    {-
    (c) The AQUA Project, Glasgow University, 1993-1998

    \section[SimplMonad]{The simplifier Monad}
    -}

    {-# LANGUAGE CPP #-}

    module SimplEnv (
            InId, InBind, InExpr, InAlt, InArg, InType, InBndr, InVar,
            OutId, OutTyVar, OutBind, OutExpr, OutAlt, OutArg, OutType, OutBndr, OutVar,
            InCoercion, OutCoercion,

            -- The simplifier mode
            setMode, getMode, updMode,

            -- Environments
            SimplEnv(..), StaticEnv, pprSimplEnv,   -- Temp not abstract
            mkSimplEnv, extendIdSubst, SimplEnv.extendTvSubst, SimplEnv.extendCvSubst,
            zapSubstEnv, setSubstEnv,
            getInScope, setInScope, setInScopeSet, modifyInScope, addNewInScopeIds,
            getSimplRules,

            SimplSR(..), mkContEx, substId, lookupRecBndr, refineFromInScope,

            substExpr,
            simplNonRecBndr, simplRecBndrs,
            simplBinder, simplBinders,
            substTy, substTyVar, getTvSubst,
            getCvSubst, substCo, substCoVar,

            -- Floats
            Floats, emptyFloats, isEmptyFloats, addNonRec, addFloats, extendFloats,
            wrapFloats, setFloats, zapFloats, addRecFloats, mapFloats,
            doFloatFromRhs, getFloatBinds
        ) where

    #include "HsVersions.h"

    import SimplMonad
    import CoreMonad        ( SimplifierMode(..) )
    import CoreSyn
    import CoreUtils
    import Var
    import VarEnv
    import VarSet
    import OrdList
    import Id
    import qualified CoreSubst
    import MkCore                   ( mkWildValBinder )
    import TysWiredIn
    import qualified Type
    import Type hiding              ( substTy, substTyVarBndr, substTyVar )
    import qualified Coercion
    import Coercion hiding          ( substCo, substTy, substCoVar, substCoVarBndr, substTyVarBndr )
    import BasicTypes
    import MonadUtils
    import Outputable
    import FastString
    import Util

    import Data.List

    {-
    ************************************************************************
    *                                                                      *
    \subsection[Simplify-types]{Type declarations}
    *                                                                      *
    ************************************************************************
    -}

    type InBndr     = CoreBndr
    type InVar      = Var                   -- Not yet cloned
    type InId       = Id                    -- Not yet cloned
    type InType     = Type                  -- Ditto
    type InBind     = CoreBind
    type InExpr     = CoreExpr
    type InAlt      = CoreAlt
    type InArg      = CoreArg
    type InCoercion = Coercion

    type OutBndr     = CoreBndr
    type OutVar      = Var                  -- Cloned
    type OutId       = Id                   -- Cloned
    type OutTyVar    = TyVar                -- Cloned
    type OutType     = Type                 -- Cloned
    type OutCoercion = Coercion
    type OutBind     = CoreBind
    type OutExpr     = CoreExpr
    type OutAlt      = CoreAlt
    type OutArg      = CoreArg

    {-
    ************************************************************************
    *                                                                      *
    \subsubsection{The @SimplEnv@ type}
    *                                                                      *
    ************************************************************************
    -}

    data SimplEnv
      = SimplEnv {
         ----------- Static part of the environment -----------
         -- Static in the sense of lexically scoped,
         -- wrt the original expression

            seMode      :: SimplifierMode,

            -- The current substitution
            seTvSubst   :: TvSubstEnv,      -- InTyVar |--> OutType
            seCvSubst   :: CvSubstEnv,      -- InCoVar |--> OutCoercion
            seIdSubst   :: SimplIdSubst,    -- InId    |--> OutExpr

         ----------- Dynamic part of the environment -----------
         -- Dynamic in the sense of describing the setup where
         -- the expression finally ends up

            -- The current set of in-scope variables
            -- They are all OutVars, and all bound in this module
            seInScope   :: InScopeSet,      -- OutVars only
                    -- Includes all variables bound by seFloats
            seFloats    :: Floats
                    -- See Note [Simplifier floats]
        }

    type StaticEnv = SimplEnv       -- Just the static part is relevant

    pprSimplEnv :: SimplEnv -> SDoc
    -- Used for debugging; selective
    pprSimplEnv env
      = vcat [ptext (sLit "TvSubst:") <+> ppr (seTvSubst env),
              ptext (sLit "IdSubst:") <+> ppr (seIdSubst env),
              ptext (sLit "InScope:") <+> vcat (map ppr_one in_scope_vars)
        ]
      where
       in_scope_vars = varEnvElts (getInScopeVars (seInScope env))
       ppr_one v | isId v = ppr v <+> ppr (idUnfolding v)
                 | otherwise = ppr v

    type SimplIdSubst = IdEnv SimplSR       -- IdId |--> OutExpr
            -- See Note [Extending the Subst] in CoreSubst

    data SimplSR
      = DoneEx OutExpr              -- Completed term
      | DoneId OutId                -- Completed term variable
      | ContEx TvSubstEnv           -- A suspended substitution
               CvSubstEnv
               SimplIdSubst
               InExpr

    instance Outputable SimplSR where
      ppr (DoneEx e) = ptext (sLit "DoneEx") <+> ppr e
      ppr (DoneId v) = ptext (sLit "DoneId") <+> ppr v
      ppr (ContEx _tv _cv _id e) = vcat [ptext (sLit "ContEx") <+> ppr e {-,
                                    ppr (filter_env tv), ppr (filter_env id) -}]
            -- where
            -- fvs = exprFreeVars e
            -- filter_env env = filterVarEnv_Directly keep env
            -- keep uniq _ = uniq `elemUFM_Directly` fvs

    {-
    Note [SimplEnv invariants]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    seInScope:
            The in-scope part of Subst includes *all* in-scope TyVars and Ids
            The elements of the set may have better IdInfo than the
            occurrences of in-scope Ids, and (more important) they will
            have a correctly-substituted type.  So we use a lookup in this
            set to replace occurrences

            The Ids in the InScopeSet are replete with their Rules,
            and as we gather info about the unfolding of an Id, we replace
            it in the in-scope set.

            The in-scope set is actually a mapping OutVar -> OutVar, and
            in case expressions we sometimes bind

    seIdSubst:
            The substitution is *apply-once* only, because InIds and OutIds
            can overlap.
            For example, we generally omit mappings
                    a77 -> a77
            from the substitution, when we decide not to clone a77, but it's quite
            legitimate to put the mapping in the substitution anyway.

            Furthermore, consider
                    let x = case k of I# x77 -> ... in
                    let y = case k of I# x77 -> ... in ...
            and suppose the body is strict in both x and y.  Then the simplifier
            will pull the first (case k) to the top; so the second (case k) will
            cancel out, mapping x77 to, well, x77!  But one is an in-Id and the
            other is an out-Id.

            Of course, the substitution *must* applied! Things in its domain
            simply aren't necessarily bound in the result.

    * substId adds a binding (DoneId new_id) to the substitution if
            the Id's unique has changed

      Note, though that the substitution isn't necessarily extended
      if the type of the Id changes.  Why not?  Because of the next point:

    * We *always, always* finish by looking up in the in-scope set
      any variable that doesn't get a DoneEx or DoneVar hit in the substitution.
      Reason: so that we never finish up with a "old" Id in the result.
      An old Id might point to an old unfolding and so on... which gives a space
      leak.

      [The DoneEx and DoneVar hits map to "new" stuff.]

    * It follows that substExpr must not do a no-op if the substitution is empty.
      substType is free to do so, however.

    * When we come to a let-binding (say) we generate new IdInfo, including an
      unfolding, attach it to the binder, and add this newly adorned binder to
      the in-scope set.  So all subsequent occurrences of the binder will get
      mapped to the full-adorned binder, which is also the one put in the
      binding site.

    * The in-scope "set" usually maps x->x; we use it simply for its domain.
      But sometimes we have two in-scope Ids that are synomyms, and should
      map to the same target:  x->x, y->x.  Notably:
            case y of x { ... }
      That's why the "set" is actually a VarEnv Var
    -}

    mkSimplEnv :: SimplifierMode -> SimplEnv
    mkSimplEnv mode
      = SimplEnv { seMode = mode
                 , seInScope = init_in_scope
                 , seFloats = emptyFloats
                 , seTvSubst = emptyVarEnv
                 , seCvSubst = emptyVarEnv
                 , seIdSubst = emptyVarEnv }
            -- The top level "enclosing CC" is "SUBSUMED".

    init_in_scope :: InScopeSet
    init_in_scope = mkInScopeSet (unitVarSet (mkWildValBinder unitTy))
                  -- See Note [WildCard binders]

    {-
    Note [WildCard binders]
    ~~~~~~~~~~~~~~~~~~~~~~~
    The program to be simplified may have wild binders
        case e of wild { p -> ... }
    We want to *rename* them away, so that there are no
    occurrences of 'wild-id' (with wildCardKey).  The easy
    way to do that is to start of with a representative
    Id in the in-scope set

    There can be be *occurrences* of wild-id.  For example,
    MkCore.mkCoreApp transforms
       e (a /# b)   -->   case (a /# b) of wild { DEFAULT -> e wild }
    This is ok provided 'wild' isn't free in 'e', and that's the delicate
    thing. Generally, you want to run the simplifier to get rid of the
    wild-ids before doing much else.

    It's a very dark corner of GHC.  Maybe it should be cleaned up.
    -}

    getMode :: SimplEnv -> SimplifierMode
    getMode env = seMode env

    setMode :: SimplifierMode -> SimplEnv -> SimplEnv
    setMode mode env = env { seMode = mode }

    updMode :: (SimplifierMode -> SimplifierMode) -> SimplEnv -> SimplEnv
    updMode upd env = env { seMode = upd (seMode env) }

    ---------------------
    extendIdSubst :: SimplEnv -> Id -> SimplSR -> SimplEnv
    extendIdSubst env@(SimplEnv {seIdSubst = subst}) var res
      = ASSERT2( isId var && not (isCoVar var), ppr var )
        env {seIdSubst = extendVarEnv subst var res}

    extendTvSubst :: SimplEnv -> TyVar -> Type -> SimplEnv
    extendTvSubst env@(SimplEnv {seTvSubst = subst}) var res
      = env {seTvSubst = extendVarEnv subst var res}

    extendCvSubst :: SimplEnv -> CoVar -> Coercion -> SimplEnv
    extendCvSubst env@(SimplEnv {seCvSubst = subst}) var res
      = env {seCvSubst = extendVarEnv subst var res}

    ---------------------
    getInScope :: SimplEnv -> InScopeSet
    getInScope env = seInScope env

    setInScopeSet :: SimplEnv -> InScopeSet -> SimplEnv
    setInScopeSet env in_scope = env {seInScope = in_scope}

    setInScope :: SimplEnv -> SimplEnv -> SimplEnv
    -- Set the in-scope set, and *zap* the floats
    setInScope env env_with_scope
      = env { seInScope = seInScope env_with_scope,
              seFloats = emptyFloats }

    setFloats :: SimplEnv -> SimplEnv -> SimplEnv
    -- Set the in-scope set *and* the floats
    setFloats env env_with_floats
      = env { seInScope = seInScope env_with_floats,
              seFloats  = seFloats  env_with_floats }

    addNewInScopeIds :: SimplEnv -> [CoreBndr] -> SimplEnv
            -- The new Ids are guaranteed to be freshly allocated
    addNewInScopeIds env@(SimplEnv { seInScope = in_scope, seIdSubst = id_subst }) vs
      = env { seInScope = in_scope `extendInScopeSetList` vs,
              seIdSubst = id_subst `delVarEnvList` vs }
            -- Why delete?  Consider
            --      let x = a*b in (x, \x -> x+3)
            -- We add [x |-> a*b] to the substitution, but we must
            -- _delete_ it from the substitution when going inside
            -- the (\x -> ...)!

    modifyInScope :: SimplEnv -> CoreBndr -> SimplEnv
    -- The variable should already be in scope, but
    -- replace the existing version with this new one
    -- which has more information
    modifyInScope env@(SimplEnv {seInScope = in_scope}) v
      = env {seInScope = extendInScopeSet in_scope v}

    ---------------------
    zapSubstEnv :: SimplEnv -> SimplEnv
    zapSubstEnv env = env {seTvSubst = emptyVarEnv, seCvSubst = emptyVarEnv, seIdSubst = emptyVarEnv}

    setSubstEnv :: SimplEnv -> TvSubstEnv -> CvSubstEnv -> SimplIdSubst -> SimplEnv
    setSubstEnv env tvs cvs ids = env { seTvSubst = tvs, seCvSubst = cvs, seIdSubst = ids }

    mkContEx :: SimplEnv -> InExpr -> SimplSR
    mkContEx (SimplEnv { seTvSubst = tvs, seCvSubst = cvs, seIdSubst = ids }) e = ContEx tvs cvs ids e

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Floats}
    *                                                                      *
    ************************************************************************

    Note [Simplifier floats]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    The Floats is a bunch of bindings, classified by a FloatFlag.

    * All of them satisfy the let/app invariant

    Examples

      NonRec x (y:ys)       FltLifted
      Rec [(x,rhs)]         FltLifted

      NonRec x* (p:q)       FltOKSpec   -- RHS is WHNF.  Question: why not FltLifted?
      NonRec x# (y +# 3)    FltOkSpec   -- Unboxed, but ok-for-spec'n

      NonRec x* (f y)       FltCareful  -- Strict binding; might fail or diverge

    Can't happen:
      NonRec x# (a /# b)    -- Might fail; does not satisfy let/app
      NonRec x# (f y)       -- Might diverge; does not satisfy let/app
    -}

    data Floats = Floats (OrdList OutBind) FloatFlag
            -- See Note [Simplifier floats]

    data FloatFlag
      = FltLifted   -- All bindings are lifted and lazy
                    --  Hence ok to float to top level, or recursive

      | FltOkSpec   -- All bindings are FltLifted *or*
                    --      strict (perhaps because unlifted,
                    --      perhaps because of a strict binder),
                    --        *and* ok-for-speculation
                    --  Hence ok to float out of the RHS
                    --  of a lazy non-recursive let binding
                    --  (but not to top level, or into a rec group)

      | FltCareful  -- At least one binding is strict (or unlifted)
                    --      and not guaranteed cheap
                    --      Do not float these bindings out of a lazy let

    instance Outputable Floats where
      ppr (Floats binds ff) = ppr ff $$ ppr (fromOL binds)

    instance Outputable FloatFlag where
      ppr FltLifted = ptext (sLit "FltLifted")
      ppr FltOkSpec = ptext (sLit "FltOkSpec")
      ppr FltCareful = ptext (sLit "FltCareful")

    andFF :: FloatFlag -> FloatFlag -> FloatFlag
    andFF FltCareful _          = FltCareful
    andFF FltOkSpec  FltCareful = FltCareful
    andFF FltOkSpec  _          = FltOkSpec
    andFF FltLifted  flt        = flt

    doFloatFromRhs :: TopLevelFlag -> RecFlag -> Bool -> OutExpr -> SimplEnv -> Bool
    -- If you change this function look also at FloatIn.noFloatFromRhs
    doFloatFromRhs lvl rec str rhs (SimplEnv {seFloats = Floats fs ff})
      =  not (isNilOL fs) && want_to_float && can_float
      where
         want_to_float = isTopLevel lvl || exprIsCheap rhs || exprIsExpandable rhs
                         -- See Note [Float when cheap or expandable]
         can_float = case ff of
                       FltLifted  -> True
                       FltOkSpec  -> isNotTopLevel lvl && isNonRec rec
                       FltCareful -> isNotTopLevel lvl && isNonRec rec && str

    {-
    Note [Float when cheap or expandable]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We want to float a let from a let if the residual RHS is
       a) cheap, such as (\x. blah)
       b) expandable, such as (f b) if f is CONLIKE
    But there are
      - cheap things that are not expandable (eg \x. expensive)
      - expandable things that are not cheap (eg (f b) where b is CONLIKE)
    so we must take the 'or' of the two.
    -}

    emptyFloats :: Floats
    emptyFloats = Floats nilOL FltLifted

    unitFloat :: OutBind -> Floats
    -- This key function constructs a singleton float with the right form
    unitFloat bind = Floats (unitOL bind) (flag bind)
      where
        flag (Rec {})                = FltLifted
        flag (NonRec bndr rhs)
          | not (isStrictId bndr)    = FltLifted
          | exprOkForSpeculation rhs = FltOkSpec  -- Unlifted, and lifted but ok-for-spec (eg HNF)
          | otherwise                = ASSERT2( not (isUnLiftedType (idType bndr)), ppr bndr )
                                       FltCareful
          -- Unlifted binders can only be let-bound if exprOkForSpeculation holds

    addNonRec :: SimplEnv -> OutId -> OutExpr -> SimplEnv
    -- Add a non-recursive binding and extend the in-scope set
    -- The latter is important; the binder may already be in the
    -- in-scope set (although it might also have been created with newId)
    -- but it may now have more IdInfo
    addNonRec env id rhs
      = id `seq`   -- This seq forces the Id, and hence its IdInfo,
                   -- and hence any inner substitutions
        env { seFloats = seFloats env `addFlts` unitFloat (NonRec id rhs),
              seInScope = extendInScopeSet (seInScope env) id }

    extendFloats :: SimplEnv -> OutBind -> SimplEnv
    -- Add these bindings to the floats, and extend the in-scope env too
    extendFloats env bind
      = env { seFloats  = seFloats env `addFlts` unitFloat bind,
              seInScope = extendInScopeSetList (seInScope env) bndrs }
      where
        bndrs = bindersOf bind

    addFloats :: SimplEnv -> SimplEnv -> SimplEnv
    -- Add the floats for env2 to env1;
    -- *plus* the in-scope set for env2, which is bigger
    -- than that for env1
    addFloats env1 env2
      = env1 {seFloats = seFloats env1 `addFlts` seFloats env2,
              seInScope = seInScope env2 }

    addFlts :: Floats -> Floats -> Floats
    addFlts (Floats bs1 l1) (Floats bs2 l2)
      = Floats (bs1 `appOL` bs2) (l1 `andFF` l2)

    zapFloats :: SimplEnv -> SimplEnv
    zapFloats env = env { seFloats = emptyFloats }

    addRecFloats :: SimplEnv -> SimplEnv -> SimplEnv
    -- Flattens the floats from env2 into a single Rec group,
    -- prepends the floats from env1, and puts the result back in env2
    -- This is all very specific to the way recursive bindings are
    -- handled; see Simplify.simplRecBind
    addRecFloats env1 env2@(SimplEnv {seFloats = Floats bs ff})
      = ASSERT2( case ff of { FltLifted -> True; _ -> False }, ppr (fromOL bs) )
        env2 {seFloats = seFloats env1 `addFlts` unitFloat (Rec (flattenBinds (fromOL bs)))}

    wrapFloats :: SimplEnv -> OutExpr -> OutExpr
    -- Wrap the floats around the expression; they should all
    -- satisfy the let/app invariant, so mkLets should do the job just fine
    wrapFloats (SimplEnv {seFloats = Floats bs _}) body
      = foldrOL Let body bs

    getFloatBinds :: SimplEnv -> [CoreBind]
    getFloatBinds (SimplEnv {seFloats = Floats bs _})
      = fromOL bs

    isEmptyFloats :: SimplEnv -> Bool
    isEmptyFloats (SimplEnv {seFloats = Floats bs _})
      = isNilOL bs

    mapFloats :: SimplEnv -> ((Id,CoreExpr) -> (Id,CoreExpr)) -> SimplEnv
    mapFloats env@SimplEnv { seFloats = Floats fs ff } fun
       = env { seFloats = Floats (mapOL app fs) ff }
       where
        app (NonRec b e) = case fun (b,e) of (b',e') -> NonRec b' e'
        app (Rec bs)     = Rec (map fun bs)

    {-
    ************************************************************************
    *                                                                      *
                    Substitution of Vars
    *                                                                      *
    ************************************************************************

    Note [Global Ids in the substitution]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We look up even a global (eg imported) Id in the substitution. Consider
       case X.g_34 of b { (a,b) ->  ... case X.g_34 of { (p,q) -> ...} ... }
    The binder-swap in the occurrence analyser will add a binding
    for a LocalId version of g (with the same unique though):
       case X.g_34 of b { (a,b) ->  let g_34 = b in
                                    ... case X.g_34 of { (p,q) -> ...} ... }
    So we want to look up the inner X.g_34 in the substitution, where we'll
    find that it has been substituted by b.  (Or conceivably cloned.)
    -}

    substId :: SimplEnv -> InId -> SimplSR
    -- Returns DoneEx only on a non-Var expression
    substId (SimplEnv { seInScope = in_scope, seIdSubst = ids }) v
      = case lookupVarEnv ids v of  -- Note [Global Ids in the substitution]
            Nothing               -> DoneId (refineFromInScope in_scope v)
            Just (DoneId v)       -> DoneId (refineFromInScope in_scope v)
            Just (DoneEx (Var v)) -> DoneId (refineFromInScope in_scope v)
            Just res              -> res    -- DoneEx non-var, or ContEx

            -- Get the most up-to-date thing from the in-scope set
            -- Even though it isn't in the substitution, it may be in
            -- the in-scope set with better IdInfo
    refineFromInScope :: InScopeSet -> Var -> Var
    refineFromInScope in_scope v
      | isLocalId v = case lookupInScope in_scope v of
                      Just v' -> v'
                      Nothing -> WARN( True, ppr v ) v  -- This is an error!
      | otherwise = v

    lookupRecBndr :: SimplEnv -> InId -> OutId
    -- Look up an Id which has been put into the envt by simplRecBndrs,
    -- but where we have not yet done its RHS
    lookupRecBndr (SimplEnv { seInScope = in_scope, seIdSubst = ids }) v
      = case lookupVarEnv ids v of
            Just (DoneId v) -> v
            Just _ -> pprPanic "lookupRecBndr" (ppr v)
            Nothing -> refineFromInScope in_scope v

    {-
    ************************************************************************
    *                                                                      *
    \section{Substituting an Id binder}
    *                                                                      *
    ************************************************************************

    * sinplBndr, simplBndrs: monadic version, only so that they
                             can be made strict via seq.

    -}

    -------------
    simplBinders :: SimplEnv -> [InBndr] -> SimplM (SimplEnv, [OutBndr])
    simplBinders  env bndrs = mapAccumLM simplBinder  env bndrs

    simplBinder :: SimplEnv -> InBndr -> SimplM (SimplEnv, OutBndr)
    -- Used for lambda and case-bound variables
    -- Clone Id if necessary, substitute type
    -- Return with IdInfo already substituted, but (fragile) occurrence info zapped
    -- The substitution is extended only if the variable is cloned, because
    -- we *don't* need to use it to track occurrence info.
    simplBinder env bndr
      | isTyVar bndr  = do  { let (env', tv) = substTyVarBndr env bndr
                            ; seqTyVar tv `seq` return (env', tv) }
      | otherwise     = do  { let (env', id) = substIdBndr env bndr
                            ; seqId id `seq` return (env', id) }

    simplNonRecBndr :: SimplEnv -> InBndr -> SimplM (SimplEnv, OutBndr)
    -- A non-recursive let binder
    simplNonRecBndr env id
      = do  { let (env1, id1) = substIdBndr env id
            ; seqId id1 `seq` return (env1, id1) }

    simplRecBndrs :: SimplEnv -> [InBndr] -> SimplM SimplEnv
    -- Recursive let binders
    simplRecBndrs env@(SimplEnv {}) ids
      = do  { let (env1, ids1) = mapAccumL substIdBndr env ids
            ; seqIds ids1 `seq` return env1 }

    substIdBndr :: SimplEnv -> InBndr -> (SimplEnv, OutBndr)
    -- Might be a coercion variable
    substIdBndr env bndr
      | isCoVar bndr  = substCoVarBndr env bndr
      | otherwise     = substNonCoVarIdBndr env bndr

    ---------------
    substNonCoVarIdBndr
       :: SimplEnv
       -> InBndr    -- Env and binder to transform
       -> (SimplEnv, OutBndr)
    -- Clone Id if necessary, substitute its type
    -- Return an Id with its
    --      * Type substituted
    --      * UnfoldingInfo, Rules, WorkerInfo zapped
    --      * Fragile OccInfo (only) zapped: Note [Robust OccInfo]
    --      * Robust info, retained especially arity and demand info,
    --         so that they are available to occurrences that occur in an
    --         earlier binding of a letrec
    --
    -- For the robust info, see Note [Arity robustness]
    --
    -- Augment the substitution  if the unique changed
    -- Extend the in-scope set with the new Id
    --
    -- Similar to CoreSubst.substIdBndr, except that
    --      the type of id_subst differs
    --      all fragile info is zapped
    substNonCoVarIdBndr env@(SimplEnv { seInScope = in_scope, seIdSubst = id_subst })
                        old_id
      = ASSERT2( not (isCoVar old_id), ppr old_id )
        (env { seInScope = in_scope `extendInScopeSet` new_id,
               seIdSubst = new_subst }, new_id)
      where
        id1    = uniqAway in_scope old_id
        id2    = substIdType env id1
        new_id = zapFragileIdInfo id2       -- Zaps rules, worker-info, unfolding
                                            -- and fragile OccInfo

            -- Extend the substitution if the unique has changed,
            -- or there's some useful occurrence information
            -- See the notes with substTyVarBndr for the delSubstEnv
        new_subst | new_id /= old_id
                  = extendVarEnv id_subst old_id (DoneId new_id)
                  | otherwise
                  = delVarEnv id_subst old_id

    ------------------------------------
    seqTyVar :: TyVar -> ()
    seqTyVar b = b `seq` ()

    seqId :: Id -> ()
    seqId id = seqType (idType id)  `seq`
               idInfo id            `seq`
               ()

    seqIds :: [Id] -> ()
    seqIds []       = ()
    seqIds (id:ids) = seqId id `seq` seqIds ids

    {-
    Note [Arity robustness]
    ~~~~~~~~~~~~~~~~~~~~~~~
    We *do* transfer the arity from from the in_id of a let binding to the
    out_id.  This is important, so that the arity of an Id is visible in
    its own RHS.  For example:
            f = \x. ....g (\y. f y)....
    We can eta-reduce the arg to g, because f is a value.  But that
    needs to be visible.

    This interacts with the 'state hack' too:
            f :: Bool -> IO Int
            f = \x. case x of
                      True  -> f y
                      False -> \s -> ...
    Can we eta-expand f?  Only if we see that f has arity 1, and then we
    take advantage of the 'state hack' on the result of
    (f y) :: State# -> (State#, Int) to expand the arity one more.

    There is a disadvantage though.  Making the arity visible in the RHS
    allows us to eta-reduce
            f = \x -> f x
    to
            f = f
    which technically is not sound.   This is very much a corner case, so
    I'm not worried about it.  Another idea is to ensure that f's arity
    never decreases; its arity started as 1, and we should never eta-reduce
    below that.

    Note [Robust OccInfo]
    ~~~~~~~~~~~~~~~~~~~~~
    It's important that we *do* retain the loop-breaker OccInfo, because
    that's what stops the Id getting inlined infinitely, in the body of
    the letrec.
    -}

    {-
    ************************************************************************
    *                                                                      *
                    Impedence matching to type substitution
    *                                                                      *
    ************************************************************************
    -}

    getTvSubst :: SimplEnv -> TvSubst
    getTvSubst (SimplEnv { seInScope = in_scope, seTvSubst = tv_env })
      = mkTvSubst in_scope tv_env

    getCvSubst :: SimplEnv -> CvSubst
    getCvSubst (SimplEnv { seInScope = in_scope, seTvSubst = tv_env, seCvSubst = cv_env })
      = CvSubst in_scope tv_env cv_env

    substTy :: SimplEnv -> Type -> Type
    substTy env ty = Type.substTy (getTvSubst env) ty

    substTyVar :: SimplEnv -> TyVar -> Type
    substTyVar env tv = Type.substTyVar (getTvSubst env) tv

    substTyVarBndr :: SimplEnv -> TyVar -> (SimplEnv, TyVar)
    substTyVarBndr env tv
      = case Type.substTyVarBndr (getTvSubst env) tv of
            (TvSubst in_scope' tv_env', tv')
               -> (env { seInScope = in_scope', seTvSubst = tv_env' }, tv')

    substCoVar :: SimplEnv -> CoVar -> Coercion
    substCoVar env tv = Coercion.substCoVar (getCvSubst env) tv

    substCoVarBndr :: SimplEnv -> CoVar -> (SimplEnv, CoVar)
    substCoVarBndr env cv
      = case Coercion.substCoVarBndr (getCvSubst env) cv of
            (CvSubst in_scope' tv_env' cv_env', cv')
               -> (env { seInScope = in_scope', seTvSubst = tv_env', seCvSubst = cv_env' }, cv')

    substCo :: SimplEnv -> Coercion -> Coercion
    substCo env co = Coercion.substCo (getCvSubst env) co

    ------------------
    substIdType :: SimplEnv -> Id -> Id
    substIdType (SimplEnv { seInScope = in_scope,  seTvSubst = tv_env }) id
      | isEmptyVarEnv tv_env || isEmptyVarSet (tyVarsOfType old_ty) = id
      | otherwise = Id.setIdType id (Type.substTy (TvSubst in_scope tv_env) old_ty)
                    -- The tyVarsOfType is cheaper than it looks
                    -- because we cache the free tyvars of the type
                    -- in a Note in the id's type itself
      where
        old_ty = idType id

    substExpr :: SimplEnv -> CoreExpr -> CoreExpr
    -- See Note [Substitution in the simplifier]
    substExpr (SimplEnv { seInScope = in_scope
                        , seTvSubst = tv_env
                        , seCvSubst = cv_env
                        , seIdSubst = id_env })
      = subst_expr in_scope tv_env cv_env id_env
      where
        subst_expr :: InScopeSet -> TvSubstEnv -> CvSubstEnv -> SimplIdSubst
                   -> CoreExpr -> CoreExpr
        subst_expr is tvs cvs id_env
          = CoreSubst.substExpr (text "SimplEnv.substExpr")
                                (CoreSubst.mkGblSubst is tvs cvs lookup_id)
          where
            lookup_id in_scope v
              = case lookupVarEnv id_env v of
                  Nothing                  -> Nothing
                  Just (DoneEx e)          -> Just e
                  Just (DoneId v)          -> Just (Var v)
                  Just (ContEx tv cv id e) -> Just (subst_expr in_scope tv cv id e)

    {- Note [Substitution in the simplifier]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In just one place (sigh) we need to lazily substitute over a CoreExpr.
    For that we need CoreSubst.substExpr.  But there is a difficulty: SimplEnv
    has a SimplIdSubst, whose range is SimplSR, not just CoreExpr.

    So SimplEnv.substExpr has to perform impedence-matching, via the ambient
    substitution provided by mkGblSubst.  It seems like a lot of work for
    a small thing.  Previously we attempted to construct a (VarEnv CoreExpr)
    from the SimplIdSubst, but that had absolutely terrible performance
    (Trac #10370 comment:12).  Then I tried to write a complete new substExpr
    that used SimplIdSubst insead of (VarEnv CoreExpr), but that got out of
    hand because we need to substitute over rules and unfoldings too
    (Trac #5113, comment:7 and following).
    -}