compiler/deSugar/Desugar.lhs

   1 %
   2 % (c) The University of Glasgow 2006
   3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
   4 %
   5
   6 The Desugarer: turning HsSyn into Core.
   7
   8 \begin{code}
   9 module Desugar ( deSugar, deSugarExpr ) where
  10
  11 import DynFlags
  12 import StaticFlags
  13 import HscTypes
  14 import HsSyn
  15 import TcRnTypes
  16 import MkIface
  17 import Id
  18 import Name
  19 import CoreSyn
  20 import CoreSubst
  21 import CoqPass ( coqPassCoreToString, coqPassCoreToCore )
  22 import PprCore
  23 import DsMonad
  24 import DsExpr
  25 import DsBinds
  26 import DsForeign
  27 import DsExpr           ()      -- Forces DsExpr to be compiled; DsBinds only
  28                                 -- depends on DsExpr.hi-boot.
  29 import Module
  30 import RdrName
  31 import NameSet
  32 import NameEnv
  33 import Rules
  34 import CoreMonad        ( endPass, CoreToDo(..) )
  35 import ErrUtils
  36 import Outputable
  37 import SrcLoc
  38 import Coverage
  39 import Util
  40 import MonadUtils
  41 import OrdList
  42 import Data.List
  43 import Data.IORef
  44 import PrelNames
  45 import UniqSupply
  46 import UniqFM
  47 import CoreFVs
  48 import Type
  49 import Coercion
  50 \end{code}
  51
  52 %************************************************************************
  53 %*                                                                      *
  54 %*              The main function: deSugar
  55 %*                                                                      *
  56 %************************************************************************
  57
  58 \begin{code}
  59
  60 -- | Main entry point to the desugarer.
  61 deSugar :: HscEnv -> ModLocation -> TcGblEnv -> IO (Messages, Maybe ModGuts)
  62 -- Can modify PCS by faulting in more declarations
  63
  64 deSugar hsc_env
  65         mod_loc
  66         tcg_env@(TcGblEnv { tcg_mod          = mod,
  67                             tcg_src          = hsc_src,
  68                             tcg_type_env     = type_env,
  69                             tcg_imports      = imports,
  70                             tcg_exports      = exports,
  71                             tcg_keep         = keep_var,
  72                             tcg_rdr_env      = rdr_env,
  73                             tcg_fix_env      = fix_env,
  74                             tcg_inst_env     = inst_env,
  75                             tcg_fam_inst_env = fam_inst_env,
  76                             tcg_warns        = warns,
  77                             tcg_anns         = anns,
  78                             tcg_binds        = binds,
  79                             tcg_imp_specs    = imp_specs,
  80                             tcg_ev_binds     = ev_binds,
  81                             tcg_fords        = fords,
  82                             tcg_rules        = rules,
  83                             tcg_vects        = vects,
  84                             tcg_insts        = insts,
  85                             tcg_fam_insts    = fam_insts,
  86                             tcg_hpc          = other_hpc_info })
  87
  88   = do  { let dflags = hsc_dflags hsc_env
  89         ; showPass dflags "Desugar"
  90
  91         -- Desugar the program
  92         ; let export_set = availsToNameSet exports
  93         ; let auto_scc = mkAutoScc dflags mod export_set
  94         ; let target = hscTarget dflags
  95         ; let hpcInfo = emptyHpcInfo other_hpc_info
  96         ; (msgs, mb_res)
  97               <- case target of
  98                    HscNothing ->
  99                        return (emptyMessages,
 100                                Just ([], nilOL, [], [], NoStubs, hpcInfo, emptyModBreaks, undefined, undefined
 101                                     , undefined
 102                                     , undefined
 103                                     , undefined
 104                                     , undefined
 105                                     , undefined
 106                                     , undefined
 107                                     , undefined
 108                                     , undefined
 109                                     , undefined
 110                                     , undefined
 111                                     , undefined
 112                                     , undefined
 113                                     , undefined
 114                                     , undefined
 115                                     , undefined
 116                                     , undefined
 117                                     , undefined
 118                                     , undefined
 119                                     , undefined
 120                                     , undefined
 121                                     , undefined
 122                                     , undefined
 123                                     , undefined
 124                                     , undefined
 125                                ))
 126                    _        -> do
 127                      (binds_cvr,ds_hpc_info, modBreaks)
 128                          <- if (opt_Hpc
 129                                   || target == HscInterpreted)
 130                                && (not (isHsBoot hsc_src))
 131                               then addCoverageTicksToBinds dflags mod mod_loc
 132                                                            (typeEnvTyCons type_env) binds
 133                               else return (binds, hpcInfo, emptyModBreaks)
 134                      initDs hsc_env mod rdr_env type_env $ do
 135                        do { ds_ev_binds <- dsEvBinds ev_binds
 136                           ; core_prs <- dsTopLHsBinds auto_scc binds_cvr
 137                           ; (spec_prs, spec_rules) <- dsImpSpecs imp_specs
 138                           ; (ds_fords, foreign_prs) <- dsForeigns fords
 139                           ; ds_rules <- mapMaybeM dsRule rules
 140                           ; ds_vects <- mapM dsVect vects
 141                           ; hetmet_brak <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_brak_name else return undefined
 142                           ; hetmet_esc  <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_esc_name  else return undefined
 143                           ; hetmet_flatten <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_flatten_name else return undefined
 144                           ; hetmet_unflatten <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_unflatten_name else return undefined
 145                           ; hetmet_flattened_id <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_flattened_id_name else return undefined
 146                           ; hetmet_PGArrow <- if dopt Opt_F_coqpass dflags then dsLookupTyCon hetmet_PGArrow_name else return undefined
 147                           ; hetmet_PGArrow_unit <- if dopt Opt_F_coqpass dflags then dsLookupTyCon hetmet_PGArrow_unit_name else return undefined
 148                           ; hetmet_PGArrow_tensor <- if dopt Opt_F_coqpass dflags then dsLookupTyCon hetmet_PGArrow_tensor_name else return undefined
 149                           ; hetmet_PGArrow_exponent <- if dopt Opt_F_coqpass dflags then dsLookupTyCon hetmet_PGArrow_exponent_name else return undefined
 150                           ; hetmet_pga_id <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_id_name else return undefined
 151                           ; hetmet_pga_comp <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_comp_name else return undefined
 152                           ; hetmet_pga_first <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_first_name else return undefined
 153                           ; hetmet_pga_second <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_second_name else return undefined
 154                           ; hetmet_pga_cancell <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_cancell_name else return undefined
 155                           ; hetmet_pga_cancelr <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_cancelr_name else return undefined
 156                           ; hetmet_pga_uncancell <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_uncancell_name else return undefined
 157                           ; hetmet_pga_uncancelr <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_uncancelr_name else return undefined
 158                           ; hetmet_pga_assoc <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_assoc_name else return undefined
 159                           ; hetmet_pga_unassoc <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_unassoc_name else return undefined
 160                           ; hetmet_pga_copy <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_copy_name else return undefined
 161                           ; hetmet_pga_drop <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_drop_name else return undefined
 162                           ; hetmet_pga_swap <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_swap_name else return undefined
 163                           ; hetmet_pga_applyl <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_applyl_name else return undefined
 164                           ; hetmet_pga_applyr <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_applyr_name else return undefined
 165                           ; hetmet_pga_curryl <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_curryl_name else return undefined
 166                           ; hetmet_pga_curryr <- if dopt Opt_F_coqpass dflags then dsLookupGlobalId hetmet_pga_curryr_name else return undefined
 167                           ; let hpc_init
 168                                   | opt_Hpc   = hpcInitCode mod ds_hpc_info
 169                                   | otherwise = empty
 170                           ; return ( ds_ev_binds
 171                                    , foreign_prs `appOL` core_prs `appOL` spec_prs
 172                                    , spec_rules ++ ds_rules, ds_vects
 173                                    , ds_fords `appendStubC` hpc_init
 174                                    , ds_hpc_info, modBreaks, hetmet_brak, hetmet_esc
 175                                    , hetmet_flatten
 176                                    , hetmet_unflatten
 177                                    , hetmet_flattened_id
 178                                    , hetmet_PGArrow
 179                                    , hetmet_PGArrow_unit
 180                                    , hetmet_PGArrow_tensor
 181                                    , hetmet_PGArrow_exponent
 182                                    , hetmet_pga_id
 183                                    , hetmet_pga_comp
 184                                    , hetmet_pga_first
 185                                    , hetmet_pga_second
 186                                    , hetmet_pga_cancell
 187                                    , hetmet_pga_cancelr
 188                                    , hetmet_pga_uncancell
 189                                    , hetmet_pga_uncancelr
 190                                    , hetmet_pga_assoc
 191                                    , hetmet_pga_unassoc
 192                                    , hetmet_pga_copy
 193                                    , hetmet_pga_drop
 194                                    , hetmet_pga_swap
 195                                    , hetmet_pga_applyl
 196                                    , hetmet_pga_applyr
 197                                    , hetmet_pga_curryl
 198                                    , hetmet_pga_curryr
 199                                    ) }
 200
 201         ; case mb_res of {
 202            Nothing -> return (msgs, Nothing) ;
 203            Just (ds_ev_binds, all_prs, all_rules, ds_vects, ds_fords,ds_hpc_info, modBreaks
 204                                    , hetmet_brak, hetmet_esc
 205                                    , hetmet_flatten
 206                                    , hetmet_unflatten
 207                                    , hetmet_flattened_id
 208                                    , hetmet_PGArrow
 209                                    , hetmet_PGArrow_unit
 210                                    , hetmet_PGArrow_tensor
 211                                    , hetmet_PGArrow_exponent
 212                                    , hetmet_pga_id
 213                                    , hetmet_pga_comp
 214                                    , hetmet_pga_first
 215                                    , hetmet_pga_second
 216                                    , hetmet_pga_cancell
 217                                    , hetmet_pga_cancelr
 218                                    , hetmet_pga_uncancell
 219                                    , hetmet_pga_uncancelr
 220                                    , hetmet_pga_assoc
 221                                    , hetmet_pga_unassoc
 222                                    , hetmet_pga_copy
 223                                    , hetmet_pga_drop
 224                                    , hetmet_pga_swap
 225                                    , hetmet_pga_applyl
 226                                    , hetmet_pga_applyr
 227                                    , hetmet_pga_curryl
 228                                    , hetmet_pga_curryr) -> do
 229
 230         {       -- Add export flags to bindings
 231           keep_alive <- readIORef keep_var
 232         ; let (rules_for_locals, rules_for_imps)
 233                    = partition isLocalRule all_rules
 234               final_prs = addExportFlagsAndRules target
 235                               export_set keep_alive rules_for_locals (fromOL all_prs)
 236
 237               final_pgm = simplifyBinds $ combineEvBinds ds_ev_binds final_prs
 238         -- Notice that we put the whole lot in a big Rec, even the foreign binds
 239         -- When compiling PrelFloat, which defines data Float = F# Float#
 240         -- we want F# to be in scope in the foreign marshalling code!
 241         -- You might think it doesn't matter, but the simplifier brings all top-level
 242         -- things into the in-scope set before simplifying; so we get no unfolding for F#!
 243
 244         -- Lint result if necessary, and print
 245         ; dumpIfSet_dyn dflags Opt_D_dump_ds "Desugared, before opt" $
 246                (vcat [ pprCoreBindings final_pgm
 247                      , pprRules rules_for_imps ])
 248
 249         ; ds_binds <- if dopt Opt_F_coqpass dflags
 250                        then do { us <- mkSplitUniqSupply '~'
 251                                ; return (coqPassCoreToCore
 252                                              hetmet_brak
 253                                              hetmet_esc
 254                                              hetmet_flatten
 255                                              hetmet_unflatten
 256                                              hetmet_flattened_id
 257                                              us
 258                                              final_pgm
 259                                              hetmet_PGArrow
 260                                              hetmet_PGArrow_unit
 261                                              hetmet_PGArrow_tensor
 262                                              hetmet_PGArrow_exponent
 263                                              hetmet_pga_id
 264                                              hetmet_pga_comp
 265                                              hetmet_pga_first
 266                                              hetmet_pga_second
 267                                              hetmet_pga_cancell
 268                                              hetmet_pga_cancelr
 269                                              hetmet_pga_uncancell
 270                                              hetmet_pga_uncancelr
 271                                              hetmet_pga_assoc
 272                                              hetmet_pga_unassoc
 273                                              hetmet_pga_copy
 274                                              hetmet_pga_drop
 275                                              hetmet_pga_swap
 276                                              hetmet_pga_applyl
 277                                              hetmet_pga_applyr
 278                                              hetmet_pga_curryl
 279                                              hetmet_pga_curryr)
 280                                }
 281                        else return final_pgm
 282
 283         ; (ds_binds', ds_rules_for_imps) <- simpleOptPgm dflags ds_binds rules_for_imps
 284                          -- The simpleOptPgm gets rid of type
 285                          -- bindings plus any stupid dead code
 286
 287         ; dumpIfSet_dyn dflags Opt_D_coqpass "Coq Pass Output" $ text $ coqPassCoreToString ds_binds'
 288
 289         ; dumpIfSet_dyn dflags Opt_D_dump_coqpass "After Coq Pass" (text $ showSDoc $ pprCoreBindings ds_binds')
 290
 291         ; endPass dflags CoreDesugar ds_binds' ds_rules_for_imps
 292
 293         ; let used_names = mkUsedNames tcg_env
 294         ; deps <- mkDependencies tcg_env
 295
 296         ; let mod_guts = ModGuts {
 297                 mg_module       = mod,
 298                 mg_boot         = isHsBoot hsc_src,
 299                 mg_exports      = exports,
 300                 mg_deps         = deps,
 301                 mg_used_names   = used_names,
 302                 mg_dir_imps     = imp_mods imports,
 303                 mg_rdr_env      = rdr_env,
 304                 mg_fix_env      = fix_env,
 305                 mg_warns        = warns,
 306                 mg_anns         = anns,
 307                 mg_types        = type_env,
 308                 mg_insts        = insts,
 309                 mg_fam_insts    = fam_insts,
 310                 mg_inst_env     = inst_env,
 311                 mg_fam_inst_env = fam_inst_env,
 312                 mg_rules        = ds_rules_for_imps,
 313                 mg_binds        = ds_binds',
 314                 mg_foreign      = ds_fords,
 315                 mg_hpc_info     = ds_hpc_info,
 316                 mg_modBreaks    = modBreaks,
 317                 mg_vect_decls   = ds_vects,
 318                 mg_vect_info    = noVectInfo
 319               }
 320         ; return (msgs, Just mod_guts)
 321         }}}
 322
 323 dsImpSpecs :: [LTcSpecPrag] -> DsM (OrdList (Id,CoreExpr), [CoreRule])
 324 dsImpSpecs imp_specs
 325  = do { spec_prs <- mapMaybeM (dsSpec Nothing) imp_specs
 326       ; let (spec_binds, spec_rules) = unzip spec_prs
 327       ; return (concatOL spec_binds, spec_rules) }
 328
 329 combineEvBinds :: [DsEvBind] -> [(Id,CoreExpr)] -> [CoreBind]
 330 -- Top-level bindings can include coercion bindings, but not via superclasses
 331 -- See Note [Top-level evidence]
 332 combineEvBinds [] val_prs
 333   = [Rec val_prs]
 334 combineEvBinds (LetEvBind (NonRec b r) : bs) val_prs
 335   | isId b    = combineEvBinds bs ((b,r):val_prs)
 336   | otherwise = NonRec b r : combineEvBinds bs val_prs
 337 combineEvBinds (LetEvBind (Rec prs) : bs) val_prs
 338   = combineEvBinds bs (prs ++ val_prs)
 339 combineEvBinds (CaseEvBind x _ _ : _) _
 340   = pprPanic "topEvBindPairs" (ppr x)
 341 \end{code}
 342
 343 Note [Top-level evidence]
 344 ~~~~~~~~~~~~~~~~~~~~~~~~~
 345 Top-level evidence bindings may be mutually recursive with the top-level value
 346 bindings, so we must put those in a Rec.  But we can't put them *all* in a Rec
 347 because the occurrence analyser doesn't teke account of type/coercion variables
 348 when computing dependencies.
 349
 350 So we pull out the type/coercion variables (which are in dependency order),
 351 and Rec the rest.
 352
 353
 354 \begin{code}
 355 mkAutoScc :: DynFlags -> Module -> NameSet -> AutoScc
 356 mkAutoScc dflags mod exports
 357   | not opt_SccProfilingOn      -- No profiling
 358   = NoSccs
 359     -- Add auto-scc on all top-level things
 360   | dopt Opt_AutoSccsOnAllToplevs dflags
 361   = AddSccs mod (\id -> not $ isDerivedOccName $ getOccName id)
 362     -- See #1641.  This is pretty yucky, but I can't see a better way
 363     -- to identify compiler-generated Ids, and at least this should
 364     -- catch them all.
 365     -- Only on exported things
 366   | dopt Opt_AutoSccsOnExportedToplevs dflags
 367   = AddSccs mod (\id -> idName id `elemNameSet` exports)
 368   | otherwise
 369   = NoSccs
 370
 371 deSugarExpr :: HscEnv
 372             -> Module -> GlobalRdrEnv -> TypeEnv
 373             -> LHsExpr Id
 374             -> IO (Messages, Maybe CoreExpr)
 375 -- Prints its own errors; returns Nothing if error occurred
 376
 377 deSugarExpr hsc_env this_mod rdr_env type_env tc_expr = do
 378     let dflags = hsc_dflags hsc_env
 379     showPass dflags "Desugar"
 380
 381     -- Do desugaring
 382     (msgs, mb_core_expr) <- initDs hsc_env this_mod rdr_env type_env $
 383                                    dsLExpr tc_expr
 384
 385     case mb_core_expr of
 386       Nothing   -> return (msgs, Nothing)
 387       Just expr -> do
 388
 389         -- Dump output
 390         dumpIfSet_dyn dflags Opt_D_dump_ds "Desugared" (pprCoreExpr expr)
 391
 392         return (msgs, Just expr)
 393 \end{code}
 394
 395 %************************************************************************
 396 %*                                                                      *
 397 %*              Add rules and export flags to binders
 398 %*                                                                      *
 399 %************************************************************************
 400
 401 \begin{code}
 402 addExportFlagsAndRules
 403     :: HscTarget -> NameSet -> NameSet -> [CoreRule]
 404     -> [(Id, t)] -> [(Id, t)]
 405 addExportFlagsAndRules target exports keep_alive rules prs
 406   = mapFst add_one prs
 407   where
 408     add_one bndr = add_rules name (add_export name bndr)
 409        where
 410          name = idName bndr
 411
 412     ---------- Rules --------
 413         -- See Note [Attach rules to local ids]
 414         -- NB: the binder might have some existing rules,
 415         -- arising from specialisation pragmas
 416     add_rules name bndr
 417         | Just rules <- lookupNameEnv rule_base name
 418         = bndr `addIdSpecialisations` rules
 419         | otherwise
 420         = bndr
 421     rule_base = extendRuleBaseList emptyRuleBase rules
 422
 423     ---------- Export flag --------
 424     -- See Note [Adding export flags]
 425     add_export name bndr
 426         | dont_discard name = setIdExported bndr
 427         | otherwise         = bndr
 428
 429     dont_discard :: Name -> Bool
 430     dont_discard name = is_exported name
 431                      || name `elemNameSet` keep_alive
 432
 433         -- In interactive mode, we don't want to discard any top-level
 434         -- entities at all (eg. do not inline them away during
 435         -- simplification), and retain them all in the TypeEnv so they are
 436         -- available from the command line.
 437         --
 438         -- isExternalName separates the user-defined top-level names from those
 439         -- introduced by the type checker.
 440     is_exported :: Name -> Bool
 441     is_exported | target == HscInterpreted = isExternalName
 442                 | otherwise                = (`elemNameSet` exports)
 443 \end{code}
 444
 445
 446 Note [Adding export flags]
 447 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 448 Set the no-discard flag if either
 449         a) the Id is exported
 450         b) it's mentioned in the RHS of an orphan rule
 451         c) it's in the keep-alive set
 452
 453 It means that the binding won't be discarded EVEN if the binding
 454 ends up being trivial (v = w) -- the simplifier would usually just
 455 substitute w for v throughout, but we don't apply the substitution to
 456 the rules (maybe we should?), so this substitution would make the rule
 457 bogus.
 458
 459 You might wonder why exported Ids aren't already marked as such;
 460 it's just because the type checker is rather busy already and
 461 I didn't want to pass in yet another mapping.
 462
 463 Note [Attach rules to local ids]
 464 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 465 Find the rules for locally-defined Ids; then we can attach them
 466 to the binders in the top-level bindings
 467
 468 Reason
 469   - It makes the rules easier to look up
 470   - It means that transformation rules and specialisations for
 471     locally defined Ids are handled uniformly
 472   - It keeps alive things that are referred to only from a rule
 473     (the occurrence analyser knows about rules attached to Ids)
 474   - It makes sure that, when we apply a rule, the free vars
 475     of the RHS are more likely to be in scope
 476   - The imported rules are carried in the in-scope set
 477     which is extended on each iteration by the new wave of
 478     local binders; any rules which aren't on the binding will
 479     thereby get dropped
 480
 481
 482 %************************************************************************
 483 %*                                                                      *
 484 %*              Desugaring transformation rules
 485 %*                                                                      *
 486 %************************************************************************
 487
 488 \begin{code}
 489 dsRule :: LRuleDecl Id -> DsM (Maybe CoreRule)
 490 dsRule (L loc (HsRule name act vars lhs _tv_lhs rhs _fv_rhs))
 491   = putSrcSpanDs loc $
 492     do  { let bndrs' = [var | RuleBndr (L _ var) <- vars]
 493
 494         ; lhs' <- unsetOptM Opt_EnableRewriteRules $
 495                   unsetOptM Opt_WarnIdentities $
 496                   dsLExpr lhs   -- Note [Desugaring RULE left hand sides]
 497
 498         ; rhs' <- dsLExpr rhs
 499
 500         -- Substitute the dict bindings eagerly,
 501         -- and take the body apart into a (f args) form
 502         ; case decomposeRuleLhs bndrs' lhs' of {
 503                 Left msg -> do { warnDs msg; return Nothing } ;
 504                 Right (final_bndrs, fn_id, args) -> do
 505
 506         { let is_local = isLocalId fn_id
 507                 -- NB: isLocalId is False of implicit Ids.  This is good becuase
 508                 -- we don't want to attach rules to the bindings of implicit Ids,
 509                 -- because they don't show up in the bindings until just before code gen
 510               fn_name   = idName fn_id
 511               final_rhs = simpleOptExpr rhs'    -- De-crap it
 512               rule      = mkRule False {- Not auto -} is_local
 513                                  name act fn_name final_bndrs args final_rhs
 514         ; return (Just rule)
 515         } } }
 516 \end{code}
 517
 518 Note [Desugaring RULE left hand sides]
 519 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 520 For the LHS of a RULE we do *not* want to desugar
 521     [x]   to    build (\cn. x `c` n)
 522 We want to leave explicit lists simply as chains
 523 of cons's. We can achieve that slightly indirectly by
 524 switching off EnableRewriteRules.  See DsExpr.dsExplicitList.
 525
 526 That keeps the desugaring of list comprehensions simple too.
 527
 528 Nor do we want to warn of conversion identities on the LHS;
 529 the rule is precisly to optimise them:
 530   {-# RULES "fromRational/id" fromRational = id :: Rational -> Rational #-}
 531
 532
 533 %************************************************************************
 534 %*                                                                      *
 535 %*              Desugaring vectorisation declarations
 536 %*                                                                      *
 537 %************************************************************************
 538
 539 \begin{code}
 540 dsVect :: LVectDecl Id -> DsM CoreVect
 541 dsVect (L loc (HsVect v rhs))
 542   = putSrcSpanDs loc $
 543     do { rhs' <- fmapMaybeM dsLExpr rhs
 544        ; return $ Vect (unLoc v) rhs'
 545            }
 546 -- dsVect (L loc (HsVect v Nothing))
 547 --   = return $ Vect v Nothing
 548 -- dsVect (L loc (HsVect v (Just rhs)))
 549 --   = putSrcSpanDs loc $
 550 --     do { rhs' <- dsLExpr rhs
 551 --        ; return $ Vect v (Just rhs')
 552 --       }
 553 \end{code}
 554
 555
 556
 557 \begin{code}
 558 --
 559 -- Simplification routines run before the flattener.  We can't use
 560 -- simpleOptPgm -- it doesn't preserve the order of subexpressions or
 561 -- let-binding groups.
 562 --
 563 simplify :: Expr CoreBndr -> Expr CoreBndr
 564 simplify (Var v)                 = Var v
 565 simplify (App e1 e2)             = App (simplify e1) (simplify e2)
 566 simplify (Lit lit)               = Lit lit
 567 simplify (Note note e)           = Note note (simplify e)
 568 simplify (Cast e co)             = if tcEqType (fst $ coercionKind co) (snd $ coercionKind co)
 569                                        then simplify e
 570                                        else Cast (simplify e) co
 571 simplify (Lam v e)               = Lam v (simplify e)
 572 simplify (Type t)                = Type t
 573 simplify (Case e b ty as)        = Case (simplify e) b ty (map (\(a,b,e) -> (a,b,simplify e)) as)
 574 simplify (Let bind body)         = foldr Let (simplify body) (simplifyBind bind)
 575
 576 simplifyBind :: Bind CoreBndr -> [Bind CoreBndr]
 577 simplifyBind (NonRec b e)             = [NonRec b (simplify e)]
 578 simplifyBind (Rec [])                 = []
 579 simplifyBind (Rec (rbs@((b,e):rbs'))) =
 580     if or $ map (\x -> elemUFM x (exprFreeIds e)) (map fst rbs)
 581     then [Rec (map (\(v,e) -> (v,simplify e)) rbs)]
 582     else (NonRec b (simplify e)):(simplifyBind $ Rec rbs')
 583
 584 simplifyBinds = concatMap simplifyBind
 585 \end{code}