%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
Desugaring exporessions.
\begin{code}
module DsExpr ( dsExpr, dsLExpr, dsLocalBinds, dsValBinds, dsLit ) where
#include "HsVersions.h"
import Match
import MatchLit
import DsBinds
import DsGRHSs
import DsListComp
import DsUtils
import DsArrows
import DsMonad
import Name
import NameEnv
import FamInstEnv( topNormaliseType )
#ifdef GHCI
import DsMeta
#endif
import HsSyn
import TcType
import Coercion ( Role(..) )
import TcEvidence
import TcRnMonad
import Type
import CoreSyn
import CoreUtils
import CoreFVs
import MkCore
import DynFlags
import CostCentre
import Id
import Module
import VarSet
import VarEnv
import ConLike
import DataCon
import TysWiredIn
import BasicTypes
import Maybes
import SrcLoc
import Util
import Bag
import Outputable
import FastString
import Control.Monad
\end{code}
%************************************************************************
%* *
dsLocalBinds, dsValBinds
%* *
%************************************************************************
\begin{code}
dsLocalBinds :: HsLocalBinds Id -> CoreExpr -> DsM CoreExpr
dsLocalBinds EmptyLocalBinds body = return body
dsLocalBinds (HsValBinds binds) body = dsValBinds binds body
dsLocalBinds (HsIPBinds binds) body = dsIPBinds binds body
dsValBinds :: HsValBinds Id -> CoreExpr -> DsM CoreExpr
dsValBinds (ValBindsOut binds _) body = foldrM ds_val_bind body binds
dsValBinds (ValBindsIn _ _) _ = panic "dsValBinds ValBindsIn"
dsIPBinds :: HsIPBinds Id -> CoreExpr -> DsM CoreExpr
dsIPBinds (IPBinds ip_binds ev_binds) body
= do { ds_binds <- dsTcEvBinds ev_binds
; let inner = mkCoreLets ds_binds body
; foldrM ds_ip_bind inner ip_binds }
where
ds_ip_bind (L _ (IPBind ~(Right n) e)) body
= do e' <- dsLExpr e
return (Let (NonRec n e') body)
ds_val_bind :: (RecFlag, LHsBinds Id) -> CoreExpr -> DsM CoreExpr
ds_val_bind (NonRecursive, hsbinds) body
| [(_, L loc bind)] <- bagToList hsbinds,
strictMatchOnly bind
= putSrcSpanDs loc (dsStrictBind bind body)
ds_val_bind (_is_rec, binds) body
= do { prs <- dsLHsBinds binds
; ASSERT2( not (any (isUnLiftedType . idType . fst) prs), ppr _is_rec $$ ppr binds )
case prs of
[] -> return body
_ -> return (Let (Rec prs) body) }
dsStrictBind :: HsBind Id -> CoreExpr -> DsM CoreExpr
dsStrictBind (AbsBinds { abs_tvs = [], abs_ev_vars = []
, abs_exports = exports
, abs_ev_binds = ev_binds
, abs_binds = binds }) body
= do { let body1 = foldr bind_export body exports
bind_export export b = bindNonRec (abe_poly export) (Var (abe_mono export)) b
; body2 <- foldlBagM (\body (_, bind) -> dsStrictBind (unLoc bind) body)
body1 binds
; ds_binds <- dsTcEvBinds ev_binds
; return (mkCoreLets ds_binds body2) }
dsStrictBind (FunBind { fun_id = L _ fun, fun_matches = matches, fun_co_fn = co_fn
, fun_tick = tick, fun_infix = inf }) body
= do { (args, rhs) <- matchWrapper (FunRhs (idName fun ) inf) matches
; MASSERT( null args )
; MASSERT( isIdHsWrapper co_fn )
; let rhs' = mkOptTickBox tick rhs
; return (bindNonRec fun rhs' body) }
dsStrictBind (PatBind {pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty }) body
=
do { rhs <- dsGuarded grhss ty
; let upat = unLoc pat
eqn = EqnInfo { eqn_pats = [upat],
eqn_rhs = cantFailMatchResult body }
; var <- selectMatchVar upat
; result <- matchEquations PatBindRhs [var] [eqn] (exprType body)
; return (bindNonRec var rhs result) }
dsStrictBind bind body = pprPanic "dsLet: unlifted" (ppr bind $$ ppr body)
strictMatchOnly :: HsBind Id -> Bool
strictMatchOnly (AbsBinds { abs_binds = binds })
= anyBag (strictMatchOnly . unLoc . snd) binds
strictMatchOnly (PatBind { pat_lhs = lpat, pat_rhs_ty = ty })
= isUnLiftedType ty
|| isBangLPat lpat
|| any (isUnLiftedType . idType) (collectPatBinders lpat)
strictMatchOnly (FunBind { fun_id = L _ id })
= isUnLiftedType (idType id)
strictMatchOnly _ = False
\end{code}
%************************************************************************
%* *
\subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
%* *
%************************************************************************
\begin{code}
dsLExpr :: LHsExpr Id -> DsM CoreExpr
dsLExpr (L loc e) = putSrcSpanDs loc $ dsExpr e
dsExpr :: HsExpr Id -> DsM CoreExpr
dsExpr (HsPar e) = dsLExpr e
dsExpr (ExprWithTySigOut e _) = dsLExpr e
dsExpr (HsVar var) = return (varToCoreExpr var)
dsExpr (HsIPVar _) = panic "dsExpr: HsIPVar"
dsExpr (HsLit lit) = dsLit lit
dsExpr (HsOverLit lit) = dsOverLit lit
dsExpr (HsWrap co_fn e)
= do { e' <- dsExpr e
; wrapped_e <- dsHsWrapper co_fn e'
; dflags <- getDynFlags
; warnAboutIdentities dflags e' (exprType wrapped_e)
; return wrapped_e }
dsExpr (NegApp expr neg_expr)
= App <$> dsExpr neg_expr <*> dsLExpr expr
dsExpr (HsLam a_Match)
= uncurry mkLams <$> matchWrapper LambdaExpr a_Match
dsExpr (HsLamCase arg matches)
= do { arg_var <- newSysLocalDs arg
; ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
; return $ Lam arg_var $ bindNonRec discrim_var (Var arg_var) matching_code }
dsExpr (HsApp fun arg)
= mkCoreAppDs <$> dsLExpr fun <*> dsLExpr arg
dsExpr (HsUnboundVar _) = panic "dsExpr: HsUnboundVar"
\end{code}
Note [Desugaring vars]
~~~~~~~~~~~~~~~~~~~~~~
In one situation we can get a *coercion* variable in a HsVar, namely
the support method for an equality superclass:
class (a~b) => C a b where ...
instance (blah) => C (T a) (T b) where ..
Then we get
$dfCT :: forall ab. blah => C (T a) (T b)
$dfCT ab blah = MkC ($c$p1C a blah) ($cop a blah)
$c$p1C :: forall ab. blah => (T a ~ T b)
$c$p1C ab blah = let ...; g :: T a ~ T b = ... } in g
That 'g' in the 'in' part is an evidence variable, and when
converting to core it must become a CO.
Operator sections. At first it looks as if we can convert
\begin{verbatim}
(expr op)
\end{verbatim}
to
\begin{verbatim}
\x -> op expr x
\end{verbatim}
But no! expr might be a redex, and we can lose laziness badly this
way. Consider
\begin{verbatim}
map (expr op) xs
\end{verbatim}
for example. So we convert instead to
\begin{verbatim}
let y = expr in \x -> op y x
\end{verbatim}
If \tr{expr} is actually just a variable, say, then the simplifier
will sort it out.
\begin{code}
dsExpr (OpApp e1 op _ e2)
=
mkCoreAppsDs <$> dsLExpr op <*> mapM dsLExpr [e1, e2]
dsExpr (SectionL expr op)
= mkCoreAppDs <$> dsLExpr op <*> dsLExpr expr
dsExpr (SectionR op expr) = do
core_op <- dsLExpr op
let (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
y_core <- dsLExpr expr
x_id <- newSysLocalDs x_ty
y_id <- newSysLocalDs y_ty
return (bindNonRec y_id y_core $
Lam x_id (mkCoreAppsDs core_op [Var x_id, Var y_id]))
dsExpr (ExplicitTuple tup_args boxity)
= do { let go (lam_vars, args) (Missing ty)
= do { lam_var <- newSysLocalDs ty
; return (lam_var : lam_vars, Var lam_var : args) }
go (lam_vars, args) (Present expr)
= do { core_expr <- dsLExpr expr
; return (lam_vars, core_expr : args) }
; (lam_vars, args) <- foldM go ([], []) (reverse tup_args)
; return $ mkCoreLams lam_vars $
mkConApp (tupleCon (boxityNormalTupleSort boxity) (length tup_args))
(map (Type . exprType) args ++ args) }
dsExpr (HsSCC cc expr@(L loc _)) = do
mod_name <- getModule
count <- goptM Opt_ProfCountEntries
uniq <- newUnique
Tick (ProfNote (mkUserCC cc mod_name loc uniq) count True) <$> dsLExpr expr
dsExpr (HsCoreAnn _ expr)
= dsLExpr expr
dsExpr (HsCase discrim matches)
= do { core_discrim <- dsLExpr discrim
; ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
; return (bindNonRec discrim_var core_discrim matching_code) }
dsExpr (HsLet binds body) = do
body' <- dsLExpr body
dsLocalBinds binds body'
dsExpr (HsDo ListComp stmts res_ty) = dsListComp stmts res_ty
dsExpr (HsDo PArrComp stmts _) = dsPArrComp (map unLoc stmts)
dsExpr (HsDo DoExpr stmts _) = dsDo stmts
dsExpr (HsDo GhciStmtCtxt stmts _) = dsDo stmts
dsExpr (HsDo MDoExpr stmts _) = dsDo stmts
dsExpr (HsDo MonadComp stmts _) = dsMonadComp stmts
dsExpr (HsIf mb_fun guard_expr then_expr else_expr)
= do { pred <- dsLExpr guard_expr
; b1 <- dsLExpr then_expr
; b2 <- dsLExpr else_expr
; case mb_fun of
Just fun -> do { core_fun <- dsExpr fun
; return (mkCoreApps core_fun [pred,b1,b2]) }
Nothing -> return $ mkIfThenElse pred b1 b2 }
dsExpr (HsMultiIf res_ty alts)
| null alts
= mkErrorExpr
| otherwise
= do { match_result <- liftM (foldr1 combineMatchResults)
(mapM (dsGRHS IfAlt res_ty) alts)
; error_expr <- mkErrorExpr
; extractMatchResult match_result error_expr }
where
mkErrorExpr = mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID res_ty
(ptext (sLit "multi-way if"))
\end{code}
\noindent
\underline{\bf Various data construction things}
% ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\begin{code}
dsExpr (ExplicitList elt_ty wit xs)
= dsExplicitList elt_ty wit xs
dsExpr (ExplicitPArr ty []) = do
emptyP <- dsDPHBuiltin emptyPVar
return (Var emptyP `App` Type ty)
dsExpr (ExplicitPArr ty xs) = do
singletonP <- dsDPHBuiltin singletonPVar
appP <- dsDPHBuiltin appPVar
xs' <- mapM dsLExpr xs
return . foldr1 (binary appP) $ map (unary singletonP) xs'
where
unary fn x = mkApps (Var fn) [Type ty, x]
binary fn x y = mkApps (Var fn) [Type ty, x, y]
dsExpr (ArithSeq expr witness seq)
= case witness of
Nothing -> dsArithSeq expr seq
Just fl -> do {
; fl' <- dsExpr fl
; newArithSeq <- dsArithSeq expr seq
; return (App fl' newArithSeq)}
dsExpr (PArrSeq expr (FromTo from to))
= mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
dsExpr (PArrSeq expr (FromThenTo from thn to))
= mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
dsExpr (PArrSeq _ _)
= panic "DsExpr.dsExpr: Infinite parallel array!"
\end{code}
\noindent
\underline{\bf Record construction and update}
% ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For record construction we do this (assuming T has three arguments)
\begin{verbatim}
T { op2 = e }
==>
let err = /\a -> recConErr a
T (recConErr t1 "M.lhs/230/op1")
e
(recConErr t1 "M.lhs/230/op3")
\end{verbatim}
@recConErr@ then converts its arugment string into a proper message
before printing it as
\begin{verbatim}
M.lhs, line 230: missing field op1 was evaluated
\end{verbatim}
We also handle @C{}@ as valid construction syntax for an unlabelled
constructor @C@, setting all of @C@'s fields to bottom.
\begin{code}
dsExpr (RecordCon (L _ data_con_id) con_expr rbinds) = do
con_expr' <- dsExpr con_expr
let
(arg_tys, _) = tcSplitFunTys (exprType con_expr')
mk_arg (arg_ty, lbl)
= case findField (rec_flds rbinds) lbl of
(rhs:rhss) -> ASSERT( null rhss )
dsLExpr rhs
[] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (ppr lbl)
unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty empty
labels = dataConFieldLabels (idDataCon data_con_id)
con_args <- if null labels
then mapM unlabelled_bottom arg_tys
else mapM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels)
return (mkApps con_expr' con_args)
\end{code}
Record update is a little harder. Suppose we have the decl:
\begin{verbatim}
data T = T1 {op1, op2, op3 :: Int}
| T2 {op4, op2 :: Int}
| T3
\end{verbatim}
Then we translate as follows:
\begin{verbatim}
r { op2 = e }
===>
let op2 = e in
case r of
T1 op1 _ op3 -> T1 op1 op2 op3
T2 op4 _ -> T2 op4 op2
other -> recUpdError "M.lhs/230"
\end{verbatim}
It's important that we use the constructor Ids for @T1@, @T2@ etc on the
RHSs, and do not generate a Core constructor application directly, because the constructor
might do some argument-evaluation first; and may have to throw away some
dictionaries.
Note [Update for GADTs]
~~~~~~~~~~~~~~~~~~~~~~~
Consider
data T a b where
T1 { f1 :: a } :: T a Int
Then the wrapper function for T1 has type
$WT1 :: a -> T a Int
But if x::T a b, then
x { f1 = v } :: T a b (not T a Int!)
So we need to cast (T a Int) to (T a b). Sigh.
\begin{code}
dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
cons_to_upd in_inst_tys out_inst_tys)
| null fields
= dsLExpr record_expr
| otherwise
= ASSERT2( notNull cons_to_upd, ppr expr )
do { record_expr' <- dsLExpr record_expr
; field_binds' <- mapM ds_field fields
; let upd_fld_env :: NameEnv Id
upd_fld_env = mkNameEnv [(f,l) | (f,l,_) <- field_binds']
; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
; ([discrim_var], matching_code)
<- matchWrapper RecUpd (MG { mg_alts = alts, mg_arg_tys = [in_ty], mg_res_ty = out_ty })
; return (add_field_binds field_binds' $
bindNonRec discrim_var record_expr' matching_code) }
where
ds_field :: HsRecField Id (LHsExpr Id) -> DsM (Name, Id, CoreExpr)
ds_field rec_field = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
; let fld_id = unLoc (hsRecFieldId rec_field)
; lcl_id <- newSysLocalDs (idType fld_id)
; return (idName fld_id, lcl_id, rhs) }
add_field_binds [] expr = expr
add_field_binds ((_,b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)
tycon = dataConTyCon (head cons_to_upd)
in_ty = mkTyConApp tycon in_inst_tys
out_ty = mkFamilyTyConApp tycon out_inst_tys
mk_alt upd_fld_env con
= do { let (univ_tvs, ex_tvs, eq_spec,
theta, arg_tys, _) = dataConFullSig con
subst = mkTopTvSubst (univ_tvs `zip` in_inst_tys)
; eqs_vars <- mapM newPredVarDs (substTheta subst (eqSpecPreds eq_spec))
; theta_vars <- mapM newPredVarDs (substTheta subst theta)
; arg_ids <- newSysLocalsDs (substTys subst arg_tys)
; let val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
(dataConFieldLabels con) arg_ids
mk_val_arg field_name pat_arg_id
= nlHsVar (lookupNameEnv upd_fld_env field_name `orElse` pat_arg_id)
inst_con = noLoc $ HsWrap wrap (HsVar (dataConWrapId con))
wrap = mkWpEvVarApps theta_vars <.>
mkWpTyApps (mkTyVarTys ex_tvs) <.>
mkWpTyApps [ty | (tv, ty) <- univ_tvs `zip` out_inst_tys
, not (tv `elemVarEnv` wrap_subst) ]
rhs = foldl (\a b -> nlHsApp a b) inst_con val_args
wrap_co = mkTcTyConAppCo Nominal tycon
[ lookup tv ty | (tv,ty) <- univ_tvs `zip` out_inst_tys ]
lookup univ_tv ty = case lookupVarEnv wrap_subst univ_tv of
Just co' -> co'
Nothing -> mkTcReflCo Nominal ty
wrap_subst = mkVarEnv [ (tv, mkTcSymCo (mkTcCoVarCo eq_var))
| ((tv,_),eq_var) <- eq_spec `zip` eqs_vars ]
pat = noLoc $ ConPatOut { pat_con = noLoc (RealDataCon con)
, pat_tvs = ex_tvs
, pat_dicts = eqs_vars ++ theta_vars
, pat_binds = emptyTcEvBinds
, pat_args = PrefixCon $ map nlVarPat arg_ids
, pat_ty = in_ty
, pat_wrap = idHsWrapper }
; let wrapped_rhs | null eq_spec = rhs
| otherwise = mkLHsWrap (mkWpCast (mkTcSubCo wrap_co)) rhs
; return (mkSimpleMatch [pat] wrapped_rhs) }
\end{code}
Here is where we desugar the Template Haskell brackets and escapes
\begin{code}
dsExpr (HsRnBracketOut _ _) = panic "dsExpr HsRnBracketOut"
#ifdef GHCI
dsExpr (HsTcBracketOut x ps) = dsBracket x ps
#else
dsExpr (HsTcBracketOut _ _) = panic "dsExpr HsBracketOut"
#endif
dsExpr (HsSpliceE _ s) = pprPanic "dsExpr:splice" (ppr s)
dsExpr (HsProc pat cmd) = dsProcExpr pat cmd
\end{code}
Hpc Support
\begin{code}
dsExpr (HsTick tickish e) = do
e' <- dsLExpr e
return (Tick tickish e')
dsExpr (HsBinTick ixT ixF e) = do
e2 <- dsLExpr e
do { ASSERT(exprType e2 `eqType` boolTy)
mkBinaryTickBox ixT ixF e2
}
\end{code}
\begin{code}
dsExpr (ExprWithTySig {}) = panic "dsExpr:ExprWithTySig"
dsExpr (HsBracket {}) = panic "dsExpr:HsBracket"
dsExpr (HsQuasiQuoteE {}) = panic "dsExpr:HsQuasiQuoteE"
dsExpr (HsArrApp {}) = panic "dsExpr:HsArrApp"
dsExpr (HsArrForm {}) = panic "dsExpr:HsArrForm"
dsExpr (HsTickPragma {}) = panic "dsExpr:HsTickPragma"
dsExpr (EWildPat {}) = panic "dsExpr:EWildPat"
dsExpr (EAsPat {}) = panic "dsExpr:EAsPat"
dsExpr (EViewPat {}) = panic "dsExpr:EViewPat"
dsExpr (ELazyPat {}) = panic "dsExpr:ELazyPat"
dsExpr (HsType {}) = panic "dsExpr:HsType"
dsExpr (HsDo {}) = panic "dsExpr:HsDo"
findField :: [HsRecField Id arg] -> Name -> [arg]
findField rbinds lbl
= [rhs | HsRecField { hsRecFieldId = id, hsRecFieldArg = rhs } <- rbinds
, lbl == idName (unLoc id) ]
\end{code}
%--------------------------------------------------------------------
Note [Desugaring explicit lists]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Explicit lists are desugared in a cleverer way to prevent some
fruitless allocations. Essentially, whenever we see a list literal
[x_1, ..., x_n] we:
1. Find the tail of the list that can be allocated statically (say
[x_k, ..., x_n]) by later stages and ensure we desugar that
normally: this makes sure that we don't cause a code size increase
by having the cons in that expression fused (see later) and hence
being unable to statically allocate any more
2. For the prefix of the list which cannot be allocated statically,
say [x_1, ..., x_(k-1)], we turn it into an expression involving
build so that if we find any foldrs over it it will fuse away
entirely!
So in this example we will desugar to:
build (\c n -> x_1 `c` x_2 `c` .... `c` foldr c n [x_k, ..., x_n]
If fusion fails to occur then build will get inlined and (since we
defined a RULE for foldr (:) []) we will get back exactly the
normal desugaring for an explicit list.
This optimisation can be worth a lot: up to 25% of the total
allocation in some nofib programs. Specifically
Program Size Allocs Runtime CompTime
rewrite +0.0% -26.3% 0.02 -1.8%
ansi -0.3% -13.8% 0.00 +0.0%
lift +0.0% -8.7% 0.00 -2.3%
Of course, if rules aren't turned on then there is pretty much no
point doing this fancy stuff, and it may even be harmful.
=======> Note by SLPJ Dec 08.
I'm unconvinced that we should *ever* generate a build for an explicit
list. See the comments in GHC.Base about the foldr/cons rule, which
points out that (foldr k z [a,b,c]) may generate *much* less code than
(a `k` b `k` c `k` z).
Furthermore generating builds messes up the LHS of RULES.
Example: the foldr/single rule in GHC.Base
foldr k z [x] = ...
We do not want to generate a build invocation on the LHS of this RULE!
We fix this by disabling rules in rule LHSs, and testing that
flag here; see Note [Desugaring RULE left hand sides] in Desugar
To test this I've added a (static) flag -fsimple-list-literals, which
makes all list literals be generated via the simple route.
\begin{code}
dsExplicitList :: PostTcType -> Maybe (SyntaxExpr Id) -> [LHsExpr Id] -> DsM CoreExpr
dsExplicitList elt_ty Nothing xs
= do { dflags <- getDynFlags
; xs' <- mapM dsLExpr xs
; let (dynamic_prefix, static_suffix) = spanTail is_static xs'
; if gopt Opt_SimpleListLiterals dflags
|| not (gopt Opt_EnableRewriteRules dflags)
|| null dynamic_prefix
then return $ mkListExpr elt_ty xs'
else mkBuildExpr elt_ty (mkSplitExplicitList dynamic_prefix static_suffix) }
where
is_static :: CoreExpr -> Bool
is_static e = all is_static_var (varSetElems (exprFreeVars e))
is_static_var :: Var -> Bool
is_static_var v
| isId v = isExternalName (idName v)
| otherwise = False
mkSplitExplicitList prefix suffix (c, _) (n, n_ty)
= do { let suffix' = mkListExpr elt_ty suffix
; folded_suffix <- mkFoldrExpr elt_ty n_ty (Var c) (Var n) suffix'
; return (foldr (App . App (Var c)) folded_suffix prefix) }
dsExplicitList elt_ty (Just fln) xs
= do { fln' <- dsExpr fln
; list <- dsExplicitList elt_ty Nothing xs
; dflags <- getDynFlags
; return (App (App fln' (mkIntExprInt dflags (length xs))) list) }
spanTail :: (a -> Bool) -> [a] -> ([a], [a])
spanTail f xs = (reverse rejected, reverse satisfying)
where (satisfying, rejected) = span f $ reverse xs
dsArithSeq :: PostTcExpr -> (ArithSeqInfo Id) -> DsM CoreExpr
dsArithSeq expr (From from)
= App <$> dsExpr expr <*> dsLExpr from
dsArithSeq expr (FromTo from to)
= do dflags <- getDynFlags
warnAboutEmptyEnumerations dflags from Nothing to
expr' <- dsExpr expr
from' <- dsLExpr from
to' <- dsLExpr to
return $ mkApps expr' [from', to']
dsArithSeq expr (FromThen from thn)
= mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn]
dsArithSeq expr (FromThenTo from thn to)
= do dflags <- getDynFlags
warnAboutEmptyEnumerations dflags from (Just thn) to
expr' <- dsExpr expr
from' <- dsLExpr from
thn' <- dsLExpr thn
to' <- dsLExpr to
return $ mkApps expr' [from', thn', to']
\end{code}
Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
handled in DsListComp). Basically does the translation given in the
Haskell 98 report:
\begin{code}
dsDo :: [ExprLStmt Id] -> DsM CoreExpr
dsDo stmts
= goL stmts
where
goL [] = panic "dsDo"
goL (L loc stmt:lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
go _ (LastStmt body _) stmts
= ASSERT( null stmts ) dsLExpr body
go _ (BodyStmt rhs then_expr _ _) stmts
= do { rhs2 <- dsLExpr rhs
; warnDiscardedDoBindings rhs (exprType rhs2)
; then_expr2 <- dsExpr then_expr
; rest <- goL stmts
; return (mkApps then_expr2 [rhs2, rest]) }
go _ (LetStmt binds) stmts
= do { rest <- goL stmts
; dsLocalBinds binds rest }
go _ (BindStmt pat rhs bind_op fail_op) stmts
= do { body <- goL stmts
; rhs' <- dsLExpr rhs
; bind_op' <- dsExpr bind_op
; var <- selectSimpleMatchVarL pat
; let bind_ty = exprType bind_op'
res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
res1_ty (cantFailMatchResult body)
; match_code <- handle_failure pat match fail_op
; return (mkApps bind_op' [rhs', Lam var match_code]) }
go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
, recS_rec_ids = rec_ids, recS_ret_fn = return_op
, recS_mfix_fn = mfix_op, recS_bind_fn = bind_op
, recS_rec_rets = rec_rets, recS_ret_ty = body_ty }) stmts
= goL (new_bind_stmt : stmts)
where
new_bind_stmt = L loc $ BindStmt (mkBigLHsPatTup later_pats)
mfix_app bind_op
noSyntaxExpr
tup_ids = rec_ids ++ filterOut (`elem` rec_ids) later_ids
tup_ty = mkBigCoreTupTy (map idType tup_ids)
rec_tup_pats = map nlVarPat tup_ids
later_pats = rec_tup_pats
rets = map noLoc rec_rets
mfix_app = nlHsApp (noLoc mfix_op) mfix_arg
mfix_arg = noLoc $ HsLam (MG { mg_alts = [mkSimpleMatch [mfix_pat] body]
, mg_arg_tys = [tup_ty], mg_res_ty = body_ty })
mfix_pat = noLoc $ LazyPat $ mkBigLHsPatTup rec_tup_pats
body = noLoc $ HsDo DoExpr (rec_stmts ++ [ret_stmt]) body_ty
ret_app = nlHsApp (noLoc return_op) (mkBigLHsTup rets)
ret_stmt = noLoc $ mkLastStmt ret_app
go _ (ParStmt {}) _ = panic "dsDo ParStmt"
go _ (TransStmt {}) _ = panic "dsDo TransStmt"
handle_failure :: LPat Id -> MatchResult -> SyntaxExpr Id -> DsM CoreExpr
handle_failure pat match fail_op
| matchCanFail match
= do { fail_op' <- dsExpr fail_op
; dflags <- getDynFlags
; fail_msg <- mkStringExpr (mk_fail_msg dflags pat)
; extractMatchResult match (App fail_op' fail_msg) }
| otherwise
= extractMatchResult match (error "It can't fail")
mk_fail_msg :: DynFlags -> Located e -> String
mk_fail_msg dflags pat = "Pattern match failure in do expression at " ++
showPpr dflags (getLoc pat)
\end{code}
%************************************************************************
%* *
\subsection{Errors and contexts}
%* *
%************************************************************************
\begin{code}
warnDiscardedDoBindings :: LHsExpr Id -> Type -> DsM ()
warnDiscardedDoBindings rhs rhs_ty
| Just (m_ty, elt_ty) <- tcSplitAppTy_maybe rhs_ty
= do { warn_unused <- woptM Opt_WarnUnusedDoBind
; warn_wrong <- woptM Opt_WarnWrongDoBind
; when (warn_unused || warn_wrong) $
do { fam_inst_envs <- dsGetFamInstEnvs
; let norm_elt_ty = topNormaliseType fam_inst_envs elt_ty
; if warn_unused && not (isUnitTy norm_elt_ty)
then warnDs (badMonadBind rhs elt_ty
(ptext (sLit "-fno-warn-unused-do-bind")))
else
when warn_wrong $
do { case tcSplitAppTy_maybe norm_elt_ty of
Just (elt_m_ty, _)
| m_ty `eqType` topNormaliseType fam_inst_envs elt_m_ty
-> warnDs (badMonadBind rhs elt_ty
(ptext (sLit "-fno-warn-wrong-do-bind")))
_ -> return () } } }
| otherwise
= return ()
badMonadBind :: LHsExpr Id -> Type -> SDoc -> SDoc
badMonadBind rhs elt_ty flag_doc
= vcat [ hang (ptext (sLit "A do-notation statement discarded a result of type"))
2 (quotes (ppr elt_ty))
, hang (ptext (sLit "Suppress this warning by saying"))
2 (quotes $ ptext (sLit "_ <-") <+> ppr rhs)
, ptext (sLit "or by using the flag") <+> flag_doc ]
\end{code}