% % (c) The University of Glasgow 2006 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % Arity and ete expansion \begin{code}
{-# OPTIONS -fno-warn-tabs #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and
-- detab the module (please do the detabbing in a separate patch). See
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
-- for details

-- | Arit and eta expansion
module CoreArity (
	manifestArity, exprArity, exprBotStrictness_maybe,
	exprEtaExpandArity, CheapFun, etaExpand
    ) where

#include "HsVersions.h"

import CoreSyn
import CoreFVs
import CoreUtils
import CoreSubst
import Demand
import Var
import VarEnv
import Id
import Type
import TyCon	( isRecursiveTyCon, isClassTyCon )
import Coercion
import BasicTypes
import Unique
import DynFlags ( DynFlags, DynFlag(..), dopt )
import Outputable
import FastString
import Pair
\end{code} %************************************************************************ %* * manifestArity and exprArity %* * %************************************************************************ exprArity is a cheap-and-cheerful version of exprEtaExpandArity. It tells how many things the expression can be applied to before doing any work. It doesn't look inside cases, lets, etc. The idea is that exprEtaExpandArity will do the hard work, leaving something that's easy for exprArity to grapple with. In particular, Simplify uses exprArity to compute the ArityInfo for the Id. Originally I thought that it was enough just to look for top-level lambdas, but it isn't. I've seen this foo = PrelBase.timesInt We want foo to get arity 2 even though the eta-expander will leave it unchanged, in the expectation that it'll be inlined. But occasionally it isn't, because foo is blacklisted (used in a rule). Similarly, see the ok_note check in exprEtaExpandArity. So f = __inline_me (\x -> e) won't be eta-expanded. And in any case it seems more robust to have exprArity be a bit more intelligent. But note that (\x y z -> f x y z) should have arity 3, regardless of f's arity. \begin{code}
manifestArity :: CoreExpr -> Arity
-- ^ manifestArity sees how many leading value lambdas there are
manifestArity (Lam v e) | isId v    	= 1 + manifestArity e
			| otherwise 	= manifestArity e
manifestArity (Tick t e) | not (tickishIsCode t) =  manifestArity e
manifestArity (Cast e _)            	= manifestArity e
manifestArity _                     	= 0

exprArity :: CoreExpr -> Arity
-- ^ An approximate, fast, version of 'exprEtaExpandArity'
exprArity e = go e
    go (Var v) 	       	           = idArity v
    go (Lam x e) | isId x    	   = go e + 1
    		 | otherwise 	   = go e
    go (Tick t e) | not (tickishIsCode t) = go e
    go (Cast e co)                 = go e `min` length (typeArity (pSnd (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]
	-- NB: coercions count as a value argument

    go _		       	   = 0

typeArity :: Type -> [OneShot]
-- How many value arrows are visible in the type?
-- We look through foralls, and newtypes
-- See Note [exprArity invariant]
typeArity ty 
  | Just (_, ty')  <- splitForAllTy_maybe ty 
  = typeArity ty'

  | Just (arg,res) <- splitFunTy_maybe ty    
  = isStateHackType arg : typeArity res

  | Just (tc,tys) <- splitTyConApp_maybe ty 
  , Just (ty', _) <- instNewTyCon_maybe tc tys
  , not (isRecursiveTyCon tc)
  , not (isClassTyCon tc)	-- Do not eta-expand through newtype classes
    		      		-- See Note [Newtype classes and eta expansion]
  = typeArity ty'
  	-- 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!

  | otherwise
  = []

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 getBotArity (arityType env e) of
	Nothing -> Nothing
	Just ar -> Just (ar, mkStrictSig (mkTopDmdType (replicate ar topDmd) BotRes))
    env = AE { ae_bndrs = [], ae_ped_bot = True, ae_cheap_fn = \ _ _ -> False }
                  -- For this purpose we can be very simple
\end{code} Note [exprArity invariant] ~~~~~~~~~~~~~~~~~~~~~~~~~~ exprArity has the following invariant: (1) If typeArity (exprType e) = n, then manifestArity (etaExpand e n) = n That is, etaExpand can always expand as much as typeArity says So the case analysis in etaExpand and in typeArity must match (2) exprArity e <= typeArity (exprType e) (3) Hence if (exprArity e) = n, then manifestArity (etaExpand e n) = n That is, if exprArity says "the arity is n" then etaExpand really can get "n" manifest lambdas to the top. Why is this important? Because - In TidyPgm we use exprArity to fix the *final arity* of each top-level Id, and in - In CorePrep we use etaExpand on each rhs, so that the visible lambdas actually match that arity, which in turn means that the StgRhs has the right number of lambdas An alternative would be to do the eta-expansion in TidyPgm, at least for top-level bindings, in which case we would not need the trim_arity in exprArity. That is a less local change, so I'm going to leave it for today! Note [Newtype classes and eta expansion] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We have to be careful when eta-expanding through newtypes. In general it's a good idea, but annoyingly it interacts badly with the class-op rule mechanism. Consider class C a where { op :: a -> a } instance C b => C [b] where op x = ... These translate to co :: forall a. (a->a) ~ C a $copList :: C b -> [b] -> [b] $copList d x = ... $dfList :: C b -> C [b] {-# DFunUnfolding = [$copList] #-} $dfList d = $copList d |> co@[b] Now suppose we have: dCInt :: C Int blah :: [Int] -> [Int] blah = op ($dfList dCInt) Now we want the built-in op/$dfList rule will fire to give blah = $copList dCInt But with eta-expansion 'blah' might (and in Trac #3772, which is slightly more complicated, does) turn into blah = op (\eta. ($dfList dCInt |> sym co) eta) and now it is *much* harder for the op/$dfList rule to fire, becuase exprIsConApp_maybe won't hold of the argument to op. I considered trying to *make* it hold, but it's tricky and I gave up. The test simplCore/should_compile/T3722 is an excellent example. 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 %************************************************************************ %* * Computing the "arity" of an expression %* * %************************************************************************ Note [Definition of arity] ~~~~~~~~~~~~~~~~~~~~~~~~~~ The "arity" of an expression 'e' is n if applying 'e' to *fewer* than n *value* arguments converges rapidly Or, to put it another way there is no work lost in duplicating the partial application (e x1 .. x(n-1)) In the divegent case, no work is lost by duplicating because if the thing is evaluated once, that's the end of the program. Or, to put it another way, in any context C C[ (\x1 .. xn. e x1 .. xn) ] is as efficient as C[ e ] It's all a bit more subtle than it looks: 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. Note [Dealing with bottom] ~~~~~~~~~~~~~~~~~~~~~~~~~~ A Big Deal with computing arities is expressions like f = \x -> case x of True -> \s -> e1 False -> \s -> e2 This happens all the time when f :: Bool -> IO () In this case we do eta-expand, in order to get that \s to the top, and give f arity 2. This isn't really right in the presence of seq. Consider (f bot) `seq` 1 This should diverge! But if we eta-expand, it won't. We ignore this "problem" (unless -fpedantic-bottoms is on), because being scrupulous would lose an important transformation for many programs. (See Trac #5587 for an example.) 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. 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. So these two transformations aren't always the Right Thing, and we have several tickets reporting unexpected bahaviour resulting from this transformation. So we try to limit it as much as possible: (1) Do NOT move a lambda outside a known-bottom case expression case undefined of { (a,b) -> \y -> e } This showed up in Trac #5557 (2) Do NOT move a lambda outside a case if all the branches of the case are known to return bottom. case x of { (a,b) -> \y -> error "urk" } This case is less important, but the idea is that if the fn is going to diverge eventually anyway then getting the best arity isn't an issue, so we might as well play safe (3) Do NOT move a lambda outside a case unless (a) The scrutinee is ok-for-speculation, or (b) There is an enclosing value \x, and the scrutinee is x E.g. let x = case y of ( DEFAULT -> \v -> blah } We don't move the \y out. This is pretty arbitrary; but it catches the common case of doing `seq` on y. This is the reason for the under_lam argument to arityType. See Trac #5625 Of course both (1) and (2) are readily defeated by disguising the bottoms. 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 newtype T = MkT ([T] -> Int) Suppose we have e = coerce T f where f has arity 1. Then: etaExpandArity e = 1; 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. 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. 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) 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. This arose in another guise in Trac #3959. Here we had catch# (throw exn >> return ()) Note that (throw :: forall a e. Exn e => e -> a) is called with [a = IO ()]. After inlining (>>) we get catch# (\_. throw {IO ()} exn) We must *not* eta-expand to catch# (\_ _. throw {...} exn) because 'catch#' expects to get a (# _,_ #) after applying its argument to a State#, not another function! In short, we use the state hack to allow us to push let inside a lambda, but not to introduce a new lambda. Note [ArityType] ~~~~~~~~~~~~~~~~ ArityType is the result of a compositional analysis on expressions, from which we can decide the real arity of the expression (extracted with function exprEtaExpandArity). Here is what the fields mean. If an arbitrary expression 'f' has ArityType 'at', then * If at = ABot n, then (f x1..xn) definitely diverges. Partial applications to fewer than n args may *or may not* diverge. We allow ourselves to eta-expand bottoming functions, even if doing so may lose some `seq` sharing, let x = in \y. error (g x y) ==> \y. let x = in error (g x y) * If at = ATop as, and n=length as, then expanding 'f' to (\x1..xn. f x1 .. xn) loses no sharing, assuming the calls of f respect the one-shot-ness of of its definition. NB 'f' is an arbitary expression, eg (f = g e1 e2). This 'f' can have ArityType as ATop, with length as > 0, only if e1 e2 are themselves. * In both cases, f, (f x1), ... (f x1 ... f(n-1)) are definitely really functions, or bottom, but *not* casts from a data type, in at least one case branch. (If it's a function in one case branch but an unsafe cast from a data type in another, the program is bogus.) So eta expansion is dynamically ok; see Note [State hack and bottoming functions], the part about catch# Example: f = \x\y. let v = in \s(one-shot) \t(one-shot). blah 'f' has ArityType [ManyShot,ManyShot,OneShot,OneShot] The one-shot-ness means we can, in effect, push that 'let' inside the \st. Suppose f = \xy. x+y Then f :: AT [False,False] ATop f v :: AT [False] ATop f :: AT [] ATop -------------------- Main arity code ---------------------------- \begin{code}
-- See Note [ArityType]
data ArityType = ATop [OneShot] | ABot Arity
     -- There is always an explicit lambda
     -- to justify the [OneShot], or the Arity

type OneShot = Bool    -- False <=> Know nothing
                       -- True  <=> Can definitely float inside this lambda
	               -- The 'True' case can arise either because a binder
		       -- is marked one-shot, or because it's a state lambda
		       -- and we have the state hack on

vanillaArityType :: ArityType
vanillaArityType = ATop []	-- Totally uninformative

-- ^ The Arity returned is the number of value args the
-- expression can be applied to without doing much work
exprEtaExpandArity :: DynFlags -> CheapAppFun -> CoreExpr -> Arity
-- exprEtaExpandArity is used when eta expanding
-- 	e  ==>  \xy -> e x y
exprEtaExpandArity dflags cheap_app e
  = case (arityType env e) of
      ATop (os:oss) 
        | os || has_lam e -> 1 + length oss	-- Note [Eta expanding thunks]
        | otherwise       -> 0
      ATop []             -> 0
      ABot n              -> n
    env = AE { ae_bndrs    = []
             , ae_cheap_fn = mk_cheap_fn dflags cheap_app
             , ae_ped_bot  = dopt Opt_PedanticBottoms dflags }

    has_lam (Tick _ e) = has_lam e
    has_lam (Lam b e)  = isId b || has_lam e
    has_lam _          = False

getBotArity :: ArityType -> Maybe Arity
-- Arity of a divergent function
getBotArity (ABot n) = Just n
getBotArity _        = Nothing

mk_cheap_fn :: DynFlags -> CheapAppFun -> CheapFun
mk_cheap_fn dflags cheap_app
  | not (dopt Opt_DictsCheap dflags)
  = \e _     -> exprIsCheap' cheap_app e
  | otherwise
  = \e mb_ty -> exprIsCheap' cheap_app e
             || case mb_ty of
                  Nothing -> False
                  Just ty -> isDictLikeTy ty
\end{code} Note [Eta expanding through dictionaries] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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 can thereby lose opportunities for fusion. Example: foo :: Ord a => a -> ... foo = /\a \(d:Ord a). let d' = ...d... in \(x:a). .... -- So foo has arity 1 f = \x. foo dInt $ bar x 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 Note [Eta expanding thunks] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we see f = case y of p -> \x -> blah should we eta-expand it? Well, if 'x' is a one-shot state token then 'yes' because 'f' will only be applied once. But otherwise we (conservatively) say no. My main reason is to avoid expanding PAPSs f = g d ==> f = \x. g d x because that might in turn make g inline (if it has an inline pragma), which we might not want. After all, INLINE pragmas say "inline only when saturate" so we don't want to be too gung-ho about saturating! \begin{code}
arityLam :: Id -> ArityType -> ArityType
arityLam id (ATop as) = ATop (isOneShotBndr id : as)
arityLam _  (ABot n)  = ABot (n+1)

floatIn :: Bool -> ArityType -> ArityType
-- We have something like (let x = E in b), 
-- where b has the given arity type.  
floatIn _     (ABot n)  = ABot n
floatIn True  (ATop as) = ATop as
floatIn False (ATop as) = ATop (takeWhile id as)
   -- If E is not cheap, keep arity only for one-shots

arityApp :: ArityType -> Bool -> ArityType
-- Processing (fun arg) where at is the ArityType of fun,
-- Knock off an argument and behave like 'let'
arityApp (ABot 0)      _     = ABot 0
arityApp (ABot n)      _     = ABot (n-1)
arityApp (ATop [])     _     = ATop []
arityApp (ATop (_:as)) cheap = floatIn cheap (ATop as)

andArityType :: ArityType -> ArityType -> ArityType   -- Used for branches of a 'case'
andArityType (ABot n1) (ABot n2) 
  = ABot (n1 `min` n2)
andArityType (ATop as)  (ABot _)  = ATop as
andArityType (ABot _)   (ATop bs) = ATop bs
andArityType (ATop as)  (ATop bs) = ATop (as `combine` bs)
  where	     -- See Note [Combining case branches]
    combine (a:as) (b:bs) = (a && b) : combine as bs
    combine []     bs     = take_one_shots bs
    combine as     []     = take_one_shots as

    take_one_shots [] = []
    take_one_shots (one_shot : as) 
      | one_shot  = True : take_one_shots as
      | otherwise = [] 
\end{code} Note [Combining case branches] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider go = \x. let z = go e0 go2 = \x. case x of True -> z False -> \s(one-shot). e1 in go2 x We *really* want to eta-expand go and go2. When combining the barnches of the case we have ATop [] `andAT` ATop [True] and we want to get ATop [True]. But if the inner lambda wasn't one-shot we don't want to do this. (We need a proper arity analysis to justify that.) \begin{code}
type CheapFun = CoreExpr -> Maybe Type -> Bool
     	-- How to decide if an expression is cheap
	-- If the Maybe is Just, the type is the type
	-- of the expression; Nothing means "don't know"

data ArityEnv 
  = AE { ae_bndrs :: [Id]          -- Enclosing value-lambda Ids
                                   -- See Note [Dealing with bottom (3)]
       , ae_cheap_fn :: CheapFun
       , ae_ped_bot  :: Bool       -- True <=> be pedantic about bottoms

arityType :: ArityEnv -> CoreExpr -> ArityType

arityType env (Cast e co)
  = case arityType env e of
      ATop os -> ATop (take co_arity os)
      ABot n  -> ABot (n `min` co_arity)
    co_arity = length (typeArity (pSnd (coercionKind co)))
    -- See Note [exprArity invariant] (2); must be true of
    -- arityType too, since that is how we compute the arity
    -- of variables, and they in turn affect result of exprArity
    -- Trac #5441 is a nice demo
    -- However, do make sure that ATop -> ATop and ABot -> ABot!
    --   Casts don't affect that part. Getting this wrong provoked #5475

arityType _ (Var v)
  | Just strict_sig <- idStrictness_maybe v
  , (ds, res) <- splitStrictSig strict_sig
  , let arity = length ds
  = if isBotRes res then ABot arity
                    else ATop (take arity one_shots)
  | otherwise
  = ATop (take (idArity v) one_shots)
    one_shots :: [Bool]	    -- One-shot-ness derived from the type
    one_shots = typeArity (idType v)

	-- Lambdas; increase arity
arityType env (Lam x e)
  | isId x    = arityLam x (arityType env' e)
  | otherwise = arityType env e
    env' = env { ae_bndrs = x : ae_bndrs env }

	-- Applications; decrease arity, except for types
arityType env (App fun (Type _))
   = arityType env fun
arityType env (App fun arg )
   = arityApp (arityType env' fun) (ae_cheap_fn env arg Nothing) 
     env' = env { ae_bndrs = case ae_bndrs env of
                                { [] -> []; (_:xs) -> xs } }

	-- 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
	--	f x = case x of { (a,b) -> \y. e }
	--  ===>
	--	f x y = case x of { (a,b) -> e }
	-- The difference is observable using 'seq'
arityType env (Case scrut _ _ alts)
  | exprIsBottom scrut || null alts
  = ABot 0     -- Do not eta expand
               -- See Note [Dealing with bottom (1)]
  | otherwise
  = case alts_type of
     ABot n  | n>0       -> ATop []    -- Don't eta expand 
     	     | otherwise -> ABot 0     -- if RHS is bottomming
    			               -- See Note [Dealing with bottom (2)]

     ATop as | not (ae_ped_bot env)    -- Check -fpedantic-bottoms
             , is_under scrut             -> ATop as
             | exprOkForSpeculation scrut -> ATop as
             | otherwise                  -> ATop (takeWhile id as)	    
    -- is_under implements Note [Dealing with bottom (3)]
    is_under (Var f)           = f `elem` ae_bndrs env
    is_under (App f (Type {})) = is_under f
    is_under (Cast f _)        = is_under f
    is_under _                 = False

    alts_type = foldr1 andArityType [arityType env rhs | (_,_,rhs) <- alts]

arityType env (Let b e) 
  = floatIn (cheap_bind b) (arityType env e)
    cheap_bind (NonRec b e) = is_cheap (b,e)
    cheap_bind (Rec prs)    = all is_cheap prs
    is_cheap (b,e) = ae_cheap_fn env e (Just (idType b))

arityType env (Tick t e)
  | not (tickishIsCode t)     = arityType env e

arityType _ _ = vanillaArityType
\end{code} %************************************************************************ %* * The main eta-expander %* * %************************************************************************ We go for: f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym (n >= 0) where (in both cases) * The xi can include type variables * The yi are all value variables * N is a NORMAL FORM (i.e. no redexes anywhere) wanting a suitable number of extra args. The biggest reason for doing this is for cases like f = \x -> case x of True -> \y -> e1 False -> \y -> e2 Here we want to get the lambdas together. A good exmaple is the nofib program fibheaps, which gets 25% more allocation if you don't do this eta-expansion. We may have to sandwich some coerces between the lambdas to make the types work. exprEtaExpandArity looks through coerces when computing arity; and etaExpand adds the coerces as necessary when actually computing the expansion. Note [No crap in eta-expanded code] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The eta expander is careful not to introduce "crap". In particular, given a CoreExpr satisfying the 'CpeRhs' invariant (in CorePrep), it returns a CoreExpr satisfying the same invariant. See Note [Eta expansion and the CorePrep invariants] in CorePrep. This means the eta-expander has to do a bit of on-the-fly 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@.
-- Given:
-- > e' = etaExpand n us e ty
-- We should have that:
-- > ty = exprType e = exprType e'
etaExpand :: Arity	  	-- ^ Result should have this number of value args
	  -> CoreExpr	        -- ^ Expression to expand
	  -> CoreExpr
-- etaExpand deals with for-alls. For example:
--		etaExpand 1 E
-- where  E :: forall a. a -> a
-- would return
--	(/\b. \y::a -> E b y)
-- It deals with coerces too, though they are now rare
-- so perhaps the extra code isn't worth it

etaExpand n orig_expr
  = go n orig_expr
      -- 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 (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)
			    in_scope = mkInScopeSet (exprFreeVars expr)
			    (in_scope', etas) = mkEtaWW n orig_expr in_scope (exprType expr)
			    subst' = mkEmptySubst in_scope'

      	      	       	        -- Wrapper    Unwrapper
data EtaInfo = EtaVar Var	-- /\a. [],   [] a
     	                 	-- \x.  [],   [] x
	     | EtaCo Coercion   -- [] |> co,  [] |> (sym co)

instance Outputable EtaInfo where
   ppr (EtaVar v) = ptext (sLit "EtaVar") <+> ppr v
   ppr (EtaCo co) = ptext (sLit "EtaCo")  <+> ppr co

pushCoercion :: Coercion -> [EtaInfo] -> [EtaInfo]
pushCoercion co1 (EtaCo co2 : eis)
  | isReflCo co = eis
  | otherwise	= EtaCo co : eis
    co = co1 `mkTransCo` co2

pushCoercion co eis = EtaCo co : eis

etaInfoAbs :: [EtaInfo] -> CoreExpr -> CoreExpr
etaInfoAbs []               expr = expr
etaInfoAbs (EtaVar v : eis) expr = Lam v (etaInfoAbs eis expr)
etaInfoAbs (EtaCo co : eis) expr = Cast (etaInfoAbs eis expr) (mkSymCo co)

etaInfoApp :: Subst -> CoreExpr -> [EtaInfo] -> CoreExpr
-- (etaInfoApp s e eis) returns something equivalent to 
-- 	       ((substExpr s e) `appliedto` eis)

etaInfoApp subst (Lam v1 e) (EtaVar v2 : eis) 
  = etaInfoApp (CoreSubst.extendSubstWithVar subst v1 v2) e eis

etaInfoApp subst (Cast e co1) eis
  = etaInfoApp subst e (pushCoercion co' eis)
    co' = CoreSubst.substCo subst co1

etaInfoApp subst (Case e b ty alts) eis 
  = Case (subst_expr subst e) b1 (mk_alts_ty (CoreSubst.substTy subst ty) eis) alts'
    (subst1, b1) = substBndr subst b
    alts' = map subst_alt alts
    subst_alt (con, bs, rhs) = (con, bs', etaInfoApp subst2 rhs eis) 
	      	 (subst2,bs') = substBndrs subst1 bs

    mk_alts_ty ty []               = ty
    mk_alts_ty ty (EtaVar v : eis) = mk_alts_ty (applyTypeToArg ty (varToCoreExpr v)) eis
    mk_alts_ty _  (EtaCo co : eis) = mk_alts_ty (pSnd (coercionKind co)) eis
etaInfoApp subst (Let b e) eis 
  = Let b' (etaInfoApp subst' e eis)
    (subst', b') = subst_bind subst b

etaInfoApp subst (Tick t e) eis
  = Tick (substTickish subst t) (etaInfoApp subst e eis)

etaInfoApp subst e eis
  = go (subst_expr subst e) eis
    go e []                  = e
    go e (EtaVar v    : eis) = go (App e (varToCoreExpr v)) eis
    go e (EtaCo co    : eis) = go (Cast e co) eis

mkEtaWW :: Arity -> CoreExpr -> InScopeSet -> Type
	-> (InScopeSet, [EtaInfo])
	-- EtaInfo contains fresh variables,
	--   not free in the incoming CoreExpr
	-- Outgoing InScopeSet includes the EtaInfo vars
	--   and the original free vars

mkEtaWW orig_n orig_expr in_scope orig_ty
  = go orig_n empty_subst orig_ty []
    empty_subst = TvSubst in_scope emptyTvSubstEnv

    go n subst ty eis	    -- See Note [exprArity invariant]
       | n == 0
       = (getTvInScope subst, reverse eis)

       | Just (tv,ty') <- splitForAllTy_maybe ty
       , let (subst', tv') = Type.substTyVarBndr subst tv
           -- Avoid free vars of the original expression
       = go n subst' ty' (EtaVar tv' : eis)

       | Just (arg_ty, res_ty) <- splitFunTy_maybe ty
       , let (subst', eta_id') = freshEtaId n subst arg_ty 
           -- Avoid free vars of the original expression
       = go (n-1) subst' res_ty (EtaVar eta_id' : eis)
       | Just(ty',co) <- splitNewTypeRepCo_maybe ty
       = 	-- Given this:
       		-- 	newtype T = MkT ([T] -> Int)
       		-- Consider eta-expanding this
       		--  	eta_expand 1 e T
       		-- We want to get
       		--	coerce T (\x::[T] -> (coerce ([T]->Int) e) x)
         go n subst ty' (EtaCo co : eis)

       | otherwise	 -- We have an expression of arity > 0, 
       	 		 -- but its type isn't a function. 		   
       = WARN( True, (ppr orig_n <+> ppr orig_ty) $$ ppr orig_expr )
         (getTvInScope subst, reverse eis)
    	-- 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).
	-- So we simply decline to eta-expand.  Otherwise we'd end up
	-- with an explicit lambda having a non-function type

-- Avoiding unnecessary substitution; use short-cutting versions

subst_expr :: Subst -> CoreExpr -> CoreExpr
subst_expr = substExprSC (text "CoreArity:substExpr")

subst_bind :: Subst -> CoreBind -> (Subst, CoreBind)
subst_bind = substBindSC

freshEtaId :: Int -> TvSubst -> Type -> (TvSubst, Id)
-- Make a fresh Id, with specified type (after applying substitution)
-- It should be "fresh" in the sense that it's not in the in-scope set
-- of the TvSubstEnv; and it should itself then be added to the in-scope
-- set of the TvSubstEnv
-- The Int is just a reasonable starting point for generating a unique;
-- it does not necessarily have to be unique itself.
freshEtaId n subst ty
      = (subst', eta_id')
        ty'     = Type.substTy subst ty
	eta_id' = uniqAway (getTvInScope subst) $
		  mkSysLocal (fsLit "eta") (mkBuiltinUnique n) ty'
	subst'  = extendTvInScope subst eta_id'