Functional.hs

{-# LANGUAGE CPP #-}
{-# LANGUAGE UndecidableInstances #-}

#ifndef MIN_VERSION_GLASGOW_HASKELL
#define MIN_VERSION_GLASGOW_HASKELL(a,b,c,d) 0
#endif
  -- MIN_VERSION_GLASGOW_HASKELL was introduced in GHC 7.10

#if MIN_VERSION_GLASGOW_HASKELL(7,10,0,0)
#else
{-# LANGUAGE OverlappingInstances #-}
#endif

#if __GLASGOW_HASKELL__ < 708
#define TYPEABLE Typeable1
#else
#define TYPEABLE Typeable
#endif

-- | Basics for implementing functional EDSLs

module Language.Syntactic.Functional
    ( -- * Syntactic constructs
      Name (..)
    , Literal (..)
    , Construct (..)
    , Binding (..)
    , maxLam
    , lam_template
    , lam
    , fromDeBruijn
    , BindingT (..)
    , maxLamT
    , lamT_template
    , lamT
    , lamTyped
    , BindingDomain (..)
    , Let (..)
    , MONAD (..)
    , Remon (..)
    , desugarMonad
    , desugarMonadTyped
      -- * Free and bound variables
    , freeVars
    , allVars
    , renameUnique'
    , renameUnique
      -- * Alpha-equivalence
    , AlphaEnv
    , alphaEq'
    , alphaEq
      -- * Evaluation
    , Denotation
    , Eval (..)
    , evalDen
    , DenotationM
    , liftDenotationM
    , RunEnv
    , EvalEnv (..)
    , compileSymDefault
    , evalOpen
    , evalClosed
    ) where



#if MIN_VERSION_GLASGOW_HASKELL(7,10,0,0)
#else
import Control.Applicative
#endif
import Control.DeepSeq
import Control.Monad.Cont
import Control.Monad.Reader
import Control.Monad.State
import Data.Dynamic
import Data.List (genericIndex)
#if MIN_VERSION_GLASGOW_HASKELL(7,10,0,0)
#else
import Data.Proxy  -- Needed by GHC < 7.8
#endif
import Data.Map (Map)
import qualified Data.Map as Map
import Data.Set (Set)
import qualified Data.Set as Set
import Data.Tree

import Data.Hash (hashInt)

import Language.Syntactic



----------------------------------------------------------------------------------------------------
-- * Syntactic constructs
----------------------------------------------------------------------------------------------------

-- | Literal
data Literal sig
  where
    Literal :: Show a => a -> Literal (Full a)

instance Symbol Literal
  where
    symSig (Literal _) = signature

instance Render Literal
  where
    renderSym (Literal a) = show a

instance Equality Literal
instance StringTree Literal

-- | Generic N-ary syntactic construct
--
-- 'Construct' gives a quick way to introduce a syntactic construct by giving its name and semantic
-- function.
data Construct sig
  where
    Construct :: Signature sig => String -> Denotation sig -> Construct sig
  -- There is no `NFData1` instance for `Construct` because that would give rise
  -- to a constraint `NFData (Denotation sig)`, which easily spreads to other
  -- functions.

instance Symbol Construct
  where
    symSig (Construct _ _) = signature

instance Render Construct
  where
    renderSym (Construct name _) = name
    renderArgs = renderArgsSmart

instance Equality Construct
  where
    equal = equalDefault
    hash  = hashDefault

instance StringTree Construct

-- | Variable name
newtype Name = Name Integer
  deriving (Eq, Ord, Num, Enum, Real, Integral, NFData)

instance Show Name
  where
    show (Name n) = show n

-- | Variables and binders
data Binding sig
  where
    Var :: Name -> Binding (Full a)
    Lam :: Name -> Binding (b :-> Full (a -> b))

instance Symbol Binding
  where
    symSig (Var _) = signature
    symSig (Lam _) = signature

instance NFData1 Binding
  where
    rnf1 (Var v) = rnf v
    rnf1 (Lam v) = rnf v

-- | 'equal' does strict identifier comparison; i.e. no alpha equivalence.
--
-- 'hash' assigns the same hash to all variables and binders. This is a valid over-approximation
-- that enables the following property:
--
-- @`alphaEq` a b ==> `hash` a == `hash` b@
instance Equality Binding
  where
    equal (Var v1) (Var v2) = v1==v2
    equal (Lam v1) (Lam v2) = v1==v2
    equal _ _ = False

    hash (Var _) = hashInt 0
    hash (Lam _) = hashInt 0

instance Render Binding
  where
    renderSym (Var v) = 'v' : show v
    renderSym (Lam v) = "Lam v" ++ show v
    renderArgs []     (Var v) = 'v' : show v
    renderArgs [body] (Lam v) = "(\\" ++ ('v':show v) ++ " -> " ++ body ++ ")"

instance StringTree Binding
  where
    stringTreeSym []     (Var v) = Node ('v' : show v) []
    stringTreeSym [body] (Lam v) = Node ("Lam " ++ 'v' : show v) [body]

-- | Get the highest name bound by the first 'Lam' binders at every path from the root. If the term
-- has /ordered binders/ \[1\], 'maxLam' returns the highest name introduced in the whole term.
--
-- \[1\] Ordered binders means that the names of 'Lam' nodes are decreasing along every path from
-- the root.
maxLam :: (Project Binding s) => AST s a -> Name
maxLam (Sym lam :$ _) | Just (Lam v) <- prj lam = v
maxLam (s :$ a) = maxLam s `Prelude.max` maxLam a
maxLam _ = 0

-- | Higher-order interface for variable binding for domains based on 'Binding'
--
-- Assumptions:
--
--   * The body @f@ does not inspect its argument.
--
--   * Applying @f@ to a term with ordered binders results in a term with /ordered binders/ \[1\].
--
-- \[1\] Ordered binders means that the names of 'Lam' nodes are decreasing along every path from
-- the root.
--
-- See \"Using Circular Programs for Higher-Order Syntax\"
-- (ICFP 2013, <http://www.cse.chalmers.se/~emax/documents/axelsson2013using.pdf>).
lam_template :: (Project Binding sym)
    => (Name -> sym (Full a))
         -- ^ Variable symbol constructor
    -> (Name -> ASTF sym b -> ASTF sym (a -> b))
         -- ^ Lambda constructor
    -> (ASTF sym a -> ASTF sym b) -> ASTF sym (a -> b)
lam_template mkVar mkLam f = mkLam v body
  where
    body = f $ Sym $ mkVar v
    v    = succ $ maxLam body

-- | Higher-order interface for variable binding
--
-- This function is 'lamT_template' specialized to domains @sym@ satisfying
-- @(`Binding` `:<:` sym)@.
lam :: (Binding :<: sym) => (ASTF sym a -> ASTF sym b) -> ASTF sym (a -> b)
lam = lam_template (inj . Var) (\v a -> Sym (inj (Lam v)) :$ a)

-- | Convert from a term with De Bruijn indexes to one with explicit names
--
-- In the argument term, variable 'Name's are treated as De Bruijn indexes, and lambda 'Name's are
-- ignored. (Ideally, one should use a different type for De Bruijn terms.)
fromDeBruijn :: (Binding :<: sym) => ASTF sym a -> ASTF sym a
fromDeBruijn = go []
  where
    go :: (Binding :<: sym) => [Name] -> ASTF sym a -> (ASTF sym a)
    go vs var           | Just (Var i) <- prj var = inj $ Var $ genericIndex vs i
    go vs (lam :$ body) | Just (Lam _) <- prj lam = inj (Lam v) :$ body'
      where
        body' = go (v:vs) body
        v     = succ $ maxLam body'
          -- Same trick as in `lam`
    go vs a = gmapT (go vs) a

-- | Typed variables and binders
data BindingT sig
  where
    VarT :: Typeable a => Name -> BindingT (Full a)
    LamT :: Typeable a => Name -> BindingT (b :-> Full (a -> b))

instance Symbol BindingT
  where
    symSig (VarT _) = signature
    symSig (LamT _) = signature

instance NFData1 BindingT
  where
    rnf1 (VarT v) = rnf v
    rnf1 (LamT v) = rnf v

-- | 'equal' does strict identifier comparison; i.e. no alpha equivalence.
--
-- 'hash' assigns the same hash to all variables and binders. This is a valid over-approximation
-- that enables the following property:
--
-- @`alphaEq` a b ==> `hash` a == `hash` b@
instance Equality BindingT
  where
    equal (VarT v1) (VarT v2) = v1==v2
    equal (LamT v1) (LamT v2) = v1==v2
    equal _ _ = False

    hash (VarT _) = hashInt 0
    hash (LamT _) = hashInt 0

instance Render BindingT
  where
    renderSym (VarT v) = renderSym (Var v)
    renderSym (LamT v) = renderSym (Lam v)
    renderArgs args (VarT v) = renderArgs args (Var v)
    renderArgs args (LamT v) = renderArgs args (Lam v)

instance StringTree BindingT
  where
    stringTreeSym args (VarT v) = stringTreeSym args (Var v)
    stringTreeSym args (LamT v) = stringTreeSym args (Lam v)

-- | Get the highest name bound by the first 'LamT' binders at every path from the root. If the term
-- has /ordered binders/ \[1\], 'maxLamT' returns the highest name introduced in the whole term.
--
-- \[1\] Ordered binders means that the names of 'LamT' nodes are decreasing along every path from
-- the root.
maxLamT :: Project BindingT sym => AST sym a -> Name
maxLamT (Sym lam :$ _) | Just (LamT n :: BindingT (b :-> a)) <- prj lam = n
maxLamT (s :$ a) = maxLamT s `Prelude.max` maxLamT a
maxLamT _ = 0

-- | Higher-order interface for variable binding
--
-- Assumptions:
--
--   * The body @f@ does not inspect its argument.
--
--   * Applying @f@ to a term with ordered binders results in a term with /ordered binders/ \[1\].
--
-- \[1\] Ordered binders means that the names of 'LamT' nodes are decreasing along every path from
-- the root.
--
-- See \"Using Circular Programs for Higher-Order Syntax\"
-- (ICFP 2013, <http://www.cse.chalmers.se/~emax/documents/axelsson2013using.pdf>).
lamT_template :: Project BindingT sym
    => (Name -> sym (Full a))
         -- ^ Variable symbol constructor
    -> (Name -> ASTF sym b -> ASTF sym (a -> b))
         -- ^ Lambda constructor
    -> (ASTF sym a -> ASTF sym b) -> ASTF sym (a -> b)
lamT_template mkVar mkLam f = mkLam v body
  where
    body = f $ Sym $ mkVar v
    v    = succ $ maxLamT body

-- | Higher-order interface for variable binding
--
-- This function is 'lamT_template' specialized to domains @sym@ satisfying
-- @(`BindingT` `:<:` sym)@.
lamT :: (BindingT :<: sym, Typeable a) =>
    (ASTF sym a -> ASTF sym b) -> ASTF sym (a -> b)
lamT = lamT_template (inj . VarT) (\v a -> Sym (inj (LamT v)) :$ a)

-- | Higher-order interface for variable binding
--
-- This function is 'lamT_template' specialized to domains @sym@ satisfying
-- @(sym ~ `Typed` s, `BindingT` `:<:` s)@.
lamTyped :: (sym ~ Typed s, BindingT :<: s, Typeable a, Typeable b) =>
    (ASTF sym a -> ASTF sym b) -> ASTF sym (a -> b)
lamTyped = lamT_template
    (Typed . inj . VarT)
    (\v a -> Sym (Typed (inj (LamT v))) :$ a)

-- | Domains that \"might\" include variables and binders
class BindingDomain sym
  where
    prVar :: sym sig -> Maybe Name
    prLam :: sym sig -> Maybe Name

    -- | Rename a variable or a lambda (no effect for other symbols)
    renameBind :: (Name -> Name) -> sym sig -> sym sig

instance {-# OVERLAPPING #-}
         (BindingDomain sym1, BindingDomain sym2) => BindingDomain (sym1 :+: sym2)
  where
    prVar (InjL s) = prVar s
    prVar (InjR s) = prVar s
    prLam (InjL s) = prLam s
    prLam (InjR s) = prLam s
    renameBind re (InjL s) = InjL $ renameBind re s
    renameBind re (InjR s) = InjR $ renameBind re s

instance {-# OVERLAPPING #-} BindingDomain sym => BindingDomain (Typed sym)
  where
    prVar (Typed s) = prVar s
    prLam (Typed s) = prLam s
    renameBind re (Typed s) = Typed $ renameBind re s

instance {-# OVERLAPPING #-} BindingDomain sym => BindingDomain (sym :&: i)
  where
    prVar = prVar . decorExpr
    prLam = prLam . decorExpr
    renameBind re (s :&: d) = renameBind re s :&: d

instance {-# OVERLAPPING #-} BindingDomain sym => BindingDomain (AST sym)
  where
    prVar (Sym s) = prVar s
    prVar _       = Nothing
    prLam (Sym s) = prLam s
    prLam _       = Nothing
    renameBind re (Sym s) = Sym $ renameBind re s

instance {-# OVERLAPPING #-} BindingDomain Binding
  where
    prVar (Var v) = Just v
    prLam (Lam v) = Just v
    renameBind re (Var v) = Var $ re v
    renameBind re (Lam v) = Lam $ re v

instance {-# OVERLAPPING #-} BindingDomain BindingT
  where
    prVar (VarT v) = Just v
    prLam (LamT v) = Just v
    renameBind re (VarT v) = VarT $ re v
    renameBind re (LamT v) = LamT $ re v

instance {-# OVERLAPPING #-} BindingDomain sym
  where
    prVar _ = Nothing
    prLam _ = Nothing
    renameBind _ a = a

-- | A symbol for let bindings
--
-- This symbol is just an application operator. The actual binding has to be
-- done by a lambda that constructs the second argument.
--
-- The provided string is just a tag and has nothing to do with the name of the
-- variable of the second argument (if that argument happens to be a lambda).
-- However, a back end may use the tag to give a sensible name to the generated
-- variable.
--
-- The string tag may be empty.
data Let sig
  where
    Let :: String -> Let (a :-> (a -> b) :-> Full b)

instance Symbol Let where symSig (Let _) = signature

instance Render Let
  where
    renderSym (Let "") = "Let"
    renderSym (Let nm) = "Let " ++ nm

instance Equality Let
  where
    equal = equalDefault
    hash  = hashDefault

instance StringTree Let
  where
    stringTreeSym [a, Node lam [body]] letSym
        | ("Lam",v) <- splitAt 3 lam = Node (renderSym letSym ++ v) [a,body]
    stringTreeSym [a,f] letSym = Node (renderSym letSym) [a,f]

-- | Monadic constructs
--
-- See \"Generic Monadic Constructs for Embedded Languages\" (Persson et al., IFL 2011
-- <http://www.cse.chalmers.se/~emax/documents/persson2011generic.pdf>).
data MONAD m sig
  where
    Return :: MONAD m (a :-> Full (m a))
    Bind   :: MONAD m (m a :-> (a -> m b) :-> Full (m b))

instance Symbol (MONAD m)
  where
    symSig Return = signature
    symSig Bind   = signature

instance Render (MONAD m)
  where
    renderSym Return = "return"
    renderSym Bind   = "(>>=)"
    renderArgs = renderArgsSmart

instance Equality (MONAD m)
  where
    equal = equalDefault
    hash  = hashDefault

instance StringTree (MONAD m)

-- | Reifiable monad
--
-- See \"Generic Monadic Constructs for Embedded Languages\" (Persson et al.,
-- IFL 2011 <http://www.cse.chalmers.se/~emax/documents/persson2011generic.pdf>).
--
-- It is advised to convert to/from 'Remon' using the 'Syntactic' instance
-- provided in the modules "Language.Syntactic.Sugar.Monad" or
-- "Language.Syntactic.Sugar.MonadT".
newtype Remon sym m a
  where
    Remon
        :: { unRemon :: forall r . Typeable r => Cont (ASTF sym (m r)) a }
        -> Remon sym m a
  deriving (Functor)
  -- The `Typeable` constraint is a bit unfortunate. It's only needed when using
  -- a `Typed` domain. Since this is probably the most common case I decided to
  -- bake in `Typeable` here. A more flexible solution would be to parameterize
  -- `Remon` on the constraint.

instance Applicative (Remon sym m)
  where
    pure a  = Remon $ pure a
    f <*> a = Remon $ unRemon f <*> unRemon a

instance Monad (Remon dom m)
  where
    return a = Remon $ return a
    ma >>= f = Remon $ unRemon ma >>= unRemon . f

-- | One-layer desugaring of monadic actions
desugarMonad
    :: ( MONAD m :<: sym
       , Typeable a
       , TYPEABLE m
       )
    => Remon sym m (ASTF sym a) -> ASTF sym (m a)
desugarMonad = flip runCont (sugarSym Return) . unRemon

-- | One-layer desugaring of monadic actions
desugarMonadTyped
    :: ( MONAD m :<: s
       , sym ~ Typed s
       , Typeable a
       , TYPEABLE m
       )
    => Remon sym m (ASTF sym a) -> ASTF sym (m a)
desugarMonadTyped = flip runCont (sugarSymTyped Return) . unRemon



----------------------------------------------------------------------------------------------------
-- * Free and bound variables
----------------------------------------------------------------------------------------------------

-- | Get the set of free variables in an expression
freeVars :: BindingDomain sym => AST sym sig -> Set Name
freeVars var
    | Just v <- prVar var = Set.singleton v
freeVars (lam :$ body)
    | Just v <- prLam lam = Set.delete v (freeVars body)
freeVars (s :$ a) = Set.union (freeVars s) (freeVars a)
freeVars _ = Set.empty

-- | Get the set of variables (free, bound and introduced by lambdas) in an
-- expression
allVars :: BindingDomain sym => AST sym sig -> Set Name
allVars var
    | Just v <- prVar var = Set.singleton v
allVars (lam :$ body)
    | Just v <- prLam lam = Set.insert v (allVars body)
allVars (s :$ a) = Set.union (allVars s) (allVars a)
allVars _ = Set.empty

-- | Generate an infinite list of fresh names given a list of allocated names
--
-- The argument is assumed to be sorted and not contain an infinite number of adjacent names.
freshVars :: [Name] -> [Name]
freshVars as = go 0 as
  where
    go c [] = [c..]
    go c (v:as)
      | c < v     = c : go (c+1) (v:as)
      | c == v    = go (c+1) as
      | otherwise = go c as

freshVar :: MonadState [Name] m => m Name
freshVar = do
    v:vs <- get
    put vs
    return v

-- | Rename the bound variables in a term
--
-- The free variables are left untouched. The bound variables are given unique
-- names using as small names as possible. The first argument is a list of
-- reserved names. Reserved names and names of free variables are not used when
-- renaming bound variables.
renameUnique' :: forall sym a . BindingDomain sym =>
    [Name] -> ASTF sym a -> ASTF sym a
renameUnique' vs a = flip evalState fs $ go Map.empty a
  where
    fs = freshVars $ Set.toAscList (freeVars a `Set.union` Set.fromList vs)

    go :: Map Name Name -> AST sym sig -> State [Name] (AST sym sig)
    go env var
      | Just v <- prVar var = case Map.lookup v env of
          Just w -> return $ renameBind (\_ -> w) var
          _ -> return var  -- Free variable
    go env (lam :$ body)
      | Just v <- prLam lam = do
          w     <- freshVar
          body' <- go (Map.insert v w env) body
          return $ renameBind (\_ -> w) lam :$ body'
    go env (s :$ a) = liftM2 (:$) (go env s) (go env a)
    go env s = return s

-- | Rename the bound variables in a term
--
-- The free variables are left untouched. The bound variables are given unique
-- names using as small names as possible. Names of free variables are not used
-- when renaming bound variables.
renameUnique :: BindingDomain sym => ASTF sym a -> ASTF sym a
renameUnique = renameUnique' []



----------------------------------------------------------------------------------------------------
-- * Alpha-equivalence
----------------------------------------------------------------------------------------------------

-- | Environment used by 'alphaEq''
type AlphaEnv = [(Name,Name)]

alphaEq' :: (Equality sym, BindingDomain sym) => AlphaEnv -> ASTF sym a -> ASTF sym b -> Bool
alphaEq' env var1 var2
    | Just v1 <- prVar var1
    , Just v2 <- prVar var2
    = case (lookup v1 env, lookup v2 env') of
        (Nothing, Nothing)   -> v1==v2  -- Free variables
        (Just v2', Just v1') -> v1==v1' && v2==v2'
        _                    -> False
  where
    env' = [(v2,v1) | (v1,v2) <- env]
alphaEq' env (lam1 :$ body1) (lam2 :$ body2)
    | Just v1 <- prLam lam1
    , Just v2 <- prLam lam2
    = alphaEq' ((v1,v2):env) body1 body2
alphaEq' env a b = simpleMatch (alphaEq'' env b) a

alphaEq'' :: (Equality sym, BindingDomain sym) =>
    AlphaEnv -> ASTF sym b -> sym a -> Args (AST sym) a -> Bool
alphaEq'' env b a aArgs = simpleMatch (alphaEq''' env a aArgs) b

alphaEq''' :: (Equality sym, BindingDomain sym) =>
    AlphaEnv -> sym a -> Args (AST sym) a -> sym b -> Args (AST sym) b -> Bool
alphaEq''' env a aArgs b bArgs
    | equal a b = alphaEqChildren env a' b'
    | otherwise = False
  where
    a' = appArgs (Sym undefined) aArgs
    b' = appArgs (Sym undefined) bArgs

alphaEqChildren :: (Equality sym, BindingDomain sym) => AlphaEnv -> AST sym a -> AST sym b -> Bool
alphaEqChildren _ (Sym _) (Sym _) = True
alphaEqChildren env (s :$ a) (t :$ b) = alphaEqChildren env s t && alphaEq' env a b
alphaEqChildren _ _ _ = False

-- | Alpha-equivalence
alphaEq :: (Equality sym, BindingDomain sym) => ASTF sym a -> ASTF sym b -> Bool
alphaEq = alphaEq' []



----------------------------------------------------------------------------------------------------
-- * Evaluation
----------------------------------------------------------------------------------------------------

-- | Semantic function type of the given symbol signature
type family   Denotation sig
type instance Denotation (Full a)    = a
type instance Denotation (a :-> sig) = a -> Denotation sig

class Eval s
  where
    evalSym :: s sig -> Denotation sig

instance (Eval s, Eval t) => Eval (s :+: t)
  where
    evalSym (InjL s) = evalSym s
    evalSym (InjR s) = evalSym s

instance Eval Empty
  where
    evalSym = error "evalSym: Empty"

instance Eval sym => Eval (sym :&: info)
  where
    evalSym = evalSym . decorExpr

instance Eval Literal
  where
    evalSym (Literal a) = a

instance Eval Construct
  where
    evalSym (Construct _ d) = d

instance Eval Let
  where
    evalSym (Let _) = flip ($)

instance Monad m => Eval (MONAD m)
  where
    evalSym Return = return
    evalSym Bind   = (>>=)

-- | Evaluation
evalDen :: Eval s => AST s sig -> Denotation sig
evalDen = go
  where
    go :: Eval s => AST s sig -> Denotation sig
    go (Sym s)  = evalSym s
    go (s :$ a) = go s $ go a

-- | Monadic denotation; mapping from a symbol signature
--
-- > a :-> b :-> Full c
--
-- to
--
-- > m a -> m b -> m c
type family   DenotationM (m :: * -> *) sig
type instance DenotationM m (Full a)    = m a
type instance DenotationM m (a :-> sig) = m a -> DenotationM m sig

-- | Lift a 'Denotation' to 'DenotationM'
liftDenotationM :: forall m sig proxy1 proxy2 . Monad m =>
    SigRep sig -> proxy1 m -> proxy2 sig -> Denotation sig -> DenotationM m sig
liftDenotationM sig _ _ = help2 sig . help1 sig
  where
    help1 :: Monad m =>
        SigRep sig' -> Denotation sig' -> Args (WrapFull m) sig' -> m (DenResult sig')
    help1 SigFull f _ = return f
    help1 (SigMore sig) f (WrapFull ma :* as) = do
        a <- ma
        help1 sig (f a) as

    help2 :: SigRep sig' -> (Args (WrapFull m) sig' -> m (DenResult sig')) -> DenotationM m sig'
    help2 SigFull f = f Nil
    help2 (SigMore sig) f = \a -> help2 sig (\as -> f (WrapFull a :* as))

-- | Runtime environment
type RunEnv = [(Name, Dynamic)]
  -- TODO Use a more efficient data structure?

-- | Evaluation
class EvalEnv sym env
  where
    compileSym :: proxy env -> sym sig -> DenotationM (Reader env) sig

    default compileSym :: (Symbol sym, Eval sym) =>
        proxy env -> sym sig -> DenotationM (Reader env) sig
    compileSym p s = compileSymDefault (symSig s) p s

-- | Simple implementation of `compileSym` from a 'Denotation'
compileSymDefault :: forall proxy env sym sig . Eval sym =>
    SigRep sig -> proxy env -> sym sig -> DenotationM (Reader env) sig
compileSymDefault sig p s = liftDenotationM sig (Proxy :: Proxy (Reader env)) s (evalSym s)

instance (EvalEnv sym1 env, EvalEnv sym2 env) => EvalEnv (sym1 :+: sym2) env
  where
    compileSym p (InjL s) = compileSym p s
    compileSym p (InjR s) = compileSym p s

instance EvalEnv Empty env
  where
    compileSym = error "compileSym: Empty"

instance EvalEnv sym env => EvalEnv (Typed sym) env
  where
    compileSym p (Typed s) = compileSym p s

instance EvalEnv sym env => EvalEnv (sym :&: info) env
  where
    compileSym p = compileSym p . decorExpr

instance EvalEnv Literal env

instance EvalEnv Construct env

instance EvalEnv Let env

instance Monad m => EvalEnv (MONAD m) env

instance EvalEnv BindingT RunEnv
  where
    compileSym _ (VarT v) = reader $ \env ->
        case lookup v env of
          Nothing -> error $ "compileSym: Variable " ++ show v ++ " not in scope"
          Just d  -> case fromDynamic d of
            Nothing -> error "compileSym: type error"  -- TODO Print types
            Just a -> a
    compileSym _ (LamT v) = \body -> reader $ \env a -> runReader body ((v, toDyn a) : env)

-- | \"Compile\" a term to a Haskell function
compile :: EvalEnv sym env => proxy env -> AST sym sig -> DenotationM (Reader env) sig
compile p (Sym s)  = compileSym p s
compile p (s :$ a) = compile p s $ compile p a
  -- This use of the term \"compile\" comes from \"Typing Dynamic Typing\" (Baars and Swierstra,
  -- ICFP 2002, <http://doi.acm.org/10.1145/581478.581494>)

-- | Evaluation of open terms
evalOpen :: EvalEnv sym env => env -> ASTF sym a -> a
evalOpen env a = runReader (compile Proxy a) env

-- | Evaluation of closed terms where 'RunEnv' is used as the internal environment
--
-- (Note that there is no guarantee that the term is actually closed.)
evalClosed :: EvalEnv sym RunEnv => ASTF sym a -> a
evalClosed a = runReader (compile (Proxy :: Proxy RunEnv) a) []