Document CoreSubst

diff --git a/compiler/coreSyn/CoreSubst.lhs b/compiler/coreSyn/CoreSubst.lhs
index ac95987..906d3f9 100644 (file)
--- a/compiler/coreSyn/CoreSubst.lhs
+++ b/compiler/coreSyn/CoreSubst.lhs
@@ -7,20 +7,22 @@ Utility functions on @Core@ syntax
 
 \begin{code}
 module CoreSubst (
 
 \begin{code}
 module CoreSubst (

-       -- Substitution stuff
+       -- * Main data types

        Subst, TvSubstEnv, IdSubstEnv, InScopeSet,
 
        Subst, TvSubstEnv, IdSubstEnv, InScopeSet,
 

+        -- ** Substituting into expressions and related types

        deShadowBinds,
        substTy, substExpr, substSpec, substWorker,
        lookupIdSubst, lookupTvSubst, 
 
        deShadowBinds,
        substTy, substExpr, substSpec, substWorker,
        lookupIdSubst, lookupTvSubst, 
 

+        -- ** Operations on substitutions

        emptySubst, mkEmptySubst, mkSubst, substInScope, isEmptySubst, 
        extendIdSubst, extendIdSubstList, extendTvSubst, extendTvSubstList,
        extendSubst, extendSubstList, zapSubstEnv,
        extendInScope, extendInScopeList, extendInScopeIds, 
        isInScope,
 
        emptySubst, mkEmptySubst, mkSubst, substInScope, isEmptySubst, 
        extendIdSubst, extendIdSubstList, extendTvSubst, extendTvSubstList,
        extendSubst, extendSubstList, zapSubstEnv,
        extendInScope, extendInScopeList, extendInScopeIds, 
        isInScope,
 

-       -- Binders
+       -- ** Substituting and cloning binders

        substBndr, substBndrs, substRecBndrs,
        cloneIdBndr, cloneIdBndrs, cloneRecIdBndrs
     ) where
        substBndr, substBndrs, substRecBndrs,
        cloneIdBndr, cloneIdBndrs, cloneRecIdBndrs
     ) where

@@ -56,23 +58,32 @@ import Data.List
 %************************************************************************
 
 \begin{code}
 %************************************************************************
 
 \begin{code}

+-- | A substitution environment, containing both 'Id' and 'TyVar' substitutions.
+--
+-- Some invariants apply to how you use the substitution:
+--
+-- 1. #in_scope_invariant# The in-scope set contains at least those 'Id's and 'TyVar's that will be in scope /after/
+-- applying the substitution to a term. Precisely, the in-scope set must be a superset of the free vars of the
+-- substitution range that might possibly clash with locally-bound variables in the thing being substituted in.
+--
+-- 2. #apply_once# You may apply the substitution only /once/
+--
+-- There are various ways of setting up the in-scope set such that the first of these invariants hold:
+--
+-- * Arrange that the in-scope set really is all the things in scope
+--
+-- * Arrange that it's the free vars of the range of the substitution
+--
+-- * Make it empty, if you know that all the free vars of the substitution are fresh, and hence can't possibly clash

 data Subst 
   = Subst InScopeSet  -- Variables in in scope (both Ids and TyVars) /after/
                       -- applying the substitution
           IdSubstEnv  -- Substitution for Ids
           TvSubstEnv  -- Substitution for TyVars
 
 data Subst 
   = Subst InScopeSet  -- Variables in in scope (both Ids and TyVars) /after/
                       -- applying the substitution
           IdSubstEnv  -- Substitution for Ids
           TvSubstEnv  -- Substitution for TyVars
 

-       -- INVARIANT 1: The (domain of the) in-scope set is a superset
-       --              of the free vars of the range of the substitution
-       --              that might possibly clash with locally-bound variables
-       --              in the thing being substituted in.
+       -- INVARIANT 1: See #in_scope_invariant#

        -- This is what lets us deal with name capture properly
        -- It's a hard invariant to check...
        -- This is what lets us deal with name capture properly
        -- It's a hard invariant to check...

-       -- There are various ways of causing it to happen:
-       --      - arrange that the in-scope set really is all the things in scope
-       --      - arrange that it's the free vars of the range of the substitution
-       --      - make it empty because all the free vars of the subst are fresh,
-       --              and hence can't possibly clash.a

        --
        -- INVARIANT 2: The substitution is apply-once; see Note [Apply once] with
        --              Types.TvSubstEnv
        --
        -- INVARIANT 2: The substitution is apply-once; see Note [Apply once] with
        --              Types.TvSubstEnv

@@ -120,6 +131,7 @@ Why do we make a different choice for the IdSubstEnv than the TvSubstEnv?
   easy to spot
 -}
 
   easy to spot
 -}
 

+-- | An environment for substituting for 'Id's

 type IdSubstEnv = IdEnv CoreExpr
 
 ----------------------------
 type IdSubstEnv = IdEnv CoreExpr
 
 ----------------------------

@@ -144,35 +156,48 @@ mkSubst in_scope tvs ids = Subst in_scope ids tvs
 -- setTvSubstEnv :: Subst -> TvSubstEnv -> Subst
 -- setTvSubstEnv (Subst in_scope ids _) tvs = Subst in_scope ids tvs
 
 -- setTvSubstEnv :: Subst -> TvSubstEnv -> Subst
 -- setTvSubstEnv (Subst in_scope ids _) tvs = Subst in_scope ids tvs
 

+-- | Find the in-scope set: see "CoreSubst#in_scope_invariant"

 substInScope :: Subst -> InScopeSet
 substInScope (Subst in_scope _ _) = in_scope
 
 substInScope :: Subst -> InScopeSet
 substInScope (Subst in_scope _ _) = in_scope
 

+-- | Remove all substitutions for 'Id's and 'Var's that might have been built up
+-- while preserving the in-scope set

 zapSubstEnv :: Subst -> Subst
 zapSubstEnv (Subst in_scope _ _) = Subst in_scope emptyVarEnv emptyVarEnv
 
 zapSubstEnv :: Subst -> Subst
 zapSubstEnv (Subst in_scope _ _) = Subst in_scope emptyVarEnv emptyVarEnv
 

--- ToDo: add an ASSERT that fvs(subst-result) is already in the in-scope set
+-- | Add a substitution for an 'Id' to the 'Subst': you must ensure that the in-scope set is
+-- such that the "CoreSubst#in_scope_invariant" is true after extending the substitution like this

 extendIdSubst :: Subst -> Id -> CoreExpr -> Subst
 extendIdSubst :: Subst -> Id -> CoreExpr -> Subst

+-- ToDo: add an ASSERT that fvs(subst-result) is already in the in-scope set

 extendIdSubst (Subst in_scope ids tvs) v r = Subst in_scope (extendVarEnv ids v r) tvs
 
 extendIdSubst (Subst in_scope ids tvs) v r = Subst in_scope (extendVarEnv ids v r) tvs
 

+-- | Adds multiple 'Id' substitutions to the 'Subst': see also 'extendIdSubst'

 extendIdSubstList :: Subst -> [(Id, CoreExpr)] -> Subst
 extendIdSubstList (Subst in_scope ids tvs) prs = Subst in_scope (extendVarEnvList ids prs) tvs
 
 extendIdSubstList :: Subst -> [(Id, CoreExpr)] -> Subst
 extendIdSubstList (Subst in_scope ids tvs) prs = Subst in_scope (extendVarEnvList ids prs) tvs
 

+-- | Add a substitution for a 'TyVar' to the 'Subst': you must ensure that the in-scope set is
+-- such that the "CoreSubst#in_scope_invariant" is true after extending the substitution like this

 extendTvSubst :: Subst -> TyVar -> Type -> Subst
 extendTvSubst (Subst in_scope ids tvs) v r = Subst in_scope ids (extendVarEnv tvs v r) 
 
 extendTvSubst :: Subst -> TyVar -> Type -> Subst
 extendTvSubst (Subst in_scope ids tvs) v r = Subst in_scope ids (extendVarEnv tvs v r) 
 

+-- | Adds multiple 'TyVar' substitutions to the 'Subst': see also 'extendTvSubst'

 extendTvSubstList :: Subst -> [(TyVar,Type)] -> Subst
 extendTvSubstList (Subst in_scope ids tvs) prs = Subst in_scope ids (extendVarEnvList tvs prs)
 
 extendTvSubstList :: Subst -> [(TyVar,Type)] -> Subst
 extendTvSubstList (Subst in_scope ids tvs) prs = Subst in_scope ids (extendVarEnvList tvs prs)
 

-extendSubstList :: Subst -> [(Var,CoreArg)] -> Subst
-extendSubstList subst []             = subst
-extendSubstList subst ((var,rhs):prs) = extendSubstList (extendSubst subst var rhs) prs
-
+-- | Add a substitution for a 'TyVar' or 'Id' as appropriate to the 'Var' being added. See also
+-- 'extendIdSubst' and 'extendTvSubst'

 extendSubst :: Subst -> Var -> CoreArg -> Subst
 extendSubst (Subst in_scope ids tvs) tv (Type ty)
   = ASSERT( isTyVar tv ) Subst in_scope ids (extendVarEnv tvs tv ty)
 extendSubst (Subst in_scope ids tvs) id expr
   = ASSERT( isId id ) Subst in_scope (extendVarEnv ids id expr) tvs
 
 extendSubst :: Subst -> Var -> CoreArg -> Subst
 extendSubst (Subst in_scope ids tvs) tv (Type ty)
   = ASSERT( isTyVar tv ) Subst in_scope ids (extendVarEnv tvs tv ty)
 extendSubst (Subst in_scope ids tvs) id expr
   = ASSERT( isId id ) Subst in_scope (extendVarEnv ids id expr) tvs
 

+-- | Add a substitution for a 'TyVar' or 'Id' as appropriate to all the 'Var's being added. See also 'extendSubst'
+extendSubstList :: Subst -> [(Var,CoreArg)] -> Subst
+extendSubstList subst []             = subst
+extendSubstList subst ((var,rhs):prs) = extendSubstList (extendSubst subst var rhs) prs
+
+-- | Find the substitution for an 'Id' in the 'Subst'

 lookupIdSubst :: Subst -> Id -> CoreExpr
 lookupIdSubst (Subst in_scope ids _) v
   | not (isLocalId v) = Var v
 lookupIdSubst :: Subst -> Id -> CoreExpr
 lookupIdSubst (Subst in_scope ids _) v
   | not (isLocalId v) = Var v

@@ -182,6 +207,7 @@ lookupIdSubst (Subst in_scope ids _) v
   | otherwise = WARN( True, ptext (sLit "CoreSubst.lookupIdSubst") <+> ppr v ) 
                Var v
 
   | otherwise = WARN( True, ptext (sLit "CoreSubst.lookupIdSubst") <+> ppr v ) 
                Var v
 

+-- | Find the substitution for a 'TyVar' in the 'Subst'

 lookupTvSubst :: Subst -> TyVar -> Type
 lookupTvSubst (Subst _ _ tvs) v = lookupVarEnv tvs v `orElse` Type.mkTyVarTy v
 
 lookupTvSubst :: Subst -> TyVar -> Type
 lookupTvSubst (Subst _ _ tvs) v = lookupVarEnv tvs v `orElse` Type.mkTyVarTy v
 

@@ -189,16 +215,20 @@ lookupTvSubst (Subst _ _ tvs) v = lookupVarEnv tvs v `orElse` Type.mkTyVarTy v
 isInScope :: Var -> Subst -> Bool
 isInScope v (Subst in_scope _ _) = v `elemInScopeSet` in_scope
 
 isInScope :: Var -> Subst -> Bool
 isInScope v (Subst in_scope _ _) = v `elemInScopeSet` in_scope
 

+-- | Add the 'Var' to the in-scope set: as a side effect, removes any existing substitutions for it

 extendInScope :: Subst -> Var -> Subst
 extendInScope (Subst in_scope ids tvs) v
   = Subst (in_scope `extendInScopeSet` v) 
          (ids `delVarEnv` v) (tvs `delVarEnv` v)
 
 extendInScope :: Subst -> Var -> Subst
 extendInScope (Subst in_scope ids tvs) v
   = Subst (in_scope `extendInScopeSet` v) 
          (ids `delVarEnv` v) (tvs `delVarEnv` v)
 

+-- | Add the 'Var's to the in-scope set: see also 'extendInScope'

 extendInScopeList :: Subst -> [Var] -> Subst
 extendInScopeList (Subst in_scope ids tvs) vs
   = Subst (in_scope `extendInScopeSetList` vs) 
          (ids `delVarEnvList` vs) (tvs `delVarEnvList` vs)
 
 extendInScopeList :: Subst -> [Var] -> Subst
 extendInScopeList (Subst in_scope ids tvs) vs
   = Subst (in_scope `extendInScopeSetList` vs) 
          (ids `delVarEnvList` vs) (tvs `delVarEnvList` vs)
 

+-- | Optimized version of 'extendInScopeList' that can be used if you are certain 
+-- all the things being added are 'Id's and hence none are 'TyVar's

 extendInScopeIds :: Subst -> [Id] -> Subst
 extendInScopeIds (Subst in_scope ids tvs) vs 
   = Subst (in_scope `extendInScopeSetList` vs) 
 extendInScopeIds :: Subst -> [Id] -> Subst
 extendInScopeIds (Subst in_scope ids tvs) vs 
   = Subst (in_scope `extendInScopeSetList` vs) 

@@ -224,6 +254,7 @@ instance Outputable Subst where
 %************************************************************************
 
 \begin{code}
 %************************************************************************
 
 \begin{code}

+-- | Apply a substititon to an entire 'CoreExpr'. Rememeber, you may only apply the substitution /once/: see "CoreSubst#apply_once"

 substExpr :: Subst -> CoreExpr -> CoreExpr
 substExpr subst expr
   = go expr
 substExpr :: Subst -> CoreExpr -> CoreExpr
 substExpr subst expr
   = go expr

@@ -252,6 +283,8 @@ substExpr subst expr
 
     go_note note            = note
 
 
     go_note note            = note
 

+-- | Apply a substititon to an entire 'CoreBind', additionally returning an updated 'Subst'
+-- that should be used by subsequent substitutons.

 substBind :: Subst -> CoreBind -> (Subst, CoreBind)
 substBind subst (NonRec bndr rhs) = (subst', NonRec bndr' (substExpr subst rhs))
                                  where
 substBind :: Subst -> CoreBind -> (Subst, CoreBind)
 substBind subst (NonRec bndr rhs) = (subst', NonRec bndr' (substExpr subst rhs))
                                  where

@@ -264,14 +297,13 @@ substBind subst (Rec pairs) = (subst', Rec pairs')
                                rhss'   = map (substExpr subst' . snd) pairs
 \end{code}
 
                                rhss'   = map (substExpr subst' . snd) pairs
 \end{code}
 

-De-shadowing the program is sometimes a useful pre-pass.  It can be done simply
-by running over the bindings with an empty substitution, becuase substitution
-returns a result that has no-shadowing guaranteed.
-
-(Actually, within a single *type* there might still be shadowing, because 
-substType is a no-op for the empty substitution, but that's OK.)
-

 \begin{code}
 \begin{code}

+-- | De-shadowing the program is sometimes a useful pre-pass. It can be done simply
+-- by running over the bindings with an empty substitution, becuase substitution
+-- returns a result that has no-shadowing guaranteed.
+--
+-- (Actually, within a single /type/ there might still be shadowing, because 
+-- 'substTy' is a no-op for the empty substitution, but that's probably OK.)

 deShadowBinds :: [CoreBind] -> [CoreBind]
 deShadowBinds binds = snd (mapAccumL substBind emptySubst binds)
 \end{code}
 deShadowBinds :: [CoreBind] -> [CoreBind]
 deShadowBinds binds = snd (mapAccumL substBind emptySubst binds)
 \end{code}

@@ -289,16 +321,20 @@ preserve all IdInfo (suitably substituted).  For example, we *want* to
 preserve occ info in rules.
 
 \begin{code}
 preserve occ info in rules.
 
 \begin{code}

+-- | Substitutes a 'Var' for another one according to the 'Subst' given, returning
+-- the result and an updated 'Subst' that should be used by subsequent substitutons.
+-- 'IdInfo' is preserved by this process, although it is substituted into appropriately.

 substBndr :: Subst -> Var -> (Subst, Var)
 substBndr subst bndr
   | isTyVar bndr  = substTyVarBndr subst bndr
   | otherwise     = substIdBndr subst subst bndr
 
 substBndr :: Subst -> Var -> (Subst, Var)
 substBndr subst bndr
   | isTyVar bndr  = substTyVarBndr subst bndr
   | otherwise     = substIdBndr subst subst bndr
 

+-- | Applies 'substBndr' to a number of 'Var's, accumulating a new 'Subst' left-to-right

 substBndrs :: Subst -> [Var] -> (Subst, [Var])
 substBndrs subst bndrs = mapAccumL substBndr subst bndrs
 
 substBndrs :: Subst -> [Var] -> (Subst, [Var])
 substBndrs subst bndrs = mapAccumL substBndr subst bndrs
 

+-- | Substitute in a mutually recursive group of 'Id's

 substRecBndrs :: Subst -> [Id] -> (Subst, [Id])
 substRecBndrs :: Subst -> [Id] -> (Subst, [Id])

--- Substitute a mutually recursive group

 substRecBndrs subst bndrs 
   = (new_subst, new_bndrs)
   where                -- Here's the reason we need to pass rec_subst to subst_id
 substRecBndrs subst bndrs 
   = (new_subst, new_bndrs)
   where                -- Here's the reason we need to pass rec_subst to subst_id

@@ -307,9 +343,9 @@ substRecBndrs subst bndrs
 
 
 \begin{code}
 
 
 \begin{code}

-substIdBndr :: Subst           -- Substitution to use for the IdInfo
-           -> Subst -> Id      -- Substitition and Id to transform
-           -> (Subst, Id)      -- Transformed pair
+substIdBndr :: Subst           -- ^ Substitution to use for the IdInfo
+           -> Subst -> Id      -- ^ Substitition and Id to transform
+           -> (Subst, Id)      -- ^ Transformed pair

                                -- NB: unfolding may be zapped
 
 substIdBndr rec_subst subst@(Subst in_scope env tvs) old_id
                                -- NB: unfolding may be zapped
 
 substIdBndr rec_subst subst@(Subst in_scope env tvs) old_id

@@ -344,14 +380,19 @@ Now a variant that unconditionally allocates a new unique.
 It also unconditionally zaps the OccInfo.
 
 \begin{code}
 It also unconditionally zaps the OccInfo.
 
 \begin{code}

+-- | Very similar to 'substBndr', but it always allocates a new 'Unique' for
+-- each variable in its output and removes all 'IdInfo'

 cloneIdBndr :: Subst -> UniqSupply -> Id -> (Subst, Id)
 cloneIdBndr subst us old_id
   = clone_id subst subst (old_id, uniqFromSupply us)
 
 cloneIdBndr :: Subst -> UniqSupply -> Id -> (Subst, Id)
 cloneIdBndr subst us old_id
   = clone_id subst subst (old_id, uniqFromSupply us)
 

+-- | Applies 'cloneIdBndr' to a number of 'Id's, accumulating a final
+-- substitution from left to right

 cloneIdBndrs :: Subst -> UniqSupply -> [Id] -> (Subst, [Id])
 cloneIdBndrs subst us ids
   = mapAccumL (clone_id subst) subst (ids `zip` uniqsFromSupply us)
 
 cloneIdBndrs :: Subst -> UniqSupply -> [Id] -> (Subst, [Id])
 cloneIdBndrs subst us ids
   = mapAccumL (clone_id subst) subst (ids `zip` uniqsFromSupply us)
 

+-- | Clone a mutually recursive group of 'Id's

 cloneRecIdBndrs :: Subst -> UniqSupply -> [Id] -> (Subst, [Id])
 cloneRecIdBndrs subst us ids
   = (subst', ids')
 cloneRecIdBndrs :: Subst -> UniqSupply -> [Id] -> (Subst, [Id])
 cloneRecIdBndrs subst us ids
   = (subst', ids')

@@ -391,6 +432,7 @@ substTyVarBndr (Subst in_scope id_env tv_env) tv
        (TvSubst in_scope' tv_env', tv') 
           -> (Subst in_scope' id_env tv_env', tv')
 
        (TvSubst in_scope' tv_env', tv') 
           -> (Subst in_scope' id_env tv_env', tv')
 

+-- | See 'Type.substTy'

 substTy :: Subst -> Type -> Type 
 substTy (Subst in_scope _id_env tv_env) ty
   = Type.substTy (TvSubst in_scope tv_env) ty
 substTy :: Subst -> Type -> Type 
 substTy (Subst in_scope _id_env tv_env) ty
   = Type.substTy (TvSubst in_scope tv_env) ty

@@ -415,8 +457,9 @@ substIdType subst@(Subst _ _ tv_env) id
     old_ty = idType id
 
 ------------------
     old_ty = idType id
 
 ------------------

-substIdInfo :: Subst -> Id -> IdInfo -> Maybe IdInfo
+-- | Substitute into some 'IdInfo' with regard to the supplied new 'Id'.

 -- Always zaps the unfolding, to save substitution work
 -- Always zaps the unfolding, to save substitution work

+substIdInfo :: Subst -> Id -> IdInfo -> Maybe IdInfo

 substIdInfo subst new_id info
   | nothing_to_do = Nothing
   | otherwise     = Just (info `setSpecInfo`             substSpec subst new_id old_rules
 substIdInfo subst new_id info
   | nothing_to_do = Nothing
   | otherwise     = Just (info `setSpecInfo`             substSpec subst new_id old_rules

@@ -431,6 +474,7 @@ substIdInfo subst new_id info
     
 
 ------------------
     
 
 ------------------

+-- | Substitutes for the 'Id's within the 'WorkerInfo'

 substWorker :: Subst -> WorkerInfo -> WorkerInfo
        -- Seq'ing on the returned WorkerInfo is enough to cause all the 
        -- substitutions to happen completely
 substWorker :: Subst -> WorkerInfo -> WorkerInfo
        -- Seq'ing on the returned WorkerInfo is enough to cause all the 
        -- substitutions to happen completely

@@ -446,6 +490,7 @@ substWorker subst (HasWorker w a)
                                --  via postInlineUnconditionally, hence warning)
 
 ------------------
                                --  via postInlineUnconditionally, hence warning)
 
 ------------------

+-- | Substitutes for the 'Id's within the 'WorkerInfo' given the new function 'Id'

 substSpec :: Subst -> Id -> SpecInfo -> SpecInfo
 
 substSpec subst new_fn spec@(SpecInfo rules rhs_fvs)
 substSpec :: Subst -> Id -> SpecInfo -> SpecInfo
 
 substSpec subst new_fn spec@(SpecInfo rules rhs_fvs)