datafix-0.0.0.1: exprs/lambda.hs
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
module Expr where
-- From Mark: marku@cs.waikato.ac.nz [Nov 2001]
-- This program contrasts the cost of direct and
-- state-monadic style of computation.
-- Experiments with Higher-Order mappings over terms.
-- Speed comparisons of passing environments in a monad, versus explicitly.
--
-- With Hugs98 Feb 2001: (K reductions/ K cells)
-- N mainSimple mainMonad
-- Redns Cells Redns Cells
-- 10 4.7 8 15 28
-- 20 16.0 27 49 92
-- 30 33.5 57 102 192
-- 40 58 98 175 329
-- 50 89 150 268 502
-- 80 221 373 664 1242
-- 160 865 1456 2582 4826
-- 320 3419 5754 10181 19021
--
-- With GHC 5.00.1
-- N mainSimple mainMonad
-- secs Kbytes secs Kbytes
-- 100 0.02 1180 0.08 8213
-- 200 0.080 4529 0.30 31997
-- 400 0.310 17850 1.36 126333
-- 800 1.54 70928 6.6 502096
-- 1 1600 7.66 282833 34.6 2001963
-- 2 1600 28.76 1725128 1 with newtype
-- 3 1600 28.1 1694119 2 + eval covers ALL cases
-- 4 1600 28.5 1742828 3 + recursion calls eval
-- 5 1600 7.13 282832 1 with no Add typechecks
--
-- Conclusion: mainMonad is about 4 times slower than mainSimple
-- and about 7 times more memory.
--
import System.Environment
import Control.Monad.Trans.State.Strict
import Data.Functor.Identity
----------------------------------------------------------------------
-- A directly recursive Eval, with explicit environment
----------------------------------------------------------------------
-- A trivial monad so that we can use monad syntax.
newtype Id a = Id (Identity a)
deriving (Applicative, Functor, Monad)
instance Show a => Show (Id a) where
show (Id i) = show (runIdentity i)
data Term
= Var String
| Con Int
| Incr
| Add Term Term
| Lam String Term
| App Term Term
| IfZero Term Term Term
-- the following terms are used internally
| Thunk Term Env -- a closure
deriving (Eq,Read,Show)
type Env = [(String,Term)]
-----------------------------------------------------------------
-- This class extends Monad to have the standard features
-- we expect while evaluating/manipulating expressions.
----------------------------------------------------
class (Monad m) => EvalEnvMonad m where
incr :: m () -- example of a state update function
-- these defines the traversal!
traverseTerm :: Term -> m Term
--traversePred :: Pred -> m Pred
lookupVar :: String -> m Term
pushVar :: String -> Term -> m a -> m a
currEnv :: m Env -- returns the current environment
withEnv :: Env -> m a -> m a -- uses the given environment
pushVar v t m = do env <- currEnv; withEnv ((v,t):env) m
instance EvalEnvMonad (State Env) where
incr = return ()
traverseTerm = undefined -- eval
lookupVar v = do
env <- get
return $ lookup2 env
where
lookup2 env = maybe (error ("undefined var: " ++ v)) id (lookup v env)
currEnv = get
withEnv tmp m = return (evalState m tmp)
expr :: IO ()
expr = do { mainSimple ; mainMonad }
where
mainSimple =
do args <- getArgs
if null args
then putStrLn "Args: number-to-sum-up-to"
else putStrLn (show (simpleEval [] (App sum0 (Con (read(head args))))))
mainMonad =
do args <- getArgs
if null args
then putStrLn "Args: number-to-sum-up-to"
else (ev (App sum0 (Con (read(head args))))) >> return ()
------------------------------------------------------------
-- Data structures
------------------------------------------------------------
--instance Show (a -> b) where
-- show f = "<function>"
----------------------------------------------------------------------
-- Evaluate a term
----------------------------------------------------------------------
ev :: Term -> IO (Env,Term)
ev t =
do let (t2, env) = runState (traverseTerm t :: State Env Term) []
putStrLn (pp t2 ++ " " ++ ppenv env)
return (env,t2)
eval :: (EvalEnvMonad m) => Term -> m Term
eval (Var x) =
do e <- currEnv
t <- lookupVar x
traverseTerm t
eval (Add u v) =
do {Con u' <- traverseTerm u;
Con v' <- traverseTerm v;
return (Con (u'+v'))}
eval (Thunk t e) =
withEnv e (traverseTerm t)
eval f@(Lam x b) =
do env <- currEnv
return (Thunk f env) -- return a closure!
eval (App u v) =
do {u' <- traverseTerm u;
-- call-by-name, so we do not evaluate the argument v
apply u' v
}
eval (IfZero c a b) =
do {val <- traverseTerm c;
if val == Con 0
then traverseTerm a
else traverseTerm b}
eval (Con i) = return (Con i)
eval (Incr) = incr >> return (Con 0)
--apply :: Term -> Term -> StateMonad2 Term
apply (Thunk (Lam x b) e) a =
do orig <- currEnv
withEnv e (pushVar x (Thunk a orig) (traverseTerm b))
apply a b = fail ("bad application: " ++ pp a ++
" [ " ++ pp b ++ " ].")
simpleEval :: Env -> Term -> Id Term
simpleEval env (Var v) =
simpleEval env (maybe (error ("undefined var: " ++ v)) id (lookup v env))
simpleEval env e@(Con _) =
return e
simpleEval env e@Incr =
return (Con 0)
simpleEval env (Add u v) =
do {Con u' <- simpleEval env u;
Con v' <- simpleEval env v;
return (Con (u' + v'))}
where
addCons (Con a) (Con b) = return (Con (a+b))
addCons (Con _) b = fail ("type error in second arg of Add: " ++ pp b)
addCons a (Con _) = fail ("type error in first arg of Add: " ++ pp a)
simpleEval env f@(Lam x b) =
return (Thunk f env) -- return a closure!
simpleEval env (App u v) =
do {u' <- simpleEval env u;
-- call-by-name, so we do not evaluate the argument v
simpleApply env u' v
}
simpleEval env (IfZero c a b) =
do {val <- simpleEval env c;
if val == Con 0
then simpleEval env a
else simpleEval env b}
simpleEval env (Thunk t e) =
simpleEval e t
simpleApply :: Env -> Term -> Term -> Id Term
simpleApply env (Thunk (Lam x b) e) a =
simpleEval env2 b
where
env2 = (x, Thunk a env) : e
simpleApply env a b = fail ("bad application: " ++ pp a ++
" [ " ++ pp b ++ " ].")
------------------------------------------------------------
-- Utility functions for printing terms and envs.
------------------------------------------------------------
ppenv env = "[" ++ concatMap (\(v,t) -> v ++ "=" ++ pp t ++ ", ") env ++ "]"
pp :: Term -> String
pp = ppn 0
-- Precedences:
-- 0 = Lam and If (contents never bracketed)
-- 1 = Add
-- 2 = App
-- 3 = atomic and bracketed things
ppn :: Int -> Term -> String
ppn _ (Var v) = v
ppn _ (Con i) = show i
ppn _ (Incr) = "INCR"
ppn n (Lam v t) = bracket n 0 ("@" ++ v ++ ". " ++ ppn (-1) t)
ppn n (Add a b) = bracket n 1 (ppn 1 a ++ " + " ++ ppn 1 b)
ppn n (App a b) = bracket n 2 (ppn 2 a ++ " " ++ ppn 2 b)
ppn n (IfZero c a b) = bracket n 0
("IF " ++ ppn 0 c ++ " THEN " ++ ppn 0 a ++ " ELSE " ++ ppn 0 b)
ppn n (Thunk t e) = bracket n 0 (ppn 3 t ++ "::" ++ ppenv e)
bracket outer this t | this <= outer = "(" ++ t ++ ")"
| otherwise = t
------------------------------------------------------------
-- Test Data
------------------------------------------------------------
x = (Var "x")
y = (Var "y")
a1 = (Lam "x" (Add (Var "x") (Con 1)))
aa = (Lam "x" (Add (Var "x") (Var "x")))
-- These should all return 1
iftrue = (IfZero (Con 0) (Con 1) (Con 2))
iffalse = (IfZero (Con 1) (Con 2) (Con 1))
-- This function sums all the numbers from 0 upto its argument.
sum0 :: Term
sum0 = (App fix partialSum0)
partialSum0 = (Lam "sum"
(Lam "n"
(IfZero (Var "n")
(Con 0)
(Add (Var "n") (App (Var "sum") nMinus1)))))
nMinus1 = (Add (Var "n") (Con (-1)))
lfxx :: Term
lfxx = (Lam "x" (App (Var "F") (App (Var "x") (Var "x"))))
-- This is the fix point combinator: Y
fix :: Term
fix = (Lam "F" (App lfxx lfxx))