types/OptCoercion.lhs

% % (c) The University of Glasgow 2006 % \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

module OptCoercion ( optCoercion ) where 

#include "HsVersions.h"

import Coercion
import Type hiding( substTyVarBndr, substTy, extendTvSubst )
import TcType       ( exactTyVarsOfType )
import TyCon
import Var
import VarSet
import VarEnv
import StaticFlags	( opt_NoOptCoercion )
import Outputable
import Pair
import Maybes( allMaybes )
import FastString
import Util

\end{code} %************************************************************************ %* * Optimising coercions %* * %************************************************************************ Note [Subtle shadowing in coercions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Supose we optimising a coercion optCoercion (forall (co_X5:t1~t2). ...co_B1...) The co_X5 is a wild-card; the bound variable of a coercion for-all should never appear in the body of the forall. Indeed we often write it like this optCoercion ( (t1~t2) => ...co_B1... ) Just because it's a wild-card doesn't mean we are free to choose whatever variable we like. For example it'd be wrong for optCoercion to return forall (co_B1:t1~t2). ...co_B1... because now the co_B1 (which is really free) has been captured, and subsequent substitutions will go wrong. That's why we can't use mkCoPredTy in the ForAll case, where this note appears. \begin{code}

optCoercion :: CvSubst -> Coercion -> NormalCo
-- ^ optCoercion applies a substitution to a coercion, 
--   *and* optimises it to reduce its size
optCoercion env co 
  | opt_NoOptCoercion = substCo env co
  | otherwise         = opt_co env False co

type NormalCo = Coercion
  -- Invariants: 
  --  * The substitution has been fully applied
  --  * For trans coercions (co1 `trans` co2)
  --       co1 is not a trans, and neither co1 nor co2 is identity
  --  * If the coercion is the identity, it has no CoVars of CoTyCons in it (just types)

type NormalNonIdCo = NormalCo  -- Extra invariant: not the identity

opt_co, opt_co' :: CvSubst
       		-> Bool	       -- True <=> return (sym co)
       		-> Coercion
       		-> NormalCo	
opt_co = opt_co'
{-
opt_co env sym co
 = pprTrace "opt_co {" (ppr sym <+> ppr co $$ ppr env) $
   co1 `seq`
   pprTrace "opt_co done }" (ppr co1) $
   (WARN( not same_co_kind, ppr co  <+> dcolon <+> pprEqPred (Pair s1 t1)
                         $$ ppr co1 <+> dcolon <+> pprEqPred (Pair s2 t2) )
    WARN( not (coreEqCoercion co1 simple_result),
           (text "env=" <+> ppr env) $$
           (text "input=" <+> ppr co) $$
           (text "simple=" <+> ppr simple_result) $$
           (text "opt=" <+> ppr co1) )
   co1)
 where
   co1 = opt_co' env sym co
   same_co_kind = s1 `eqType` s2 && t1 `eqType` t2
   Pair s t = coercionKind (substCo env co)
   (s1,t1) | sym = (t,s)
           | otherwise = (s,t)
   Pair s2 t2 = coercionKind co1

   simple_result | sym = mkSymCo (substCo env co)
                 | otherwise = substCo env co
-}

opt_co' env _   (Refl ty)           = Refl (substTy env ty)
opt_co' env sym (SymCo co)          = opt_co env (not sym) co
opt_co' env sym (TyConAppCo tc cos) = mkTyConAppCo tc (map (opt_co env sym) cos)
opt_co' env sym (AppCo co1 co2)     = mkAppCo (opt_co env sym co1) (opt_co env sym co2)
opt_co' env sym (ForAllCo tv co)    = case substTyVarBndr env tv of
                                         (env', tv') -> mkForAllCo tv' (opt_co env' sym co)
     -- Use the "mk" functions to check for nested Refls

opt_co' env sym (CoVarCo cv)
  | Just co <- lookupCoVar env cv
  = opt_co (zapCvSubstEnv env) sym co

  | Just cv1 <- lookupInScope (getCvInScope env) cv
  = ASSERT( isCoVar cv1 ) wrapSym sym (CoVarCo cv1)
                -- cv1 might have a substituted kind!

  | otherwise = WARN( True, ptext (sLit "opt_co: not in scope:") <+> ppr cv $$ ppr env)
                ASSERT( isCoVar cv )
                wrapSym sym (CoVarCo cv)

opt_co' env sym (AxiomInstCo con cos)
    -- Do *not* push sym inside top-level axioms
    -- e.g. if g is a top-level axiom
    --   g a : f a ~ a
    -- then (sym (g ty)) /= g (sym ty) !!
  = wrapSym sym $ AxiomInstCo con (map (opt_co env False) cos)
      -- Note that the_co does *not* have sym pushed into it

opt_co' env sym (UnsafeCo ty1 ty2)
  | ty1' `eqType` ty2' = Refl ty1'
  | sym                = mkUnsafeCo ty2' ty1'
  | otherwise          = mkUnsafeCo ty1' ty2'
  where
    ty1' = substTy env ty1
    ty2' = substTy env ty2

opt_co' env sym (TransCo co1 co2)
  | sym       = opt_trans in_scope opt_co2 opt_co1   -- sym (g `o` h) = sym h `o` sym g
  | otherwise = opt_trans in_scope opt_co1 opt_co2
  where
    opt_co1 = opt_co env sym co1
    opt_co2 = opt_co env sym co2
    in_scope = getCvInScope env

opt_co' env sym (NthCo n co)
  | TyConAppCo tc cos <- co'
  , isDecomposableTyCon tc		-- Not synonym families
  = ASSERT( n < length cos )
    cos !! n
  | otherwise
  = NthCo n co'
  where
    co' = opt_co env sym co

opt_co' env sym (InstCo co ty)
    -- See if the first arg is already a forall
    -- ...then we can just extend the current substitution
  | Just (tv, co_body) <- splitForAllCo_maybe co
  = opt_co (extendTvSubst env tv ty') sym co_body

     -- See if it is a forall after optimization
     -- If so, do an inefficient one-variable substitution
  | Just (tv, co'_body) <- splitForAllCo_maybe co'
  = substCoWithTy (getCvInScope env) tv ty' co'_body   

  | otherwise = InstCo co' ty'

  where
    co' = opt_co env sym co
    ty' = substTy env ty

-------------
opt_transList :: InScopeSet -> [NormalCo] -> [NormalCo] -> [NormalCo]
opt_transList is = zipWith (opt_trans is)

opt_trans :: InScopeSet -> NormalCo -> NormalCo -> NormalCo
opt_trans is co1 co2
  | isReflCo co1 = co2
  | otherwise    = opt_trans1 is co1 co2

opt_trans1 :: InScopeSet -> NormalNonIdCo -> NormalCo -> NormalCo
-- First arg is not the identity
opt_trans1 is co1 co2
  | isReflCo co2 = co1
  | otherwise    = opt_trans2 is co1 co2

opt_trans2 :: InScopeSet -> NormalNonIdCo -> NormalNonIdCo -> NormalCo
-- Neither arg is the identity
opt_trans2 is (TransCo co1a co1b) co2
    -- Don't know whether the sub-coercions are the identity
  = opt_trans is co1a (opt_trans is co1b co2)  

opt_trans2 is co1 co2 
  | Just co <- opt_trans_rule is co1 co2
  = co

opt_trans2 is co1 (TransCo co2a co2b)
  | Just co1_2a <- opt_trans_rule is co1 co2a
  = if isReflCo co1_2a
    then co2b
    else opt_trans1 is co1_2a co2b

opt_trans2 _ co1 co2
  = mkTransCo co1 co2

------
-- Optimize coercions with a top-level use of transitivity.
opt_trans_rule :: InScopeSet -> NormalNonIdCo -> NormalNonIdCo -> Maybe NormalCo

-- push transitivity down through matching top-level constructors.
opt_trans_rule is in_co1@(TyConAppCo tc1 cos1) in_co2@(TyConAppCo tc2 cos2)
  | tc1 == tc2 = fireTransRule "PushTyConApp" in_co1 in_co2 $
                 TyConAppCo tc1 (opt_transList is cos1 cos2)

-- push transitivity through matching destructors
opt_trans_rule is in_co1@(NthCo d1 co1) in_co2@(NthCo d2 co2)
  | d1 == d2
  , co1 `compatible_co` co2
  = fireTransRule "PushNth" in_co1 in_co2 $
    mkNthCo d1 (opt_trans is co1 co2)

-- Push transitivity inside instantiation
opt_trans_rule is in_co1@(InstCo co1 ty1) in_co2@(InstCo co2 ty2)
  | ty1 `eqType` ty2
  , co1 `compatible_co` co2
  = fireTransRule "TrPushInst" in_co1 in_co2 $
    mkInstCo (opt_trans is co1 co2) ty1
 
-- Push transitivity inside apply
opt_trans_rule is in_co1@(AppCo co1a co1b) in_co2@(AppCo co2a co2b)
  = fireTransRule "TrPushApp" in_co1 in_co2 $
    mkAppCo (opt_trans is co1a co2a) (opt_trans is co1b co2b)

opt_trans_rule is co1@(TyConAppCo tc cos1) co2
  | Just cos2 <- etaTyConAppCo_maybe tc co2
  = ASSERT( length cos1 == length cos2 )
    fireTransRule "EtaCompL" co1 co2 $
    TyConAppCo tc (opt_transList is cos1 cos2)

opt_trans_rule is co1 co2@(TyConAppCo tc cos2)
  | Just cos1 <- etaTyConAppCo_maybe tc co1
  = ASSERT( length cos1 == length cos2 )
    fireTransRule "EtaCompR" co1 co2 $
    TyConAppCo tc (opt_transList is cos1 cos2)

-- Push transitivity inside forall
opt_trans_rule is co1 co2
  | Just (tv1,r1) <- splitForAllCo_maybe co1
  , Just (tv2,r2) <- etaForAllCo_maybe co2
  , let r2' = substCoWithTy is' tv2 (mkTyVarTy tv1) r2
        is' = is `extendInScopeSet` tv1
  = fireTransRule "EtaAllL" co1 co2 $
    mkForAllCo tv1 (opt_trans2 is' r1 r2')

  | Just (tv2,r2) <- splitForAllCo_maybe co2
  , Just (tv1,r1) <- etaForAllCo_maybe co1
  , let r1' = substCoWithTy is' tv1 (mkTyVarTy tv2) r1
        is' = is `extendInScopeSet` tv2
  = fireTransRule "EtaAllR" co1 co2 $
    mkForAllCo tv1 (opt_trans2 is' r1' r2)

-- Push transitivity inside axioms
opt_trans_rule is co1 co2

  -- TrPushAxR/TrPushSymAxR
  | Just (sym, con, cos1) <- co1_is_axiom_maybe
  , Just cos2 <- matchAxiom sym con co2
  = fireTransRule "TrPushAxR" co1 co2 $
    if sym 
    then SymCo $ AxiomInstCo con (opt_transList is (map mkSymCo cos2) cos1)
    else         AxiomInstCo con (opt_transList is cos1 cos2)

  -- TrPushAxL/TrPushSymAxL
  | Just (sym, con, cos2) <- co2_is_axiom_maybe
  , Just cos1 <- matchAxiom (not sym) con co1
  = fireTransRule "TrPushAxL" co1 co2 $
    if sym 
    then SymCo $ AxiomInstCo con (opt_transList is cos2 (map mkSymCo cos1))
    else         AxiomInstCo con (opt_transList is cos1 cos2)

  -- TrPushAxSym/TrPushSymAx
  | Just (sym1, con1, cos1) <- co1_is_axiom_maybe
  , Just (sym2, con2, cos2) <- co2_is_axiom_maybe
  , con1 == con2
  , sym1 == not sym2
  , let qtvs = co_ax_tvs con1
        lhs  = co_ax_lhs con1 
        rhs  = co_ax_rhs con1 
        pivot_tvs = exactTyVarsOfType (if sym2 then rhs else lhs)
  , all (`elemVarSet` pivot_tvs) qtvs
  = fireTransRule "TrPushAxSym" co1 co2 $
    if sym2
    then liftCoSubstWith qtvs (opt_transList is cos1 (map mkSymCo cos2)) lhs  -- TrPushAxSym
    else liftCoSubstWith qtvs (opt_transList is (map mkSymCo cos1) cos2) rhs  -- TrPushSymAx
  where
    co1_is_axiom_maybe = isAxiom_maybe co1
    co2_is_axiom_maybe = isAxiom_maybe co2

opt_trans_rule _ co1 co2	-- Identity rule
  | Pair ty1 _ <- coercionKind co1
  , Pair _ ty2 <- coercionKind co2
  , ty1 `eqType` ty2
  = fireTransRule "RedTypeDirRefl" co1 co2 $
    Refl ty2

opt_trans_rule _ _ _ = Nothing

fireTransRule :: String -> Coercion -> Coercion -> Coercion -> Maybe Coercion
fireTransRule _rule _co1 _co2 res
  = -- pprTrace ("Trans rule fired: " ++ _rule) (vcat [ppr _co1, ppr _co2, ppr res]) $
    Just res

-----------
wrapSym :: Bool -> Coercion -> Coercion
wrapSym sym co | sym       = SymCo co
               | otherwise = co

-----------
isAxiom_maybe :: Coercion -> Maybe (Bool, CoAxiom, [Coercion])
isAxiom_maybe (SymCo co) 
  | Just (sym, con, cos) <- isAxiom_maybe co
  = Just (not sym, con, cos)
isAxiom_maybe (AxiomInstCo con cos)
  = Just (False, con, cos)
isAxiom_maybe _ = Nothing

matchAxiom :: Bool -- True = match LHS, False = match RHS
           -> CoAxiom -> Coercion -> Maybe [Coercion]
-- If we succeed in matching, then *all the quantified type variables are bound*
-- E.g.   if tvs = [a,b], lhs/rhs = [b], we'll fail
matchAxiom sym (CoAxiom { co_ax_tvs = qtvs, co_ax_lhs = lhs, co_ax_rhs = rhs }) co
  = case liftCoMatch (mkVarSet qtvs) (if sym then lhs else rhs) co of
      Nothing    -> Nothing
      Just subst -> allMaybes (map (liftCoSubstTyVar subst) qtvs)

-------------
compatible_co :: Coercion -> Coercion -> Bool
-- Check whether (co1 . co2) will be well-kinded
compatible_co co1 co2
  = x1 `eqType` x2		
  where
    Pair _ x1 = coercionKind co1
    Pair x2 _ = coercionKind co2

-------------
etaForAllCo_maybe :: Coercion -> Maybe (TyVar, Coercion)
-- Try to make the coercion be of form (forall tv. co)
etaForAllCo_maybe co
  | Just (tv, r) <- splitForAllCo_maybe co
  = Just (tv, r)

  | Pair ty1 ty2  <- coercionKind co
  , Just (tv1, _) <- splitForAllTy_maybe ty1
  , Just (tv2, _) <- splitForAllTy_maybe ty2
  , tyVarKind tv1 `eqKind` tyVarKind tv2
  = Just (tv1, mkInstCo co (mkTyVarTy tv1))

  | otherwise
  = Nothing

etaTyConAppCo_maybe :: TyCon -> Coercion -> Maybe [Coercion]
-- If possible, split a coercion 
--       g :: T s1 .. sn ~ T t1 .. tn
-- into [ Nth 0 g :: s1~t1, ..., Nth (n-1) g :: sn~tn ] 
etaTyConAppCo_maybe tc (TyConAppCo tc2 cos2)
  = ASSERT( tc == tc2 ) Just cos2

etaTyConAppCo_maybe tc co
  | isDecomposableTyCon tc
  , Pair ty1 ty2     <- coercionKind co
  , Just (tc1, tys1) <- splitTyConApp_maybe ty1
  , Just (tc2, tys2) <- splitTyConApp_maybe ty2
  , tc1 == tc2
  , let n = length tys1
  = ASSERT( tc == tc1 ) 
    ASSERT( n == length tys2 )
    Just (decomposeCo n co)  
    -- NB: n might be <> tyConArity tc
    -- e.g.   data family T a :: * -> *
    --        g :: T a b ~ T c d

  | otherwise
  = Nothing

\end{code}