Allow the old [$foo| ... |] syntax for quasi-quotes

[ghc-hetmet.git] / compiler / simplCore / SimplUtils.lhs

diff --git a/compiler/simplCore/SimplUtils.lhs b/compiler/simplCore/SimplUtils.lhs
index a37cfe9..1618525 100644 (file)
--- a/compiler/simplCore/SimplUtils.lhs
+++ b/compiler/simplCore/SimplUtils.lhs
@@ -6,7 +6,7 @@
 \begin{code}
 module SimplUtils (
        -- Rebuilding
 \begin{code}
 module SimplUtils (
        -- Rebuilding

-       mkLam, mkCase, prepareAlts, 
+       mkLam, mkCase, prepareAlts, tryEtaExpand,

 
        -- Inlining,
        preInlineUnconditionally, postInlineUnconditionally, 
 
        -- Inlining,
        preInlineUnconditionally, postInlineUnconditionally, 

@@ -15,8 +15,9 @@ module SimplUtils (
 
        -- The continuation type
        SimplCont(..), DupFlag(..), ArgInfo(..),
 
        -- The continuation type
        SimplCont(..), DupFlag(..), ArgInfo(..),

+        isSimplified,

        contIsDupable, contResultType, contIsTrivial, contArgs, dropArgs, 
        contIsDupable, contResultType, contIsTrivial, contArgs, dropArgs, 

-       pushArgs, countValArgs, countArgs, addArgTo,
+       pushSimplifiedArgs, countValArgs, countArgs, addArgTo,

        mkBoringStop, mkRhsStop, mkLazyArgStop, contIsRhsOrArg,
        interestingCallContext, 
 
        mkBoringStop, mkRhsStop, mkLazyArgStop, contIsRhsOrArg,
        interestingCallContext, 
 

@@ -40,7 +41,7 @@ import CoreArity
 import CoreUnfold
 import Name
 import Id
 import CoreUnfold
 import Name
 import Id

-import Var     ( Var, isCoVar )
+import Var

 import Demand
 import SimplMonad
 import Type    hiding( substTy )
 import Demand
 import SimplMonad
 import Type    hiding( substTy )

@@ -99,12 +100,12 @@ data SimplCont
        SimplCont
 
   | ApplyTo            -- C arg
        SimplCont
 
   | ApplyTo            -- C arg

-       DupFlag 
-       InExpr StaticEnv                -- The argument and its static env
+       DupFlag                 -- See Note [DupFlag invariants]
+       InExpr StaticEnv        -- The argument and its static env

        SimplCont
 
   | Select             -- case C of alts
        SimplCont
 
   | Select             -- case C of alts

-       DupFlag 
+       DupFlag                 -- See Note [DupFlag invariants]

        InId [InAlt] StaticEnv  -- The case binder, alts, and subst-env
        SimplCont
 
        InId [InAlt] StaticEnv  -- The case binder, alts, and subst-env
        SimplCont
 

@@ -151,14 +152,31 @@ instance Outputable SimplCont where
                                       (nest 2 $ vcat [ppr (seTvSubst se), ppr alts]) $$ ppr cont 
   ppr (CoerceIt co cont)            = (ptext (sLit "CoerceIt") <+> ppr co) $$ ppr cont
 
                                       (nest 2 $ vcat [ppr (seTvSubst se), ppr alts]) $$ ppr cont 
   ppr (CoerceIt co cont)            = (ptext (sLit "CoerceIt") <+> ppr co) $$ ppr cont
 

-data DupFlag = OkToDup | NoDup
+data DupFlag = NoDup       -- Unsimplified, might be big
+             | Simplified  -- Simplified
+             | OkToDup     -- Simplified and small
+
+isSimplified :: DupFlag -> Bool
+isSimplified NoDup = False
+isSimplified _     = True      -- Invariant: the subst-env is empty

 
 instance Outputable DupFlag where
 
 instance Outputable DupFlag where

-  ppr OkToDup = ptext (sLit "ok")
-  ppr NoDup   = ptext (sLit "nodup")
+  ppr OkToDup    = ptext (sLit "ok")
+  ppr NoDup      = ptext (sLit "nodup")
+  ppr Simplified = ptext (sLit "simpl")
+\end{code}

 
 

+Note [DupFlag invariants]
+~~~~~~~~~~~~~~~~~~~~~~~~~
+In both (ApplyTo dup _ env k)
+   and  (Select dup _ _ env k)
+the following invariants hold

 
 

+  (a) if dup = OkToDup, then continuation k is also ok-to-dup
+  (b) if dup = OkToDup or Simplified, the subst-env is empty
+      (and and hence no need to re-simplify)

 
 

+\begin{code}

 -------------------
 mkBoringStop :: SimplCont
 mkBoringStop = Stop BoringCtxt
 -------------------
 mkBoringStop :: SimplCont
 mkBoringStop = Stop BoringCtxt

@@ -179,8 +197,8 @@ contIsRhsOrArg _               = False
 -------------------
 contIsDupable :: SimplCont -> Bool
 contIsDupable (Stop {})                  = True
 -------------------
 contIsDupable :: SimplCont -> Bool
 contIsDupable (Stop {})                  = True

-contIsDupable (ApplyTo  OkToDup _ _ _)   = True
-contIsDupable (Select   OkToDup _ _ _ _) = True
+contIsDupable (ApplyTo  OkToDup _ _ _)   = True        -- See Note [DupFlag invariants]
+contIsDupable (Select   OkToDup _ _ _ _) = True -- ...ditto...

 contIsDupable (CoerceIt _ cont)          = contIsDupable cont
 contIsDupable _                          = False
 
 contIsDupable (CoerceIt _ cont)          = contIsDupable cont
 contIsDupable _                          = False
 

@@ -238,9 +256,10 @@ contArgs cont@(ApplyTo {})
 
 contArgs cont = (True, [], cont)
 
 
 contArgs cont = (True, [], cont)
 

-pushArgs :: SimplEnv -> [CoreExpr] -> SimplCont -> SimplCont
-pushArgs _env []         cont = cont
-pushArgs env  (arg:args) cont = ApplyTo NoDup arg env (pushArgs env args cont)
+pushSimplifiedArgs :: SimplEnv -> [CoreExpr] -> SimplCont -> SimplCont
+pushSimplifiedArgs _env []         cont = cont
+pushSimplifiedArgs env  (arg:args) cont = ApplyTo Simplified arg env (pushSimplifiedArgs env args cont)
+                  -- The env has an empty SubstEnv

 
 dropArgs :: Int -> SimplCont -> SimplCont
 dropArgs 0 cont = cont
 
 dropArgs :: Int -> SimplCont -> SimplCont
 dropArgs 0 cont = cont

@@ -433,10 +452,9 @@ interestingArgContext rules call_cont
 \end{code}
 
 
 \end{code}
 
 

-

 %************************************************************************
 %*                                                                     *
 %************************************************************************
 %*                                                                     *

-\subsection{Decisions about inlining}
+                  Gentle mode                                                                  

 %*                                                                     *
 %************************************************************************
 
 %*                                                                     *
 %************************************************************************
 

@@ -457,9 +475,33 @@ Gentle mode has a separate boolean flag to control
        a) inlining (sm_inline flag)
        b) rules    (sm_rules  flag)
 A key invariant about Gentle mode is that it is treated as the EARLIEST
        a) inlining (sm_inline flag)
        b) rules    (sm_rules  flag)
 A key invariant about Gentle mode is that it is treated as the EARLIEST

-phase.  Something is inlined if the sm_inline flag is on AND the thing
-is inlinable in the earliest phase.  This is important. Example
+phase.  

 
 

+\begin{code}
+simplEnvForGHCi :: SimplEnv
+simplEnvForGHCi = mkSimplEnv allOffSwitchChecker $
+                  SimplGently { sm_rules = True, sm_inline = False }
+   -- Do not do any inlining, in case we expose some unboxed
+   -- tuple stuff that confuses the bytecode interpreter
+
+simplEnvForRules :: SimplEnv
+simplEnvForRules = mkSimplEnv allOffSwitchChecker $
+                   SimplGently { sm_rules = True, sm_inline = False }
+
+updModeForInlineRules :: Activation -> SimplifierMode -> SimplifierMode
+-- See Note [Simplifying inside InlineRules]
+updModeForInlineRules _inline_rule_act _current_mode
+  = SimplGently { sm_rules = True, sm_inline = True }
+\end{code}
+
+Note [Inlining in gentle mode]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Something is inlined if 
+   (i)   the sm_inline flag is on, AND
+   (ii)  the thing has an INLINE pragma, AND
+   (iii) the thing is inlinable in the earliest phase.  
+
+Example of why (iii) is important:

   {-# INLINE [~1] g #-}
   g = ...
   
   {-# INLINE [~1] g #-}
   g = ...
   

@@ -505,6 +547,9 @@ RULES are enabled when doing "gentle" simplification.  Two reasons:
 But watch out: list fusion can prevent floating.  So use phase control
 to switch off those rules until after floating.
 
 But watch out: list fusion can prevent floating.  So use phase control
 to switch off those rules until after floating.
 

+Currently (Oct10) I think that sm_rules is always True, so we
+could remove it.
+

 Note [Simplifying inside InlineRules]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 We must take care with simplification inside InlineRules (which come from
 Note [Simplifying inside InlineRules]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 We must take care with simplification inside InlineRules (which come from

@@ -524,57 +569,7 @@ one; see OccurAnal.addRuleUsage.
 Second, we do want *do* to some modest rules/inlining stuff in InlineRules,
 partly to eliminate senseless crap, and partly to break the recursive knots
 generated by instance declarations.  To keep things simple, we always set 
 Second, we do want *do* to some modest rules/inlining stuff in InlineRules,
 partly to eliminate senseless crap, and partly to break the recursive knots
 generated by instance declarations.  To keep things simple, we always set 

-the phase to 'gentle' when processing InlineRules.  OK, so suppose we have
-       {-# INLINE <act> f #-}
-       f = <rhs>
-meaning "inline f in phases p where activation <act>(p) holds". 
-Then what inlinings/rules can we apply to the copy of <rhs> captured in
-f's InlineRule?  Our model is that literally <rhs> is substituted for
-f when it is inlined.  So our conservative plan (implemented by 
-updModeForInlineRules) is this:
-
-  -------------------------------------------------------------
-  When simplifying the RHS of an InlineRule,
-  If the InlineRule becomes active in phase p, then
-    if the current phase is *earlier than* p, 
-       make no inlinings or rules active when simplifying the RHS
-    otherwise 
-       set the phase to p when simplifying the RHS
-  -------------------------------------------------------------
-
-That ensures that
-
-  a) Rules/inlinings that *cease* being active before p will 
-     not apply to the InlineRule rhs, consistent with it being
-     inlined in its *original* form in phase p.
-
-  b) Rules/inlinings that only become active *after* p will
-     not apply to the InlineRule rhs, again to be consistent with
-     inlining the *original* rhs in phase p.
-
-For example, 
-       {-# INLINE f #-}
-       f x = ...g...
-
-       {-# NOINLINE [1] g #-}
-       g y = ...
-
-       {-# RULE h g = ... #-}
-Here we must not inline g into f's RHS, even when we get to phase 0,
-because when f is later inlined into some other module we want the
-rule for h to fire.
-
-Similarly, consider
-       {-# INLINE f #-}
-       f x = ...g...
-
-       g y = ...
-and suppose that there are auto-generated specialisations and a strictness
-wrapper for g.  The specialisations get activation AlwaysActive, and the
-strictness wrapper get activation (ActiveAfter 0).  So the strictness
-wrepper fails the test and won't be inlined into f's InlineRule. That
-means f can inline, expose the specialised call to g, so the specialisation
-rules can fire.
+the phase to 'gentle' when processing InlineRules.  

 
 A note about wrappers
 ~~~~~~~~~~~~~~~~~~~~~
 
 A note about wrappers
 ~~~~~~~~~~~~~~~~~~~~~

@@ -588,41 +583,155 @@ mark it 'demanded', so when the RHS is simplified, it'll get an ArgOf
 continuation. 
 
 \begin{code}
 continuation. 
 
 \begin{code}

-simplEnvForGHCi :: SimplEnv
-simplEnvForGHCi = mkSimplEnv allOffSwitchChecker $
-                  SimplGently { sm_rules = False, sm_inline = False }
-   -- Do not do any inlining, in case we expose some unboxed
-   -- tuple stuff that confuses the bytecode interpreter
+activeUnfolding :: SimplEnv -> IdUnfoldingFun
+activeUnfolding env
+  = case getMode env of
+      SimplGently { sm_inline = False } -> active_unfolding_minimal
+      SimplGently { sm_inline = True  } -> active_unfolding_gentle
+      SimplPhase n _                    -> active_unfolding n

 
 

-simplEnvForRules :: SimplEnv
-simplEnvForRules = mkSimplEnv allOffSwitchChecker $
-                   SimplGently { sm_rules = True, sm_inline = False }
+activeUnfInRule :: SimplEnv -> IdUnfoldingFun
+-- When matching in RULE, we want to "look through" an unfolding
+-- (to see a constructor) if *rules* are on, even if *inlinings* 
+-- are not.  A notable example is DFuns, which really we want to 
+-- match in rules like (op dfun) in gentle mode. Another example
+-- is 'otherwise' which we want exprIsConApp_maybe to be able to
+-- see very early on
+activeUnfInRule env
+  = case getMode env of
+      SimplGently { sm_rules = False } -> active_unfolding_minimal
+      SimplGently { sm_rules = True  } -> active_unfolding_early
+      SimplPhase n _                   -> active_unfolding n
+  where
+    active_unfolding_early id
+      | isEarlyActive (idInlineActivation id) = idUnfolding id
+      | otherwise                             = idUnfolding id

 
 

-updModeForInlineRules :: Activation -> SimplifierMode -> SimplifierMode
--- See Note [Simplifying inside InlineRules]
---    Treat Gentle as phase "infinity"
---    If current_phase `earlier than` inline_rule_start_phase 
---      then no_op
---    else 
---    if current_phase `same phase` inline_rule_start_phase 
---      then current_phase   (keep gentle flags)
---      else inline_rule_start_phase
-updModeForInlineRules inline_rule_act current_mode
-  = case inline_rule_act of
-      NeverActive     -> no_op
-      AlwaysActive    -> mk_gentle current_mode
-      ActiveBefore {} -> mk_gentle current_mode
-      ActiveAfter n   -> mk_phase n current_mode
+active_unfolding_minimal :: IdUnfoldingFun
+-- Compuslory unfoldings only
+-- Ignore SimplGently, because we want to inline regardless;
+-- the Id has no top-level binding at all
+--
+-- NB: we used to have a second exception, for data con wrappers.
+-- On the grounds that we use gentle mode for rule LHSs, and 
+-- they match better when data con wrappers are inlined.
+-- But that only really applies to the trivial wrappers (like (:)),
+-- and they are now constructed as Compulsory unfoldings (in MkId)
+-- so they'll happen anyway.
+active_unfolding_minimal id
+  | isCompulsoryUnfolding unf = unf
+  | otherwise                 = NoUnfolding

   where
   where

-    no_op = SimplGently { sm_rules = False, sm_inline = False }
+    unf = idUnfolding id

 
 

-    mk_gentle (SimplGently {}) = current_mode
-    mk_gentle _                = SimplGently { sm_rules = True, sm_inline = True }
+active_unfolding_gentle :: IdUnfoldingFun
+-- Anything that is early-active
+-- See Note [Gentle mode]
+active_unfolding_gentle id
+  | isStableUnfolding unf
+  , isEarlyActive (idInlineActivation id) = unf
+       -- NB: wrappers are not early-active
+  | otherwise                             = NoUnfolding
+  where
+    unf = idUnfolding id
+      -- idUnfolding checks for loop-breakers
+      -- Things with an INLINE pragma may have 
+      -- an unfolding *and* be a loop breaker  
+      -- (maybe the knot is not yet untied)
+
+active_unfolding :: CompilerPhase -> IdUnfoldingFun
+active_unfolding n id
+  | isActive n (idInlineActivation id) = idUnfolding id
+  | otherwise                          = NoUnfolding

 
 

-    mk_phase n (SimplPhase _ ss) = SimplPhase n ss
-    mk_phase n (SimplGently {})  = SimplPhase n ["gentle-rules"]
+activeRule :: DynFlags -> SimplEnv -> Maybe (Activation -> Bool)
+-- Nothing => No rules at all
+activeRule dflags env
+  | not (dopt Opt_EnableRewriteRules dflags)
+  = Nothing    -- Rewriting is off
+  | otherwise
+  = case getMode env of
+      SimplGently { sm_rules = rules_on } 
+        | rules_on  -> Just isEarlyActive      -- Note [RULEs enabled in SimplGently]
+        | otherwise -> Nothing
+      SimplPhase n _ -> Just (isActive n)

 \end{code}
 
 \end{code}
 

+--------------------------------------------------------------
+                OLD NOTES, now wrong
+           Preserved just for now (Oct 10)
+--------------------------------------------------------------
+
+      OK, so suppose we have
+       {-# INLINE <act> f #-}
+       f = <rhs>
+      meaning "inline f in phases p where activation <act>(p) holds". 
+      Then what inlinings/rules can we apply to the copy of <rhs> captured in
+      f's InlineRule?  Our model is that literally <rhs> is substituted for
+      f when it is inlined.  So our conservative plan (implemented by 
+      updModeForInlineRules) is this:
+
+        -------------------------------------------------------------
+        When simplifying the RHS of an InlineRule,
+        If the InlineRule becomes active in phase p, then
+          if the current phase is *earlier than* p, 
+             make no inlinings or rules active when simplifying the RHS
+          otherwise 
+             set the phase to p when simplifying the RHS
+
+      --    Treat Gentle as phase "infinity"
+      --    If current_phase `earlier than` inline_rule_start_phase 
+      --      then no_op
+      --    else 
+      --    if current_phase `same phase` inline_rule_start_phase 
+      --      then current_phase   (keep gentle flags)
+      --      else inline_rule_start_phase
+        -------------------------------------------------------------
+
+      That ensures that
+
+        a) Rules/inlinings that *cease* being active before p will 
+           not apply to the InlineRule rhs, consistent with it being
+           inlined in its *original* form in phase p.
+
+        b) Rules/inlinings that only become active *after* p will
+           not apply to the InlineRule rhs, again to be consistent with
+           inlining the *original* rhs in phase p.
+
+      For example, 
+               {-# INLINE f #-}
+       f x = ...g...
+
+       {-# NOINLINE [1] g #-}
+       g y = ...
+
+       {-# RULE h g = ... #-}
+      Here we must not inline g into f's RHS, even when we get to phase 0,
+      because when f is later inlined into some other module we want the
+      rule for h to fire.
+
+      Similarly, consider
+               {-# INLINE f #-}
+       f x = ...g...
+
+       g y = ...
+      and suppose that there are auto-generated specialisations and a strictness
+      wrapper for g.  The specialisations get activation AlwaysActive, and the
+      strictness wrapper get activation (ActiveAfter 0).  So the strictness
+      wrepper fails the test and won't be inlined into f's InlineRule. That
+      means f can inline, expose the specialised call to g, so the specialisation
+      rules can fire.
+
+--------------------------------------------------------------
+                END OF OLD NOTES
+--------------------------------------------------------------
+
+
+%************************************************************************
+%*                                                                     *
+                  preInlineUnconditionally
+%*                                                                     *
+%************************************************************************

 
 preInlineUnconditionally
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 preInlineUnconditionally
 ~~~~~~~~~~~~~~~~~~~~~~~~

@@ -795,6 +904,12 @@ preInlineUnconditionally env top_lvl bndr rhs
 
 \end{code}
 
 
 \end{code}
 

+%************************************************************************
+%*                                                                     *
+                  postInlineUnconditionally
+%*                                                                     *
+%************************************************************************
+

 postInlineUnconditionally
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 @postInlineUnconditionally@ decides whether to unconditionally inline
 postInlineUnconditionally
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 @postInlineUnconditionally@ decides whether to unconditionally inline

@@ -904,69 +1019,6 @@ postInlineUnconditionally env top_lvl bndr occ_info rhs unfolding
                        -- See Note [pre/postInlineUnconditionally in gentle mode]
                   SimplPhase n _ -> isActive n act
     act = idInlineActivation bndr
                        -- See Note [pre/postInlineUnconditionally in gentle mode]
                   SimplPhase n _ -> isActive n act
     act = idInlineActivation bndr

-
-activeUnfolding :: SimplEnv -> IdUnfoldingFun
-activeUnfolding env
-  = case getMode env of
-      SimplGently { sm_inline = False } -> active_unfolding_minimal
-      SimplGently { sm_inline = True  } -> active_unfolding_gentle
-      SimplPhase n _                    -> active_unfolding n
-
-activeUnfInRule :: SimplEnv -> IdUnfoldingFun
--- When matching in RULE, we want to "look through" an unfolding
--- if *rules* are on, even if *inlinings* are not.  A notable example
--- is DFuns, which really we want to match in rules like (op dfun)
--- in gentle mode.
-activeUnfInRule env
-  = case getMode env of
-      SimplGently { sm_rules = False } -> active_unfolding_minimal
-      SimplGently { sm_rules = True  } -> active_unfolding_gentle
-      SimplPhase n _                   -> active_unfolding n
-
-active_unfolding_minimal :: IdUnfoldingFun
--- Compuslory unfoldings only
--- Ignore SimplGently, because we want to inline regardless;
--- the Id has no top-level binding at all
---
--- NB: we used to have a second exception, for data con wrappers.
--- On the grounds that we use gentle mode for rule LHSs, and 
--- they match better when data con wrappers are inlined.
--- But that only really applies to the trivial wrappers (like (:)),
--- and they are now constructed as Compulsory unfoldings (in MkId)
--- so they'll happen anyway.
-active_unfolding_minimal id
-  | isCompulsoryUnfolding unf = unf
-  | otherwise                 = NoUnfolding
-  where
-    unf = realIdUnfolding id   -- Never a loop breaker
-
-active_unfolding_gentle :: IdUnfoldingFun
--- Anything that is early-active
--- See Note [Gentle mode]
-active_unfolding_gentle id
-  | isEarlyActive (idInlineActivation id) = idUnfolding id
-  | otherwise                             = NoUnfolding
-      -- idUnfolding checks for loop-breakers
-      -- Things with an INLINE pragma may have 
-      -- an unfolding *and* be a loop breaker  
-      -- (maybe the knot is not yet untied)
-
-active_unfolding :: CompilerPhase -> IdUnfoldingFun
-active_unfolding n id
-  | isActive n (idInlineActivation id) = idUnfolding id
-  | otherwise                          = NoUnfolding
-
-activeRule :: DynFlags -> SimplEnv -> Maybe (Activation -> Bool)
--- Nothing => No rules at all
-activeRule dflags env
-  | not (dopt Opt_EnableRewriteRules dflags)
-  = Nothing    -- Rewriting is off
-  | otherwise
-  = case getMode env of
-      SimplGently { sm_rules = rules_on } 
-        | rules_on  -> Just isEarlyActive      -- Note [RULEs enabled in SimplGently]
-        | otherwise -> Nothing
-      SimplPhase n _ -> Just (isActive n)

 \end{code}
 
 Note [Top level and postInlineUnconditionally]
 \end{code}
 
 Note [Top level and postInlineUnconditionally]

@@ -1038,40 +1090,10 @@ mkLam _env bndrs body
       = do { tick (EtaReduction (head bndrs))
           ; return etad_lam }
 
       = do { tick (EtaReduction (head bndrs))
           ; return etad_lam }
 

-      | dopt Opt_DoLambdaEtaExpansion dflags
-      ,        any ok_to_expand bndrs
-      = do { let body'     = etaExpand fun_arity body
-                 fun_arity = exprEtaExpandArity dflags body
-          ; return (mkLams bndrs body') }
-   

       | otherwise 
       = return (mkLams bndrs body)
       | otherwise 
       = return (mkLams bndrs body)

-
-    ok_to_expand :: Var -> Bool        -- Note [When to eta expand]
-    ok_to_expand bndr = isId bndr && not (isDictId bndr)

 \end{code}
 
 \end{code}
 

-Note [When to eta expand]
-~~~~~~~~~~~~~~~~~~~~~~~~~
-We only eta expand if there is at least one non-tyvar, non-dict 
-binder.  The proximate cause for not eta-expanding dictionary lambdas 
-was this example:
-   genMap :: C a => ...
-   {-# INLINE genMap #-}
-   genMap f xs = ...
-
-   myMap :: D a => ...
-   {-# INLINE myMap #-}
-   myMap = genMap
-
-Notice that 'genMap' should only inline if applied to two arguments.
-In the InlineRule for myMap we'll have the unfolding 
-    (\d -> genMap Int (..d..))  
-We do not want to eta-expand to 
-    (\d f xs -> genMap Int (..d..) f xs) 
-because then 'genMap' will inline, and it really shouldn't: at least
-as far as the programmer is concerned, it's not applied to two
-arguments!

 
 Note [Casts and lambdas]
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 Note [Casts and lambdas]
 ~~~~~~~~~~~~~~~~~~~~~~~~

@@ -1105,20 +1127,65 @@ It does not make sense to transform
        /\g. e `cast` g  ==>  (/\g.e) `cast` (/\g.g)
 because the latter is not well-kinded.
 
        /\g. e `cast` g  ==>  (/\g.e) `cast` (/\g.g)
 because the latter is not well-kinded.
 

---     c) floating lets out through big lambdas 
---             [only if all tyvar lambdas, and only if this lambda
---              is the RHS of a let]
-
-{-     Sept 01: I'm experimenting with getting the
-       full laziness pass to float out past big lambdsa
- | all isTyCoVar bndrs,        -- Only for big lambdas
-   contIsRhs cont      -- Only try the rhs type-lambda floating
-                       -- if this is indeed a right-hand side; otherwise
-                       -- we end up floating the thing out, only for float-in
-                       -- to float it right back in again!
- = do (floats, body') <- tryRhsTyLam env bndrs body
-      return (floats, mkLams bndrs body')
--}
+%************************************************************************
+%*                                                                     *
+              Eta expansion                                                                     
+%*                                                                     *
+%************************************************************************
+
+\begin{code}
+tryEtaExpand :: SimplEnv -> OutId -> OutExpr -> SimplM (Arity, OutExpr)
+-- See Note [Eta-expanding at let bindings]
+tryEtaExpand env bndr rhs
+  = do { dflags <- getDOptsSmpl
+       ; (new_arity, new_rhs) <- try_expand dflags
+
+       ; WARN( new_arity < old_arity || new_arity < _dmd_arity, 
+               (ptext (sLit "Arity decrease:") <+> (ppr bndr <+> ppr old_arity
+               <+> ppr new_arity <+> ppr _dmd_arity) $$ ppr new_rhs) )
+                       -- Note [Arity decrease]
+         return (new_arity, new_rhs) }
+  where
+    try_expand dflags
+      | dopt Opt_DoLambdaEtaExpansion dflags 
+      , not (exprIsTrivial rhs)
+      , not (inGentleMode env)  -- In gentle mode don't eta-expansion
+                               -- because it can clutter up the code
+                               -- with casts etc that may not be removed
+      , let new_arity = exprEtaExpandArity dflags rhs
+      , new_arity > old_arity
+      = do { tick (EtaExpansion bndr)
+           ; return (new_arity, etaExpand new_arity rhs) }
+      | otherwise
+      = return (exprArity rhs, rhs)
+
+    old_arity  = idArity bndr
+    _dmd_arity = length $ fst $ splitStrictSig $ idStrictness bndr
+\end{code}
+
+Note [Eta-expanding at let bindings]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We now eta expand at let-bindings, which is where the payoff 
+comes. 
+
+One useful consequence is this example:
+   genMap :: C a => ...
+   {-# INLINE genMap #-}
+   genMap f xs = ...
+
+   myMap :: D a => ...
+   {-# INLINE myMap #-}
+   myMap = genMap
+
+Notice that 'genMap' should only inline if applied to two arguments.
+In the InlineRule for myMap we'll have the unfolding 
+    (\d -> genMap Int (..d..))  
+We do not want to eta-expand to 
+    (\d f xs -> genMap Int (..d..) f xs) 
+because then 'genMap' will inline, and it really shouldn't: at least
+as far as the programmer is concerned, it's not applied to two
+arguments!
+

 
 %************************************************************************
 %*                                                                     *
 
 %************************************************************************
 %*                                                                     *