2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section{Tidying up Core}
8 tidyCorePgm, tidyExpr, tidyCoreExpr,
12 #include "HsVersions.h"
14 import CmdLineOpts ( DynFlags, DynFlag(..), opt_OmitInterfacePragmas )
16 import CoreUnfold ( noUnfolding, mkTopUnfolding, okToUnfoldInHiFile )
17 import CoreFVs ( ruleSomeFreeVars, exprSomeFreeVars )
18 import PprCore ( pprIdCoreRule )
19 import CoreLint ( showPass, endPass )
20 import CoreUtils ( exprArity )
23 import Var ( Id, Var )
24 import Id ( idType, idInfo, idName, isExportedId,
25 idSpecialisation, idUnique,
26 mkVanillaGlobal, isLocalId,
27 isImplicitId, mkUserLocal, setIdInfo
29 import IdInfo {- loads of stuff -}
30 import NewDemand ( isBottomingSig, topSig )
31 import BasicTypes ( isNeverActive )
32 import Name ( getOccName, nameOccName, mkLocalName, mkGlobalName,
33 localiseName, isGlobalName, nameSrcLoc
35 import NameEnv ( filterNameEnv )
36 import OccName ( TidyOccEnv, initTidyOccEnv, tidyOccName )
37 import Type ( tidyTopType, tidyType, tidyTyVarBndr )
38 import Module ( Module, moduleName )
39 import HscTypes ( PersistentCompilerState( pcs_PRS ),
40 PersistentRenamerState( prsOrig ),
41 NameSupply( nsNames, nsUniqs ),
42 TypeEnv, extendTypeEnvList, typeEnvIds,
43 ModDetails(..), TyThing(..)
45 import FiniteMap ( lookupFM, addToFM )
46 import Maybes ( orElse )
47 import ErrUtils ( showPass, dumpIfSet_core )
48 import SrcLoc ( noSrcLoc )
49 import UniqFM ( mapUFM )
50 import UniqSupply ( splitUniqSupply, uniqFromSupply )
51 import List ( partition )
52 import Util ( mapAccumL )
53 import Maybe ( isJust )
59 %************************************************************************
61 \subsection{What goes on}
63 %************************************************************************
69 Step 1: Figure out external Ids
70 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
71 First we figure out which Ids are "external" Ids. An
72 "external" Id is one that is visible from outside the compilation
74 a) the user exported ones
75 b) ones mentioned in the unfoldings, workers,
76 or rules of externally-visible ones
77 This exercise takes a sweep of the bindings bottom to top. Actually,
78 in Step 2 we're also going to need to know which Ids should be
79 exported with their unfoldings, so we produce not an IdSet but an
83 Step 2: Tidy the program
84 ~~~~~~~~~~~~~~~~~~~~~~~~
85 Next we traverse the bindings top to bottom. For each *top-level*
88 1. Make it into a GlobalId
90 2. Give it a system-wide Unique.
91 [Even non-exported things need system-wide Uniques because the
92 byte-code generator builds a single Name->BCO symbol table.]
94 We use the NameSupply kept in the PersistentRenamerState as the
95 source of such system-wide uniques.
97 For external Ids, use the original-name cache in the NameSupply
98 to ensure that the unique assigned is the same as the Id had
99 in any previous compilation run.
101 3. If it's an external Id, make it have a global Name, otherwise
102 make it have a local Name.
103 This is used by the code generator to decide whether
104 to make the label externally visible
106 4. Give external Ids a "tidy" occurrence name. This means
107 we can print them in interface files without confusing
108 "x" (unique 5) with "x" (unique 10).
110 5. Give it its UTTERLY FINAL IdInfo; in ptic,
111 * Its IdDetails becomes VanillaGlobal, reflecting the fact that
112 from now on we regard it as a global, not local, Id
114 * its unfolding, if it should have one
116 * its arity, computed from the number of visible lambdas
118 * its CAF info, computed from what is free in its RHS
121 Finally, substitute these new top-level binders consistently
122 throughout, including in unfoldings. We also tidy binders in
123 RHSs, so that they print nicely in interfaces.
126 tidyCorePgm :: DynFlags -> Module
127 -> PersistentCompilerState
128 -> CgInfoEnv -- Information from the back end,
129 -- to be splatted into the IdInfo
131 -> IO (PersistentCompilerState, ModDetails)
133 tidyCorePgm dflags mod pcs cg_info_env
134 (ModDetails { md_types = env_tc, md_insts = insts_tc,
135 md_binds = binds_in, md_rules = orphans_in })
136 = do { showPass dflags "Tidy Core"
138 ; let ext_ids = findExternalSet binds_in orphans_in
139 ; let ext_rules = findExternalRules binds_in orphans_in ext_ids
141 -- We also make sure to avoid any exported binders. Consider
142 -- f{-u1-} = 1 -- Local decl
144 -- f{-u2-} = 2 -- Exported decl
146 -- The second exported decl must 'get' the name 'f', so we
147 -- have to put 'f' in the avoids list before we get to the first
148 -- decl. tidyTopId then does a no-op on exported binders.
149 ; let prs = pcs_PRS pcs
150 orig_ns = prsOrig prs
152 init_tidy_env = (orig_ns, initTidyOccEnv avoids, emptyVarEnv)
153 avoids = [getOccName name | bndr <- typeEnvIds env_tc,
154 let name = idName bndr,
156 -- In computing our "avoids" list, we must include
158 -- all things with global names (assigned once and for
159 -- all by the renamer)
160 -- since their names are "taken".
161 -- The type environment is a convenient source of such things.
163 ; let ((orig_ns', occ_env, subst_env), tidy_binds)
164 = mapAccumL (tidyTopBind mod ext_ids cg_info_env)
165 init_tidy_env binds_in
167 ; let tidy_rules = tidyIdRules (occ_env,subst_env) ext_rules
169 ; let prs' = prs { prsOrig = orig_ns' }
170 pcs' = pcs { pcs_PRS = prs' }
172 ; let final_ids = [ id
174 , id <- bindersOf bind
175 , isGlobalName (idName id)]
177 -- Dfuns are local Ids that might have
178 -- changed their unique during tidying
179 ; let lookup_dfun_id id = lookupVarEnv subst_env id `orElse`
180 pprPanic "lookup_dfun_id" (ppr id)
183 ; let tidy_type_env = mkFinalTypeEnv env_tc final_ids
184 tidy_dfun_ids = map lookup_dfun_id insts_tc
186 ; let tidy_details = ModDetails { md_types = tidy_type_env,
187 md_rules = tidy_rules,
188 md_insts = tidy_dfun_ids,
189 md_binds = tidy_binds }
191 ; endPass dflags "Tidy Core" Opt_D_dump_simpl tidy_binds
192 ; dumpIfSet_core dflags Opt_D_dump_simpl
194 (vcat (map pprIdCoreRule tidy_rules))
196 ; return (pcs', tidy_details)
199 tidyCoreExpr :: CoreExpr -> IO CoreExpr
200 tidyCoreExpr expr = return (tidyExpr emptyTidyEnv expr)
204 %************************************************************************
206 \subsection{Write a new interface file}
208 %************************************************************************
211 mkFinalTypeEnv :: TypeEnv -- From typechecker
215 mkFinalTypeEnv type_env final_ids
216 = extendTypeEnvList (filterNameEnv keep_it type_env)
219 -- The competed type environment is gotten from
220 -- a) keeping the types and classes
221 -- b) removing all Ids,
222 -- c) adding Ids with correct IdInfo, including unfoldings,
223 -- gotten from the bindings
224 -- From (c) we keep only those Ids with Global names;
225 -- the CoreTidy pass makes sure these are all and only
226 -- the externally-accessible ones
227 -- This truncates the type environment to include only the
228 -- exported Ids and things needed from them, which saves space
230 -- However, we do keep things like constructors, which should not appear
231 -- in interface files, because they are needed by importing modules when
232 -- using the compilation manager
234 -- We keep implicit Ids, because they won't appear
235 -- in the bindings from which final_ids are derived!
236 keep_it (AnId id) = isImplicitId id -- Remove all Ids except implicit ones
237 keep_it other = True -- Keep all TyCons and Classes
241 findExternalRules :: [CoreBind]
242 -> [IdCoreRule] -- Orphan rules
243 -> IdEnv a -- Ids that are exported, so we need their rules
245 -- The complete rules are gotten by combining
246 -- a) the orphan rules
247 -- b) rules embedded in the top-level Ids
248 findExternalRules binds orphan_rules ext_ids
249 | opt_OmitInterfacePragmas = []
251 = orphan_rules ++ local_rules
253 local_rules = [ (id, rule)
254 | id <- bindersOfBinds binds,
255 id `elemVarEnv` ext_ids,
256 rule <- rulesRules (idSpecialisation id),
257 not (isBuiltinRule rule)
258 -- We can't print builtin rules in interface files
259 -- Since they are built in, an importing module
260 -- will have access to them anyway
264 %************************************************************************
266 \subsection{Step 1: finding externals}
268 %************************************************************************
271 findExternalSet :: [CoreBind] -> [IdCoreRule]
272 -> IdEnv Bool -- In domain => external
273 -- Range = True <=> show unfolding
274 -- Step 1 from the notes above
275 findExternalSet binds orphan_rules
276 = foldr find init_needed binds
278 orphan_rule_ids :: IdSet
279 orphan_rule_ids = unionVarSets [ ruleSomeFreeVars isLocalId rule
280 | (_, rule) <- orphan_rules]
281 init_needed :: IdEnv Bool
282 init_needed = mapUFM (\_ -> False) orphan_rule_ids
283 -- The mapUFM is a bit cheesy. It is a cheap way
284 -- to turn the set of orphan_rule_ids, which we use to initialise
285 -- the sweep, into a mapping saying 'don't expose unfolding'
286 -- (When we come to the binding site we may change our mind, of course.)
288 find (NonRec id rhs) needed
289 | need_id needed id = addExternal (id,rhs) needed
291 find (Rec prs) needed = find_prs prs needed
293 -- For a recursive group we have to look for a fixed point
295 | null needed_prs = needed
296 | otherwise = find_prs other_prs new_needed
298 (needed_prs, other_prs) = partition (need_pr needed) prs
299 new_needed = foldr addExternal needed needed_prs
301 -- The 'needed' set contains the Ids that are needed by earlier
302 -- interface file emissions. If the Id isn't in this set, and isn't
303 -- exported, there's no need to emit anything
304 need_id needed_set id = id `elemVarEnv` needed_set || isExportedId id
305 need_pr needed_set (id,rhs) = need_id needed_set id
307 addExternal :: (Id,CoreExpr) -> IdEnv Bool -> IdEnv Bool
308 -- The Id is needed; extend the needed set
309 -- with it and its dependents (free vars etc)
310 addExternal (id,rhs) needed
311 = extendVarEnv (foldVarSet add_occ needed new_needed_ids)
314 add_occ id needed = extendVarEnv needed id False
315 -- "False" because we don't know we need the Id's unfolding
316 -- We'll override it later when we find the binding site
318 new_needed_ids | opt_OmitInterfacePragmas = emptyVarSet
319 | otherwise = worker_ids `unionVarSet`
320 unfold_ids `unionVarSet`
324 dont_inline = isNeverActive (inlinePragInfo idinfo)
325 loop_breaker = isLoopBreaker (occInfo idinfo)
326 bottoming_fn = isBottomingSig (newStrictnessInfo idinfo `orElse` topSig)
327 spec_ids = rulesRhsFreeVars (specInfo idinfo)
328 worker_info = workerInfo idinfo
330 -- Stuff to do with the Id's unfolding
331 -- The simplifier has put an up-to-date unfolding
332 -- in the IdInfo, but the RHS will do just as well
333 unfolding = unfoldingInfo idinfo
334 rhs_is_small = not (neverUnfold unfolding)
336 -- We leave the unfolding there even if there is a worker
337 -- In GHCI the unfolding is used by importers
338 -- When writing an interface file, we omit the unfolding
339 -- if there is a worker
340 show_unfold = not bottoming_fn && -- Not necessary
343 rhs_is_small && -- Small enough
344 okToUnfoldInHiFile rhs -- No casms etc
346 unfold_ids | show_unfold = exprSomeFreeVars isLocalId rhs
347 | otherwise = emptyVarSet
349 worker_ids = case worker_info of
350 HasWorker work_id _ -> unitVarSet work_id
351 otherwise -> emptyVarSet
355 %************************************************************************
357 \subsection{Step 2: top-level tidying}
359 %************************************************************************
363 type TopTidyEnv = (NameSupply, TidyOccEnv, VarEnv Var)
365 -- TopTidyEnv: when tidying we need to know
366 -- * ns: The NameSupply, containing a unique supply and any pre-ordained Names.
367 -- These may have arisen because the
368 -- renamer read in an interface file mentioning M.$wf, say,
369 -- and assigned it unique r77. If, on this compilation, we've
370 -- invented an Id whose name is $wf (but with a different unique)
371 -- we want to rename it to have unique r77, so that we can do easy
372 -- comparisons with stuff from the interface file
374 -- * occ_env: The TidyOccEnv, which tells us which local occurrences
377 -- * subst_env: A Var->Var mapping that substitutes the new Var for the old
382 tidyTopBind :: Module
383 -> IdEnv Bool -- Domain = Ids that should be external
384 -- True <=> their unfolding is external too
386 -> TopTidyEnv -> CoreBind
387 -> (TopTidyEnv, CoreBind)
389 tidyTopBind mod ext_ids cg_info_env top_tidy_env (NonRec bndr rhs)
390 = ((orig,occ,subst) , NonRec bndr' rhs')
392 ((orig,occ,subst), bndr')
393 = tidyTopBinder mod ext_ids cg_info_env
394 rec_tidy_env rhs' top_tidy_env bndr
395 rec_tidy_env = (occ,subst)
396 rhs' = tidyExpr rec_tidy_env rhs
398 tidyTopBind mod ext_ids cg_info_env top_tidy_env (Rec prs)
399 = (final_env, Rec prs')
401 (final_env@(_,occ,subst), prs') = mapAccumL do_one top_tidy_env prs
402 rec_tidy_env = (occ,subst)
404 do_one top_tidy_env (bndr,rhs)
405 = ((orig,occ,subst), (bndr',rhs'))
407 ((orig,occ,subst), bndr')
408 = tidyTopBinder mod ext_ids cg_info_env
409 rec_tidy_env rhs' top_tidy_env bndr
411 rhs' = tidyExpr rec_tidy_env rhs
413 tidyTopBinder :: Module -> IdEnv Bool -> CgInfoEnv
414 -> TidyEnv -> CoreExpr
415 -- The TidyEnv is used to tidy the IdInfo
416 -- The expr is the already-tided RHS
417 -- Both are knot-tied: don't look at them!
418 -> TopTidyEnv -> Id -> (TopTidyEnv, Id)
419 -- NB: tidyTopBinder doesn't affect the unique supply
421 tidyTopBinder mod ext_ids cg_info_env rec_tidy_env rhs
422 env@(ns2, occ_env2, subst_env2) id
423 -- This function is the heart of Step 2
424 -- The rec_tidy_env is the one to use for the IdInfo
425 -- It's necessary because when we are dealing with a recursive
426 -- group, a variable late in the group might be mentioned
427 -- in the IdInfo of one early in the group
429 -- The rhs is already tidied
431 = ((orig_env', occ_env', subst_env'), id')
433 (orig_env', occ_env', name') = tidyTopName mod ns2 occ_env2
436 ty' = tidyTopType (idType id)
437 idinfo = tidyTopIdInfo rec_tidy_env is_external
438 (idInfo id) unfolding arity
439 (lookupCgInfo cg_info_env name')
441 id' = mkVanillaGlobal name' ty' idinfo
443 subst_env' = extendVarEnv subst_env2 id id'
445 maybe_external = lookupVarEnv ext_ids id
446 is_external = isJust maybe_external
448 -- Usually the Id will have an accurate arity on it, because
449 -- the simplifier has just run, but not always.
450 -- One case I found was when the last thing the simplifier
451 -- did was to let-bind a non-atomic argument and then float
452 -- it to the top level. So it seems more robust just to
454 arity = exprArity rhs
455 unfolding = mkTopUnfolding rhs
459 -- tidyTopIdInfo creates the final IdInfo for top-level
460 -- binders. There are two delicate pieces:
462 -- * Arity. After CoreTidy, this arity must not change any more.
463 -- Indeed, CorePrep must eta expand where necessary to make
464 -- the manifest arity equal to the claimed arity.
466 -- * CAF info, which comes from the CoreToStg pass via a knot.
467 -- The CAF info will not be looked at by the downstream stuff:
468 -- it *generates* it, and knot-ties it back. It will only be
469 -- looked at by (a) MkIface when generating an interface file
470 -- (b) In GHCi, importing modules
471 -- Nevertheless, we add the info here so that it propagates to all
472 -- occurrences of the binders in RHSs, and hence to occurrences in
473 -- unfoldings, which are inside Ids imported by GHCi. Ditto RULES.
475 -- An alterative would be to do a second pass over the unfoldings
476 -- of Ids, and rules, right at the top, but that would be a pain.
478 tidyTopIdInfo tidy_env is_external idinfo unfold_info arity cg_info
479 | opt_OmitInterfacePragmas || not is_external
480 -- Only basic info if the Id isn't external, or if we don't have -O
483 | otherwise -- Add extra optimisation info
485 `setInlinePragInfo` inlinePragInfo idinfo
486 `setUnfoldingInfo` unfold_info
487 `setWorkerInfo` tidyWorker tidy_env (workerInfo idinfo)
488 -- NB: we throw away the Rules
489 -- They have already been extracted by findExternalRules
492 -- baasic_info is attached to every top-level binder
493 basic_info = vanillaIdInfo
496 `setNewStrictnessInfo` newStrictnessInfo idinfo
498 -- This is where we set names to local/global based on whether they really are
499 -- externally visible (see comment at the top of this module). If the name
500 -- was previously local, we have to give it a unique occurrence name if
501 -- we intend to globalise it.
502 tidyTopName mod ns occ_env external name
503 | global && internal = (ns, occ_env, localiseName name)
505 | global && external = (ns, occ_env, name)
506 -- Global names are assumed to have been allocated by the renamer,
507 -- so they already have the "right" unique
508 -- And it's a system-wide unique too
510 | local && internal = (ns_w_local, occ_env', new_local_name)
511 -- Even local, internal names must get a unique occurrence, because
512 -- if we do -split-objs we globalise the name later, in the code generator
514 -- Similarly, we must make sure it has a system-wide Unique, because
515 -- the byte-code generator builds a system-wide Name->BCO symbol table
517 | local && external = case lookupFM ns_names key of
518 Just orig -> (ns, occ_env', orig)
519 Nothing -> (ns_w_global, occ_env', new_global_name)
520 -- If we want to globalise a currently-local name, check
521 -- whether we have already assigned a unique for it.
522 -- If so, use it; if not, extend the table (ns_w_global).
523 -- This is needed when *re*-compiling a module in GHCi; we want to
524 -- use the same name for externally-visible things as we did before.
527 global = isGlobalName name
529 internal = not external
531 (occ_env', occ') = tidyOccName occ_env (nameOccName name)
532 key = (moduleName mod, occ')
533 ns_names = nsNames ns
534 ns_uniqs = nsUniqs ns
535 (us1, us2) = splitUniqSupply ns_uniqs
536 uniq = uniqFromSupply us1
537 loc = nameSrcLoc name
539 new_local_name = mkLocalName uniq occ' loc
540 new_global_name = mkGlobalName uniq mod occ' loc
542 ns_w_local = ns { nsUniqs = us2 }
543 ns_w_global = ns { nsUniqs = us2, nsNames = addToFM ns_names key new_global_name }
546 ------------ Worker --------------
547 tidyWorker tidy_env (HasWorker work_id wrap_arity)
548 = HasWorker (tidyVarOcc tidy_env work_id) wrap_arity
549 tidyWorker tidy_env other
552 ------------ Rules --------------
553 tidyIdRules :: TidyEnv -> [IdCoreRule] -> [IdCoreRule]
554 tidyIdRules env [] = []
555 tidyIdRules env ((fn,rule) : rules)
556 = tidyRule env rule =: \ rule ->
557 tidyIdRules env rules =: \ rules ->
558 ((tidyVarOcc env fn, rule) : rules)
560 tidyRule :: TidyEnv -> CoreRule -> CoreRule
561 tidyRule env rule@(BuiltinRule _ _) = rule
562 tidyRule env (Rule name act vars tpl_args rhs)
563 = tidyBndrs env vars =: \ (env', vars) ->
564 map (tidyExpr env') tpl_args =: \ tpl_args ->
565 (Rule name act vars tpl_args (tidyExpr env' rhs))
568 %************************************************************************
570 \subsection{Step 2: inner tidying
572 %************************************************************************
577 -> (TidyEnv, CoreBind)
579 tidyBind env (NonRec bndr rhs)
580 = tidyLetBndr env (bndr,rhs) =: \ (env', bndr') ->
581 (env', NonRec bndr' (tidyExpr env' rhs))
583 tidyBind env (Rec prs)
584 = mapAccumL tidyLetBndr env prs =: \ (env', bndrs') ->
585 map (tidyExpr env') (map snd prs) =: \ rhss' ->
586 (env', Rec (zip bndrs' rhss'))
589 tidyExpr env (Var v) = Var (tidyVarOcc env v)
590 tidyExpr env (Type ty) = Type (tidyType env ty)
591 tidyExpr env (Lit lit) = Lit lit
592 tidyExpr env (App f a) = App (tidyExpr env f) (tidyExpr env a)
593 tidyExpr env (Note n e) = Note (tidyNote env n) (tidyExpr env e)
595 tidyExpr env (Let b e)
596 = tidyBind env b =: \ (env', b') ->
597 Let b' (tidyExpr env' e)
599 tidyExpr env (Case e b alts)
600 = tidyBndr env b =: \ (env', b) ->
601 Case (tidyExpr env e) b (map (tidyAlt env') alts)
603 tidyExpr env (Lam b e)
604 = tidyBndr env b =: \ (env', b) ->
605 Lam b (tidyExpr env' e)
608 tidyAlt env (con, vs, rhs)
609 = tidyBndrs env vs =: \ (env', vs) ->
610 (con, vs, tidyExpr env' rhs)
612 tidyNote env (Coerce t1 t2) = Coerce (tidyType env t1) (tidyType env t2)
613 tidyNote env note = note
617 %************************************************************************
619 \subsection{Tidying up non-top-level binders}
621 %************************************************************************
624 tidyVarOcc (_, var_env) v = case lookupVarEnv var_env v of
628 -- tidyBndr is used for lambda and case binders
629 tidyBndr :: TidyEnv -> Var -> (TidyEnv, Var)
631 | isTyVar var = tidyTyVarBndr env var
632 | otherwise = tidyIdBndr env var
634 tidyBndrs :: TidyEnv -> [Var] -> (TidyEnv, [Var])
635 tidyBndrs env vars = mapAccumL tidyBndr env vars
637 tidyLetBndr :: TidyEnv -> (Id, CoreExpr) -> (TidyEnv, Var)
638 -- Used for local (non-top-level) let(rec)s
639 tidyLetBndr env (id,rhs)
640 = ((tidy_env,new_var_env), final_id)
642 ((tidy_env,var_env), new_id) = tidyIdBndr env id
644 -- We need to keep around any interesting strictness and demand info
645 -- because later on we may need to use it when converting to A-normal form.
647 -- f (g x), where f is strict in its argument, will be converted
648 -- into case (g x) of z -> f z by CorePrep, but only if f still
649 -- has its strictness info.
651 -- Similarly for the demand info - on a let binder, this tells
652 -- CorePrep to turn the let into a case.
654 -- Similarly arity info for eta expansion in CorePrep
655 final_id = new_id `setIdInfo` new_info
657 new_info = vanillaIdInfo
658 `setArityInfo` exprArity rhs
659 `setNewStrictnessInfo` newStrictnessInfo idinfo
660 `setNewDemandInfo` newDemandInfo idinfo
662 -- Override the env we get back from tidyId with the new IdInfo
663 -- so it gets propagated to the usage sites.
664 new_var_env = extendVarEnv var_env id final_id
666 tidyIdBndr :: TidyEnv -> Id -> (TidyEnv, Id)
667 tidyIdBndr env@(tidy_env, var_env) id
668 = -- Non-top-level variables
670 -- Give the Id a fresh print-name, *and* rename its type
671 -- The SrcLoc isn't important now,
672 -- though we could extract it from the Id
674 -- All nested Ids now have the same IdInfo, namely none,
675 -- which should save some space.
676 (tidy_env', occ') = tidyOccName tidy_env (getOccName id)
677 ty' = tidyType env (idType id)
678 id' = mkUserLocal occ' (idUnique id) ty' noSrcLoc
679 var_env' = extendVarEnv var_env id id'
681 ((tidy_env', var_env'), id')