $ Just (DEFAULT, [], fail)
mentioned_constructors = mkUniqSet $ map alt_pat sorted_alts
un_mentioned_constructors
= mkUniqSet data_cons `minusUniqSet` mentioned_constructors
exhaustive_case = isEmptyUniqSet un_mentioned_constructors
{-
************************************************************************
* *
\subsection{Desugarer's versions of some Core functions}
* *
************************************************************************
-}
mkErrorAppDs :: Id -- The error function
-> Type -- Type to which it should be applied
-> SDoc -- The error message string to pass
-> DsM CoreExpr
mkErrorAppDs err_id ty msg = do
src_loc <- getSrcSpanDs
dflags <- getDynFlags
let
full_msg = showSDoc dflags (hcat [ppr src_loc, vbar, msg])
core_msg = Lit (mkLitString full_msg)
-- mkLitString returns a result of type String#
return (mkApps (Var err_id) [Type (getRuntimeRep ty), Type ty, core_msg])
{-
'mkCoreAppDs' and 'mkCoreAppsDs' handle the special-case desugaring of 'seq'.
Note [Desugaring seq]
~~~~~~~~~~~~~~~~~~~~~
There are a few subtleties in the desugaring of `seq`:
1. (as described in #1031)
Consider,
f x y = x `seq` (y `seq` (# x,y #))
The [Core let/app invariant] means that, other things being equal, because
the argument to the outer 'seq' has an unlifted type, we'll use call-by-value thus:
f x y = case (y `seq` (# x,y #)) of v -> x `seq` v
But that is bad for two reasons:
(a) we now evaluate y before x, and
(b) we can't bind v to an unboxed pair
Seq is very, very special! So we recognise it right here, and desugar to
case x of _ -> case y of _ -> (# x,y #)
2. (as described in #2273)
Consider
let chp = case b of { True -> fst x; False -> 0 }
in chp `seq` ...chp...
Here the seq is designed to plug the space leak of retaining (snd x)
for too long.
If we rely on the ordinary inlining of seq, we'll get
let chp = case b of { True -> fst x; False -> 0 }
case chp of _ { I# -> ...chp... }
But since chp is cheap, and the case is an alluring contet, we'll
inline chp into the case scrutinee. Now there is only one use of chp,
so we'll inline a second copy. Alas, we've now ruined the purpose of
the seq, by re-introducing the space leak:
case (case b of {True -> fst x; False -> 0}) of
I# _ -> ...case b of {True -> fst x; False -> 0}...
We can try to avoid doing this by ensuring that the binder-swap in the
case happens, so we get his at an early stage:
case chp of chp2 { I# -> ...chp2... }
But this is fragile. The real culprit is the source program. Perhaps we
should have said explicitly
let !chp2 = chp in ...chp2...
But that's painful. So the code here does a little hack to make seq
more robust: a saturated application of 'seq' is turned *directly* into
the case expression, thus:
x `seq` e2 ==> case x of x -> e2 -- Note shadowing!
e1 `seq` e2 ==> case x of _ -> e2
So we desugar our example to:
let chp = case b of { True -> fst x; False -> 0 }
case chp of chp { I# -> ...chp... }
And now all is well.
The reason it's a hack is because if you define mySeq=seq, the hack
won't work on mySeq.
3. (as described in #2409)
The isLocalId ensures that we don't turn
True `seq` e
into
case True of True { ... }
which stupidly tries to bind the datacon 'True'.
-}
-- NB: Make sure the argument is not levity polymorphic
mkCoreAppDs :: SDoc -> CoreExpr -> CoreExpr -> CoreExpr
mkCoreAppDs _ (Var f `App` Type _r `App` Type ty1 `App` Type ty2 `App` arg1) arg2
| f `hasKey` seqIdKey -- Note [Desugaring seq], points (1) and (2)
= Case arg1 case_bndr ty2 [(DEFAULT,[],arg2)]
where
case_bndr = case arg1 of
Var v1 | isInternalName (idName v1)
-> v1 -- Note [Desugaring seq], points (2) and (3)
_ -> mkWildValBinder Many ty1
mkCoreAppDs s fun arg = mkCoreApp s fun arg -- The rest is done in GHC.Core.Make
-- NB: No argument can be levity polymorphic
mkCoreAppsDs :: SDoc -> CoreExpr -> [CoreExpr] -> CoreExpr
mkCoreAppsDs s fun args = foldl' (mkCoreAppDs s) fun args
mkCastDs :: CoreExpr -> Coercion -> CoreExpr
-- We define a desugarer-specific version of GHC.Core.Utils.mkCast,
-- because in the immediate output of the desugarer, we can have
-- apparently-mis-matched coercions: E.g.
-- let a = b
-- in (x :: a) |> (co :: b ~ Int)
-- Lint know about type-bindings for let and does not complain
-- So here we do not make the assertion checks that we make in
-- GHC.Core.Utils.mkCast; and we do less peephole optimisation too
mkCastDs e co | isReflCo co = e
| otherwise = Cast e co
{-
************************************************************************
* *
Tuples and selector bindings
* *
************************************************************************
This is used in various places to do with lazy patterns.
For each binder $b$ in the pattern, we create a binding:
\begin{verbatim}
b = case v of pat' -> b'
\end{verbatim}
where @pat'@ is @pat@ with each binder @b@ cloned into @b'@.
ToDo: making these bindings should really depend on whether there's
much work to be done per binding. If the pattern is complex, it
should be de-mangled once, into a tuple (and then selected from).
Otherwise the demangling can be in-line in the bindings (as here).
Boring! Boring! One error message per binder. The above ToDo is
even more helpful. Something very similar happens for pattern-bound
expressions.
Note [mkSelectorBinds]
~~~~~~~~~~~~~~~~~~~~~~
mkSelectorBinds is used to desugar a pattern binding {p = e},
in a binding group:
let { ...; p = e; ... } in body
where p binds x,y (this list of binders can be empty).
There are two cases.
------ Special case (A) -------
For a pattern that is just a variable,
let !x = e in body
==>
let x = e in x `seq` body
So we return the binding, with 'x' as the variable to seq.
------ Special case (B) -------
For a pattern that is essentially just a tuple:
* A product type, so cannot fail
* Only one level, so that
- generating multiple matches is fine
- seq'ing it evaluates the same as matching it
Then instead we generate
{ v = e
; x = case v of p -> x
; y = case v of p -> y }
with 'v' as the variable to force
------ General case (C) -------
In the general case we generate these bindings:
let { ...; p = e; ... } in body
==>
let { t = case e of p -> (x,y)
; x = case t of (x,y) -> x
; y = case t of (x,y) -> y }
in t `seq` body
Note that we return 't' as the variable to force if the pattern
is strict (i.e. with -XStrict or an outermost-bang-pattern)
Note that (A) /includes/ the situation where
* The pattern binds exactly one variable
let !(Just (Just x) = e in body
==>
let { t = case e of Just (Just v) -> Solo v
; v = case t of Solo v -> v }
in t `seq` body
The 'Solo' is a one-tuple; see Note [One-tuples] in GHC.Builtin.Types
Note that forcing 't' makes the pattern match happen,
but does not force 'v'.
* The pattern binds no variables
let !(True,False) = e in body
==>
let t = case e of (True,False) -> ()
in t `seq` body
------ Examples ----------
* !(_, (_, a)) = e
==>
t = case e of (_, (_, a)) -> Solo a
a = case t of Solo a -> a
Note that
- Forcing 't' will force the pattern to match fully;
e.g. will diverge if (snd e) is bottom
- But 'a' itself is not forced; it is wrapped in a one-tuple
(see Note [One-tuples] in GHC.Builtin.Types)
* !(Just x) = e
==>
t = case e of Just x -> Solo x
x = case t of Solo x -> x
Again, forcing 't' will fail if 'e' yields Nothing.
Note that even though this is rather general, the special cases
work out well:
* One binder, not -XStrict:
let Just (Just v) = e in body
==>
let t = case e of Just (Just v) -> Solo v
v = case t of Solo v -> v
in body
==>
let v = case (case e of Just (Just v) -> Solo v) of
Solo v -> v
in body
==>
let v = case e of Just (Just v) -> v
in body
* Non-recursive, -XStrict
let p = e in body
==>
let { t = case e of p -> (x,y)
; x = case t of (x,y) -> x
; y = case t of (x,y) -> x }
in t `seq` body
==> {inline seq, float x,y bindings inwards}
let t = case e of p -> (x,y) in
case t of t' ->
let { x = case t' of (x,y) -> x
; y = case t' of (x,y) -> x } in
body
==> {inline t, do case of case}
case e of p ->
let t = (x,y) in
let { x = case t' of (x,y) -> x
; y = case t' of (x,y) -> x } in
body
==> {case-cancellation, drop dead code}
case e of p -> body
* Special case (B) is there to avoid fruitlessly taking the tuple
apart and rebuilding it. For example, consider
{ K x y = e }
where K is a product constructor. Then general case (A) does:
{ t = case e of K x y -> (x,y)
; x = case t of (x,y) -> x
; y = case t of (x,y) -> y }
In the lazy case we can't optimise out this fruitless taking apart
and rebuilding. Instead (B) builds
{ v = e
; x = case v of K x y -> x
; y = case v of K x y -> y }
which is better.
-}
-- Remark: pattern selectors only occur in unrestricted patterns so we are free
-- to select Many as the multiplicity of every let-expression introduced.
mkSelectorBinds :: [[Tickish Id]] -- ^ ticks to add, possibly
-> LPat GhcTc -- ^ The pattern
-> CoreExpr -- ^ Expression to which the pattern is bound
-> DsM (Id,[(Id,CoreExpr)])
-- ^ Id the rhs is bound to, for desugaring strict
-- binds (see Note [Desugar Strict binds] in "GHC.HsToCore.Binds")
-- and all the desugared binds
mkSelectorBinds ticks pat val_expr
| L _ (VarPat _ (L _ v)) <- pat' -- Special case (A)
= return (v, [(v, val_expr)])
| is_flat_prod_lpat pat' -- Special case (B)
= do { let pat_ty = hsLPatType pat'
; val_var <- newSysLocalDsNoLP Many pat_ty
; let mk_bind tick bndr_var
-- (mk_bind sv bv) generates bv = case sv of { pat -> bv }
-- Remember, 'pat' binds 'bv'
= do { rhs_expr <- matchSimply (Var val_var) PatBindRhs pat'
(Var bndr_var)
(Var bndr_var) -- Neat hack
-- Neat hack: since 'pat' can't fail, the
-- "fail-expr" passed to matchSimply is not
-- used. But it /is/ used for its type, and for
-- that bndr_var is just the ticket.
; return (bndr_var, mkOptTickBox tick rhs_expr) }
; binds <- zipWithM mk_bind ticks' binders
; return ( val_var, (val_var, val_expr) : binds) }
| otherwise -- General case (C)
= do { tuple_var <- newSysLocalDs Many tuple_ty
; error_expr <- mkErrorAppDs pAT_ERROR_ID tuple_ty (ppr pat')
; tuple_expr <- matchSimply val_expr PatBindRhs pat
local_tuple error_expr
; let mk_tup_bind tick binder
= (binder, mkOptTickBox tick $
mkTupleSelector1 local_binders binder
tuple_var (Var tuple_var))
tup_binds = zipWith mk_tup_bind ticks' binders
; return (tuple_var, (tuple_var, tuple_expr) : tup_binds) }
where
pat' = strip_bangs pat
-- Strip the bangs before looking for case (A) or (B)
-- The incoming pattern may well have a bang on it
binders = collectPatBinders pat'
ticks' = ticks ++ repeat []
local_binders = map localiseId binders -- See Note [Localise pattern binders]
local_tuple = mkBigCoreVarTup1 binders
tuple_ty = exprType local_tuple
strip_bangs :: LPat (GhcPass p) -> LPat (GhcPass p)
-- Remove outermost bangs and parens
strip_bangs (L _ (ParPat _ p)) = strip_bangs p
strip_bangs (L _ (BangPat _ p)) = strip_bangs p
strip_bangs lp = lp
is_flat_prod_lpat :: LPat GhcTc -> Bool
is_flat_prod_lpat = is_flat_prod_pat . unLoc
is_flat_prod_pat :: Pat GhcTc -> Bool
is_flat_prod_pat (ParPat _ p) = is_flat_prod_lpat p
is_flat_prod_pat (TuplePat _ ps Boxed) = all is_triv_lpat ps
is_flat_prod_pat (ConPat { pat_con = L _ pcon
, pat_args = ps})
| RealDataCon con <- pcon
, isProductTyCon (dataConTyCon con)
= all is_triv_lpat (hsConPatArgs ps)
is_flat_prod_pat _ = False
is_triv_lpat :: LPat (GhcPass p) -> Bool
is_triv_lpat = is_triv_pat . unLoc
is_triv_pat :: Pat (GhcPass p) -> Bool
is_triv_pat (VarPat {}) = True
is_triv_pat (WildPat{}) = True
is_triv_pat (ParPat _ p) = is_triv_lpat p
is_triv_pat _ = False
{- *********************************************************************
* *
Creating big tuples and their types for full Haskell expressions.
They work over *Ids*, and create tuples replete with their types,
which is whey they are not in GHC.Hs.Utils.
* *
********************************************************************* -}
mkLHsPatTup :: [LPat GhcTc] -> LPat GhcTc
mkLHsPatTup [] = noLoc $ mkVanillaTuplePat [] Boxed
mkLHsPatTup [lpat] = lpat
mkLHsPatTup lpats = L (getLoc (head lpats)) $
mkVanillaTuplePat lpats Boxed
mkVanillaTuplePat :: [LPat GhcTc] -> Boxity -> Pat GhcTc
-- A vanilla tuple pattern simply gets its type from its sub-patterns
mkVanillaTuplePat pats box = TuplePat (map hsLPatType pats) pats box
-- The Big equivalents for the source tuple expressions
mkBigLHsVarTupId :: [Id] -> LHsExpr GhcTc
mkBigLHsVarTupId ids = mkBigLHsTupId (map nlHsVar ids)
mkBigLHsTupId :: [LHsExpr GhcTc] -> LHsExpr GhcTc
mkBigLHsTupId = mkChunkified mkLHsTupleExpr
-- The Big equivalents for the source tuple patterns
mkBigLHsVarPatTupId :: [Id] -> LPat GhcTc
mkBigLHsVarPatTupId bs = mkBigLHsPatTupId (map nlVarPat bs)
mkBigLHsPatTupId :: [LPat GhcTc] -> LPat GhcTc
mkBigLHsPatTupId = mkChunkified mkLHsPatTup
{-
************************************************************************
* *
Code for pattern-matching and other failures
* *
************************************************************************
Generally, we handle pattern matching failure like this: let-bind a
fail-variable, and use that variable if the thing fails:
\begin{verbatim}
let fail.33 = error "Help"
in
case x of
p1 -> ...
p2 -> fail.33
p3 -> fail.33
p4 -> ...
\end{verbatim}
Then
\begin{itemize}
\item
If the case can't fail, then there'll be no mention of @fail.33@, and the
simplifier will later discard it.
\item
If it can fail in only one way, then the simplifier will inline it.
\item
Only if it is used more than once will the let-binding remain.
\end{itemize}
There's a problem when the result of the case expression is of
unboxed type. Then the type of @fail.33@ is unboxed too, and
there is every chance that someone will change the let into a case:
\begin{verbatim}
case error "Help" of
fail.33 -> case ....
\end{verbatim}
which is of course utterly wrong. Rather than drop the condition that
only boxed types can be let-bound, we just turn the fail into a function
for the primitive case:
\begin{verbatim}
let fail.33 :: Void -> Int#
fail.33 = \_ -> error "Help"
in
case x of
p1 -> ...
p2 -> fail.33 void
p3 -> fail.33 void
p4 -> ...
\end{verbatim}
Now @fail.33@ is a function, so it can be let-bound.
We would *like* to use join points here; in fact, these "fail variables" are
paradigmatic join points! Sadly, this breaks pattern synonyms, which desugar as
CPS functions - i.e. they take "join points" as parameters. It's not impossible
to imagine extending our type system to allow passing join points around (very
carefully), but we certainly don't support it now.
99.99% of the time, the fail variables wind up as join points in short order
anyway, and the Void# doesn't do much harm.
-}
mkFailurePair :: CoreExpr -- Result type of the whole case expression
-> DsM (CoreBind, -- Binds the newly-created fail variable
-- to \ _ -> expression
CoreExpr) -- Fail variable applied to realWorld#
-- See Note [Failure thunks and CPR]
mkFailurePair expr
= do { fail_fun_var <- newFailLocalDs Many (voidPrimTy `mkVisFunTyMany` ty)
; fail_fun_arg <- newSysLocalDs Many voidPrimTy
; let real_arg = setOneShotLambda fail_fun_arg
; return (NonRec fail_fun_var (Lam real_arg expr),
App (Var fail_fun_var) (Var voidPrimId)) }
where
ty = exprType expr
-- Uses '@mkFailurePair@' to bind the failure case. Infallible matches have
-- neither a failure arg or failure "hole", so nothing is let-bound, and no
-- extraneous Core is produced.
shareFailureHandler :: MatchResult CoreExpr -> MatchResult CoreExpr
shareFailureHandler = \case
mr@(MR_Infallible _) -> mr
MR_Fallible match_fn -> MR_Fallible $ \fail_expr -> do
(fail_bind, shared_failure_handler) <- mkFailurePair fail_expr
body <- match_fn shared_failure_handler
-- Never unboxed, per the above, so always OK for `let` not `case`.
return $ Let fail_bind body
{-
Note [Failure thunks and CPR]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(This note predates join points as formal entities (hence the quotation marks).
We can't use actual join points here (see above); if we did, this would also
solve the CPR problem, since join points don't get CPR'd. See Note [Don't CPR
join points] in GHC.Core.Opt.WorkWrap.)
When we make a failure point we ensure that it
does not look like a thunk. Example:
let fail = \rw -> error "urk"
in case x of
[] -> fail realWorld#
(y:ys) -> case ys of
[] -> fail realWorld#
(z:zs) -> (y,z)
Reason: we know that a failure point is always a "join point" and is
entered at most once. Adding a dummy 'realWorld' token argument makes
it clear that sharing is not an issue. And that in turn makes it more
CPR-friendly. This matters a lot: if you don't get it right, you lose
the tail call property. For example, see #3403.
************************************************************************
* *
Ticks
* *
********************************************************************* -}
mkOptTickBox :: [Tickish Id] -> CoreExpr -> CoreExpr
mkOptTickBox = flip (foldr Tick)
mkBinaryTickBox :: Int -> Int -> CoreExpr -> DsM CoreExpr
mkBinaryTickBox ixT ixF e = do
uq <- newUnique
this_mod <- getModule
let bndr1 = mkSysLocal (fsLit "t1") uq One boolTy
-- It's always sufficient to pattern-match on a boolean with
-- multiplicity 'One'.
let
falseBox = Tick (HpcTick this_mod ixF) (Var falseDataConId)
trueBox = Tick (HpcTick this_mod ixT) (Var trueDataConId)
--
return $ Case e bndr1 boolTy
[ (DataAlt falseDataCon, [], falseBox)
, (DataAlt trueDataCon, [], trueBox)
]
-- *******************************************************************
{- Note [decideBangHood]
~~~~~~~~~~~~~~~~~~~~~~~~
With -XStrict we may make /outermost/ patterns more strict.
E.g.
let (Just x) = e in ...
==>
let !(Just x) = e in ...
and
f x = e
==>
f !x = e
This adjustment is done by decideBangHood,
* Just before constructing an EqnInfo, in GHC.HsToCore.Match
(matchWrapper and matchSinglePat)
* When desugaring a pattern-binding in GHC.HsToCore.Binds.dsHsBind
Note that it is /not/ done recursively. See the -XStrict
spec in the user manual.
Specifically:
~pat => pat -- when -XStrict (even if pat = ~pat')
!pat => !pat -- always
pat => !pat -- when -XStrict
pat => pat -- otherwise
-}
-- | Use -XStrict to add a ! or remove a ~
-- See Note [decideBangHood]
decideBangHood :: DynFlags
-> LPat GhcTc -- ^ Original pattern
-> LPat GhcTc -- Pattern with bang if necessary
decideBangHood dflags lpat
| not (xopt LangExt.Strict dflags)
= lpat
| otherwise -- -XStrict
= go lpat
where
go lp@(L l p)
= case p of
ParPat x p -> L l (ParPat x (go p))
LazyPat _ lp' -> lp'
BangPat _ _ -> lp
_ -> L l (BangPat noExtField lp)
isTrueLHsExpr :: LHsExpr GhcTc -> Maybe (CoreExpr -> DsM CoreExpr)
-- Returns Just {..} if we're sure that the expression is True
-- I.e. * 'True' datacon
-- * 'otherwise' Id
-- * Trivial wappings of these
-- The arguments to Just are any HsTicks that we have found,
-- because we still want to tick then, even it they are always evaluated.
isTrueLHsExpr (L _ (HsVar _ (L _ v)))
| v `hasKey` otherwiseIdKey
|| v `hasKey` getUnique trueDataConId
= Just return
-- trueDataConId doesn't have the same unique as trueDataCon
isTrueLHsExpr (L _ (HsConLikeOut _ con))
| con `hasKey` getUnique trueDataCon = Just return
isTrueLHsExpr (L _ (HsTick _ tickish e))
| Just ticks <- isTrueLHsExpr e
= Just (\x -> do wrapped <- ticks x
return (Tick tickish wrapped))
-- This encodes that the result is constant True for Hpc tick purposes;
-- which is specifically what isTrueLHsExpr is trying to find out.
isTrueLHsExpr (L _ (HsBinTick _ ixT _ e))
| Just ticks <- isTrueLHsExpr e
= Just (\x -> do e <- ticks x
this_mod <- getModule
return (Tick (HpcTick this_mod ixT) e))
isTrueLHsExpr (L _ (HsPar _ e)) = isTrueLHsExpr e
isTrueLHsExpr _ = Nothing