X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FcoreSyn%2FCoreArity.lhs;h=d5849cbe89f070569916c3c6e450edf13013d4ec;hb=b8ee6f14ca6e9e49015ee9b404cf8b8191fede05;hp=f39b6b92352ebe07a18c5851a536dc73423e9b1b;hpb=ac38ece1e717cb412e89354aa95fd11d44c1cefb;p=ghc-hetmet.git diff --git a/compiler/coreSyn/CoreArity.lhs b/compiler/coreSyn/CoreArity.lhs index f39b6b9..d5849cb 100644 --- a/compiler/coreSyn/CoreArity.lhs +++ b/compiler/coreSyn/CoreArity.lhs @@ -8,8 +8,8 @@ \begin{code} -- | Arit and eta expansion module CoreArity ( - manifestArity, exprArity, - exprEtaExpandArity, etaExpand + manifestArity, exprArity, exprBotStrictness_maybe, + exprEtaExpandArity, etaExpand ) where #include "HsVersions.h" @@ -17,23 +17,21 @@ module CoreArity ( import CoreSyn import CoreFVs import CoreUtils -import qualified CoreSubst -import CoreSubst ( Subst, substBndr, substBndrs, substExpr - , mkEmptySubst, isEmptySubst ) +import CoreSubst +import Demand import Var import VarEnv import Id import Type +import TyCon ( isRecursiveTyCon ) import TcType ( isDictLikeTy ) import Coercion import BasicTypes import Unique import Outputable import DynFlags +import StaticFlags ( opt_NoStateHack ) import FastString -import Maybes - -import GHC.Exts -- For `xori` \end{code} %************************************************************************ @@ -99,78 +97,93 @@ exprArity :: CoreExpr -> Arity -- ^ An approximate, fast, version of 'exprEtaExpandArity' exprArity e = go e where - go (Var v) = idArity v - go (Lam x e) | isId x = go e + 1 - | otherwise = go e - go (Note _ e) = go e - go (Cast e co) = trim_arity (go e) 0 (snd (coercionKind co)) - go (App e (Type _)) = go e - go (App f a) | exprIsCheap a = (go f - 1) `max` 0 - -- NB: exprIsCheap a! - -- f (fac x) does not have arity 2, - -- even if f has arity 3! - -- NB: `max 0`! (\x y -> f x) has arity 2, even if f is - -- unknown, hence arity 0 + go (Var v) = idArity v + go (Lam x e) | isId x = go e + 1 + | otherwise = go e + go (Note _ e) = go e + go (Cast e co) = go e `min` typeArity (snd (coercionKind co)) + -- Note [exprArity invariant] + go (App e (Type _)) = go e + go (App f a) | exprIsTrivial a = (go f - 1) `max` 0 + -- See Note [exprArity for applications] go _ = 0 - - -- Note [exprArity invariant] - trim_arity n a ty - | n==a = a - | Just (_, ty') <- splitForAllTy_maybe ty = trim_arity n a ty' - | Just (_, ty') <- splitFunTy_maybe ty = trim_arity n (a+1) ty' - | Just (ty',_) <- splitNewTypeRepCo_maybe ty = trim_arity n a ty' - | otherwise = a \end{code} +Note [exprArity for applications] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +When we come to an application we check that the arg is trivial. + eg f (fac x) does not have arity 2, + even if f has arity 3! + +* We require that is trivial rather merely cheap. Suppose f has arity 2. + Then f (Just y) + has arity 0, because if we gave it arity 1 and then inlined f we'd get + let v = Just y in \w. + which has arity 0. And we try to maintain the invariant that we don't + have arity decreases. + +* The `max 0` is important! (\x y -> f x) has arity 2, even if f is + unknown, hence arity 0 + + %************************************************************************ %* * -\subsection{Eta reduction and expansion} + Eta expansion %* * %************************************************************************ -exprEtaExpandArity is used when eta expanding - e ==> \xy -> e x y - -It returns 1 (or more) to: - case x of p -> \s -> ... -because for I/O ish things we really want to get that \s to the top. -We are prepared to evaluate x each time round the loop in order to get that - -It's all a bit more subtle than it looks: +\begin{code} +-- ^ The Arity returned is the number of value args the +-- expression can be applied to without doing much work +exprEtaExpandArity :: DynFlags -> CoreExpr -> Arity +-- exprEtaExpandArity is used when eta expanding +-- e ==> \xy -> e x y +exprEtaExpandArity dflags e + = applyStateHack e (arityType dicts_cheap e) + where + dicts_cheap = dopt Opt_DictsCheap dflags + +exprBotStrictness_maybe :: CoreExpr -> Maybe (Arity, StrictSig) +-- A cheap and cheerful function that identifies bottoming functions +-- and gives them a suitable strictness signatures. It's used during +-- float-out +exprBotStrictness_maybe e + = case arityType False e of + AT _ ATop -> Nothing + AT a ABot -> Just (a, mkStrictSig (mkTopDmdType (replicate a topDmd) BotRes)) +\end{code} + +Note [Definition of arity] +~~~~~~~~~~~~~~~~~~~~~~~~~~ +The "arity" of an expression 'e' is n if + applying 'e' to *fewer* than n *value* arguments + converges rapidly -1. One-shot lambdas +Or, to put it another way -Consider one-shot lambdas - let x = expensive in \y z -> E -We want this to have arity 2 if the \y-abstraction is a 1-shot lambda -Hence the ArityType returned by arityType + there is no work lost in duplicating the partial + application (e x1 .. x(n-1)) -2. The state-transformer hack +In the divegent case, no work is lost by duplicating because if the thing +is evaluated once, that's the end of the program. -The one-shot lambda special cause is particularly important/useful for -IO state transformers, where we often get - let x = E in \ s -> ... +Or, to put it another way, in any context C -and the \s is a real-world state token abstraction. Such abstractions -are almost invariably 1-shot, so we want to pull the \s out, past the -let x=E, even if E is expensive. So we treat state-token lambdas as -one-shot even if they aren't really. The hack is in Id.isOneShotBndr. + C[ (\x1 .. xn. e x1 .. xn) ] + is as efficient as + C[ e ] -3. Dealing with bottom +It's all a bit more subtle than it looks: -Consider also - f = \x -> error "foo" -Here, arity 1 is fine. But if it is - f = \x -> case x of - True -> error "foo" - False -> \y -> x+y -then we want to get arity 2. Tecnically, this isn't quite right, because - (f True) `seq` 1 -should diverge, but it'll converge if we eta-expand f. Nevertheless, we -do so; it improves some programs significantly, and increasing convergence -isn't a bad thing. Hence the ABot/ATop in ArityType. +Note [Arity of case expressions] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +We treat the arity of + case x of p -> \s -> ... +as 1 (or more) because for I/O ish things we really want to get that +\s to the top. We are prepared to evaluate x each time round the loop +in order to get that. -Actually, the situation is worse. Consider +This isn't really right in the presence of seq. Consider f = \x -> case x of True -> \y -> x+y False -> \y -> x-y @@ -181,9 +194,28 @@ This should diverge! But if we eta-expand, it won't. Again, we ignore this "problem", because being scrupulous would lose an important transformation for many programs. +1. Note [One-shot lambdas] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider one-shot lambdas + let x = expensive in \y z -> E +We want this to have arity 1 if the \y-abstraction is a 1-shot lambda. -4. Newtypes +3. Note [Dealing with bottom] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider + f = \x -> error "foo" +Here, arity 1 is fine. But if it is + f = \x -> case x of + True -> error "foo" + False -> \y -> x+y +then we want to get arity 2. Technically, this isn't quite right, because + (f True) `seq` 1 +should diverge, but it'll converge if we eta-expand f. Nevertheless, we +do so; it improves some programs significantly, and increasing convergence +isn't a bad thing. Hence the ABot/ATop in ArityType. +4. Note [Newtype arity] +~~~~~~~~~~~~~~~~~~~~~~~~ Non-recursive newtypes are transparent, and should not get in the way. We do (currently) eta-expand recursive newtypes too. So if we have, say @@ -197,82 +229,182 @@ that is, etaExpandArity looks through the coerce. When we eta-expand e to arity 1: eta_expand 1 e T we want to get: coerce T (\x::[T] -> (coerce ([T]->Int) e) x) -HOWEVER, note that if you use coerce bogusly you can ge - coerce Int negate -And since negate has arity 2, you might try to eta expand. But you can't -decopose Int to a function type. Hence the final case in eta_expand. + HOWEVER, note that if you use coerce bogusly you can ge + coerce Int negate + And since negate has arity 2, you might try to eta expand. But you can't + decopose Int to a function type. Hence the final case in eta_expand. + +\begin{code} +applyStateHack :: CoreExpr -> ArityType -> Arity +applyStateHack e (AT orig_arity is_bot) + | opt_NoStateHack = orig_arity + | ABot <- is_bot = orig_arity -- Note [State hack and bottoming functions] + | otherwise = go orig_ty orig_arity + where -- Note [The state-transformer hack] + orig_ty = exprType e + go :: Type -> Arity -> Arity + go ty arity -- This case analysis should match that in eta_expand + | Just (_, ty') <- splitForAllTy_maybe ty = go ty' arity + | Just (arg,res) <- splitFunTy_maybe ty + , arity > 0 || isStateHackType arg = 1 + go res (arity-1) + +-- See Note [trimCast] + | Just (tc,tys) <- splitTyConApp_maybe ty + , Just (ty', _) <- instNewTyCon_maybe tc tys + , not (isRecursiveTyCon tc) = go ty' arity + -- Important to look through non-recursive newtypes, so that, eg + -- (f x) where f has arity 2, f :: Int -> IO () + -- Here we want to get arity 1 for the result! +------- + +{- + = if arity > 0 then 1 + go res (arity-1) + else if isStateHackType arg then + pprTrace "applystatehack" (vcat [ppr orig_arity, ppr orig_ty, + ppr ty, ppr res, ppr e]) $ + 1 + go res (arity-1) + else WARN( arity > 0, ppr arity ) 0 +-} + | otherwise = WARN( arity > 0, ppr arity <+> ppr ty) 0 +\end{code} + +Note [The state-transformer hack] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Suppose we have + f = e +where e has arity n. Then, if we know from the context that f has +a usage type like + t1 -> ... -> tn -1-> t(n+1) -1-> ... -1-> tm -> ... +then we can expand the arity to m. This usage type says that +any application (x e1 .. en) will be applied to uniquely to (m-n) more args +Consider f = \x. let y = + in case x of + True -> foo + False -> \(s:RealWorld) -> e +where foo has arity 1. Then we want the state hack to +apply to foo too, so we can eta expand the case. +Then we expect that if f is applied to one arg, it'll be applied to two +(that's the hack -- we don't really know, and sometimes it's false) +See also Id.isOneShotBndr. -\begin{code} --- ^ The Arity returned is the number of value args the --- expression can be applied to without doing much work -exprEtaExpandArity :: DynFlags -> CoreExpr -> Arity -exprEtaExpandArity dflags e = arityDepth (arityType dflags e) +Note [State hack and bottoming functions] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +It's a terrible idea to use the state hack on a bottoming function. +Here's what happens (Trac #2861): + + f :: String -> IO T + f = \p. error "..." + +Eta-expand, using the state hack: + + f = \p. (\s. ((error "...") |> g1) s) |> g2 + g1 :: IO T ~ (S -> (S,T)) + g2 :: (S -> (S,T)) ~ IO T + +Extrude the g2 + + f' = \p. \s. ((error "...") |> g1) s + f = f' |> (String -> g2) + +Discard args for bottomming function + + f' = \p. \s. ((error "...") |> g1 |> g3 + g3 :: (S -> (S,T)) ~ (S,T) + +Extrude g1.g3 + + f'' = \p. \s. (error "...") + f' = f'' |> (String -> S -> g1.g3) --- A limited sort of function type -data ArityType = AFun Bool ArityType -- True <=> one-shot - | ATop -- Know nothing +And now we can repeat the whole loop. Aargh! The bug is in applying the +state hack to a function which then swallows the argument. + + +-------------------- Main arity code ---------------------------- +\begin{code} +-- If e has ArityType (AT n r), then the term 'e' +-- * Must be applied to at least n *value* args +-- before doing any significant work +-- * It will not diverge before being applied to n +-- value arguments +-- * If 'r' is ABot, then it guarantees to diverge if +-- applied to n arguments (or more) + +data ArityType = AT Arity ArityRes +data ArityRes = ATop -- Know nothing | ABot -- Diverges -arityDepth :: ArityType -> Arity -arityDepth (AFun _ ty) = 1 + arityDepth ty -arityDepth _ = 0 +vanillaArityType :: ArityType +vanillaArityType = AT 0 ATop -- Totally uninformative -andArityType :: ArityType -> ArityType -> ArityType -andArityType ABot at2 = at2 -andArityType ATop _ = ATop -andArityType (AFun t1 at1) (AFun t2 at2) = AFun (t1 && t2) (andArityType at1 at2) -andArityType at1 at2 = andArityType at2 at1 +incArity :: ArityType -> ArityType +incArity (AT a r) = AT (a+1) r -arityType :: DynFlags -> CoreExpr -> ArityType - -- (go1 e) = [b1,..,bn] - -- means expression can be rewritten \x_b1 -> ... \x_bn -> body - -- where bi is True <=> the lambda is one-shot +decArity :: ArityType -> ArityType +decArity (AT 0 r) = AT 0 r +decArity (AT a r) = AT (a-1) r -arityType dflags (Note _ e) = arityType dflags e --- Not needed any more: etaExpand is cleverer --- removed: | ok_note n = arityType dflags e --- removed: | otherwise = ATop +andArityType :: ArityType -> ArityType -> ArityType -- Used for branches of a 'case' +andArityType (AT a1 ATop) (AT a2 ATop) = AT (a1 `min` a2) ATop +andArityType (AT _ ABot) (AT a2 ATop) = AT a2 ATop +andArityType (AT a1 ATop) (AT _ ABot) = AT a1 ATop +andArityType (AT a1 ABot) (AT a2 ABot) = AT (a1 `max` a2) ABot -arityType dflags (Cast e _) = arityType dflags e +--------------------------- +trimCast :: Coercion -> ArityType -> ArityType +-- Trim the arity to be no more than allowed by the +-- arrows in ty2, where co :: ty1~ty2 +trimCast _ at = at -arityType _ (Var v) - = mk (idArity v) (arg_tys (idType v)) +{- Omitting for now Note [trimCast] +trimCast co at@(AT ar _) + | ar > co_arity = AT co_arity ATop + | otherwise = at where - mk :: Arity -> [Type] -> ArityType - -- The argument types are only to steer the "state hack" - -- Consider case x of - -- True -> foo - -- False -> \(s:RealWorld) -> e - -- where foo has arity 1. Then we want the state hack to - -- apply to foo too, so we can eta expand the case. - mk 0 tys | isBottomingId v = ABot - | (ty:_) <- tys, isStateHackType ty = AFun True ATop - | otherwise = ATop - mk n (ty:tys) = AFun (isStateHackType ty) (mk (n-1) tys) - mk n [] = AFun False (mk (n-1) []) - - arg_tys :: Type -> [Type] -- Ignore for-alls - arg_tys ty - | Just (_, ty') <- splitForAllTy_maybe ty = arg_tys ty' - | Just (arg,res) <- splitFunTy_maybe ty = arg : arg_tys res - | otherwise = [] + (_, ty2) = coercionKind co + co_arity = typeArity ty2 +-} +\end{code} + +Note [trimCast] +~~~~~~~~~~~~~~~ +When you try putting trimCast back in, comment out the snippets +flagged by the other references to Note [trimCast] + +\begin{code} +--------------------------- +trimArity :: Bool -> ArityType -> ArityType +-- We have something like (let x = E in b), where b has the given +-- arity type. Then +-- * If E is cheap we can push it inside as far as we like +-- * If b eventually diverges, we allow ourselves to push inside +-- arbitrarily, even though that is not quite right +trimArity _cheap (AT a ABot) = AT a ABot +trimArity True (AT a ATop) = AT a ATop +trimArity False (AT _ ATop) = AT 0 ATop -- Bale out + +--------------------------- +arityType :: Bool -> CoreExpr -> ArityType +arityType _ (Var v) + | Just strict_sig <- idStrictness_maybe v + , (ds, res) <- splitStrictSig strict_sig + , isBotRes res + = AT (length ds) ABot -- Function diverges + | otherwise + = AT (idArity v) ATop -- Lambdas; increase arity -arityType dflags (Lam x e) - | isId x = AFun (isOneShotBndr x) (arityType dflags e) - | otherwise = arityType dflags e +arityType dicts_cheap (Lam x e) + | isId x = incArity (arityType dicts_cheap e) + | otherwise = arityType dicts_cheap e -- Applications; decrease arity -arityType dflags (App f (Type _)) = arityType dflags f -arityType dflags (App f a) - = case arityType dflags f of - ABot -> ABot -- If function diverges, ignore argument - ATop -> ATop -- No no info about function - AFun _ xs - | exprIsCheap a -> xs - | otherwise -> ATop - +arityType dicts_cheap (App fun (Type _)) + = arityType dicts_cheap fun +arityType dicts_cheap (App fun arg ) + = trimArity (exprIsCheap arg) (decArity (arityType dicts_cheap fun)) + -- Case/Let; keep arity if either the expression is cheap -- or it's a 1-shot lambda -- The former is not really right for Haskell @@ -280,26 +412,21 @@ arityType dflags (App f a) -- ===> -- f x y = case x of { (a,b) -> e } -- The difference is observable using 'seq' -arityType dflags (Case scrut _ _ alts) - = case foldr1 andArityType [arityType dflags rhs | (_,_,rhs) <- alts] of - xs | exprIsCheap scrut -> xs - AFun one_shot _ | one_shot -> AFun True ATop - _ -> ATop - -arityType dflags (Let b e) - = case arityType dflags e of - xs | cheap_bind b -> xs - AFun one_shot _ | one_shot -> AFun True ATop - _ -> ATop +arityType dicts_cheap (Case scrut _ _ alts) + = trimArity (exprIsCheap scrut) + (foldr1 andArityType [arityType dicts_cheap rhs | (_,_,rhs) <- alts]) + +arityType dicts_cheap (Let b e) + = trimArity (cheap_bind b) (arityType dicts_cheap e) where cheap_bind (NonRec b e) = is_cheap (b,e) cheap_bind (Rec prs) = all is_cheap prs - is_cheap (b,e) = (dopt Opt_DictsCheap dflags && isDictLikeTy (idType b)) + is_cheap (b,e) = (dicts_cheap && isDictLikeTy (idType b)) || exprIsCheap e -- If the experimental -fdicts-cheap flag is on, we eta-expand through -- dictionary bindings. This improves arities. Thereby, it also -- means that full laziness is less prone to floating out the - -- application of a function to its dictionary arguments, which + -- application of a function to its dictionary arguments, which -- can thereby lose opportunities for fusion. Example: -- foo :: Ord a => a -> ... -- foo = /\a \(d:Ord a). let d' = ...d... in \(x:a). .... @@ -309,17 +436,19 @@ arityType dflags (Let b e) -- -- The (foo DInt) is floated out, and makes ineffective a RULE -- foo (bar x) = ... - -- + -- -- One could go further and make exprIsCheap reply True to any -- dictionary-typed expression, but that's more work. -- -- See Note [Dictionary-like types] in TcType.lhs for why we use -- isDictLikeTy here rather than isDictTy -arityType _ _ = ATop +arityType dicts_cheap (Note _ e) = arityType dicts_cheap e +arityType dicts_cheap (Cast e co) = trimCast co (arityType dicts_cheap e) +arityType _ _ = vanillaArityType \end{code} - - + + %************************************************************************ %* * The main eta-expander @@ -336,6 +465,13 @@ simplification but it's not too hard. The alernative, of relying on a subsequent clean-up phase of the Simplifier to de-crapify the result, means you can't really use it in CorePrep, which is painful. +Note [Eta expansion and SCCs] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Note that SCCs are not treated specially by etaExpand. If we have + etaExpand 2 (\x -> scc "foo" e) + = (\xy -> (scc "foo" e) y) +So the costs of evaluating 'e' (not 'e y') are attributed to "foo" + \begin{code} -- | @etaExpand n us e ty@ returns an expression with -- the same meaning as @e@, but with arity @n@. @@ -350,11 +486,6 @@ means you can't really use it in CorePrep, which is painful. etaExpand :: Arity -- ^ Result should have this number of value args -> CoreExpr -- ^ Expression to expand -> CoreExpr --- Note that SCCs are not treated specially. If we have --- etaExpand 2 (\x -> scc "foo" e) --- = (\xy -> (scc "foo" e) y) --- So the costs of evaluating 'e' (not 'e y') are attributed to "foo" - -- etaExpand deals with for-alls. For example: -- etaExpand 1 E -- where E :: forall a. a -> a @@ -365,16 +496,13 @@ etaExpand :: Arity -- ^ Result should have this number of value args -- so perhaps the extra code isn't worth it etaExpand n orig_expr - | manifestArity orig_expr >= n = orig_expr -- The no-op case - | otherwise = go n orig_expr where - -- Strip off existing lambdas + -- Strip off existing lambdas and casts + -- Note [Eta expansion and SCCs] go 0 expr = expr go n (Lam v body) | isTyVar v = Lam v (go n body) | otherwise = Lam v (go (n-1) body) - go n (Note InlineMe expr) = Note InlineMe (go n expr) - -- Note [Eta expansion and SCCs] go n (Cast expr co) = Cast (go n expr) co go n expr = -- pprTrace "ee" (vcat [ppr orig_expr, ppr expr, ppr etas]) $ etaInfoAbs etas (etaInfoApp subst' expr etas) @@ -456,8 +584,8 @@ mkEtaWW :: Arity -> InScopeSet -> Type -- Outgoing InScopeSet includes the EtaInfo vars -- and the original free vars -mkEtaWW n in_scope ty - = go n empty_subst ty [] +mkEtaWW orig_n in_scope orig_ty + = go orig_n empty_subst orig_ty [] where empty_subst = mkTvSubst in_scope emptyTvSubstEnv @@ -475,6 +603,7 @@ mkEtaWW n in_scope ty -- Avoid free vars of the original expression = go (n-1) subst' res_ty (EtaVar eta_id' : eis) +-- See Note [trimCast] | Just(ty',co) <- splitNewTypeRepCo_maybe ty = -- Given this: -- newtype T = MkT ([T] -> Int) @@ -482,10 +611,12 @@ mkEtaWW n in_scope ty -- eta_expand 1 e T -- We want to get -- coerce T (\x::[T] -> (coerce ([T]->Int) e) x) - go n subst ty' (EtaCo (substTy subst co) : eis) + go n subst ty' (EtaCo (Type.substTy subst co) : eis) +------- | otherwise -- We have an expression of arity > 0, - = (getTvInScope subst, reverse eis) -- but its type isn't a function. + = WARN( True, ppr orig_n <+> ppr orig_ty ) + (getTvInScope subst, reverse eis) -- but its type isn't a function. -- This *can* legitmately happen: -- e.g. coerce Int (\x. x) Essentially the programmer is -- playing fast and loose with types (Happy does this a lot). @@ -494,22 +625,13 @@ mkEtaWW n in_scope ty -------------- --- Avoiding unnecessary substitution +-- Avoiding unnecessary substitution; use short-cutting versions subst_expr :: Subst -> CoreExpr -> CoreExpr -subst_expr s e | isEmptySubst s = e - | otherwise = substExpr s e +subst_expr = substExprSC (text "CoreArity:substExpr") subst_bind :: Subst -> CoreBind -> (Subst, CoreBind) -subst_bind subst (NonRec b r) - = (subst', NonRec b' (subst_expr subst r)) - where - (subst', b') = substBndr subst b -subst_bind subst (Rec prs) - = (subst', Rec (bs1 `zip` map (subst_expr subst') rhss)) - where - (bs, rhss) = unzip prs - (subst', bs1) = substBndrs subst bs +subst_bind = substBindSC -------------- @@ -524,9 +646,9 @@ freshEtaId :: Int -> TvSubst -> Type -> (TvSubst, Id) freshEtaId n subst ty = (subst', eta_id') where - ty' = substTy subst ty + ty' = Type.substTy subst ty eta_id' = uniqAway (getTvInScope subst) $ mkSysLocal (fsLit "eta") (mkBuiltinUnique n) ty' - subst' = extendTvInScope subst [eta_id'] + subst' = extendTvInScope subst eta_id' \end{code}