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module Haskell.Rewrite
(rewrite
,betared, etared, casered
,etacase, casecase
,autoSimp
,normalise) where
import Control.Monad
import Data.List
import Data.Maybe
import qualified Data.Map.Strict as Map
import Haskell.AST
import Util
rewrite :: Name -> Expr -> Expr -> Expr
rewrite name repl = \case
App e as -> App (rewrite name repl e) (map (rewrite name repl) as)
Ref n -> if n == name then repl else Ref n
Num k -> Num k
Tup es -> Tup (map (rewrite name repl) es)
Lam ns e -> if name `elem` ns then Lam ns e else Lam ns (rewrite name repl e)
Case e as -> Case (rewrite name repl e) (map caseArm as)
where
caseArm (p, e') =
if name `elem` boundVars p then (p, e') else (p, rewrite name repl e')
boundVars :: Pat -> [Name]
boundVars PatAny = []
boundVars (PatVar n) = [n]
boundVars (PatCon n ps) = nub $ [n] ++ concatMap boundVars ps
boundVars (PatTup ps) = nub $ concatMap boundVars ps
betared :: Expr -> Expr
betared (App (Lam (n:as) bd) (arg:args)) =
App (rewrite n arg (Lam as bd)) args
betared e = recurse id betared e
etared :: Expr -> Expr
etared (Lam (n:as) (App e es@(_:_)))
| last es == Ref n =
Lam as (App e (init es))
etared e = recurse id etared e
casered :: Bool -> Expr -> Expr
casered ambig orig@(Case subj arms) =
case catMaybes [(,rhs) <$> unify p (casered ambig subj) | (p, rhs) <- arms] of
[] -> recurse id (casered ambig) orig
((mp, rhs):rest) ->
let res = foldl' (\e' (n, rhs') -> rewrite n rhs' e') rhs (Map.assocs mp)
in case (ambig, rest) of
(True, _) -> res
(False, []) -> res
(False, _) -> recurse id (casered ambig) orig
casered ambig e = recurse id (casered ambig) e
etacase :: Expr -> Expr
etacase (Case subj arms) | all (uncurry eqPE) arms = subj
etacase e = recurse id etacase e
casecase :: Expr -> Expr
casecase (Case (Case subj arms1) arms2) =
Case subj [(p, Case e arms2) | (p, e) <- arms1]
casecase e = recurse id casecase e
autoSimp :: Expr -> Expr
autoSimp expr =
let steps = [betared, casered False, etared, etacase, casecase]
in fixpoint (normalise . foldl1 (.) (intersperse normalise steps)) expr
eqPE :: Pat -> Expr -> Bool
eqPE pat expr = case unify pat expr of
Nothing -> False
Just mp -> all (uncurry isIdmap) (Map.assocs mp)
where
isIdmap n = (== Ref n)
unify :: Pat -> Expr -> Maybe (Map.Map Name Expr)
unify PatAny _ = Just Map.empty
unify (PatVar n) e = Just (Map.singleton n e)
unify (PatCon n ps) (App n' es)
| n' == Ref n, length ps == length es =
foldM (\m (p, e) -> unify p e >>= reconcile m) Map.empty (zip ps es)
unify (PatTup ps) (Tup es)
| length ps == length es =
foldM (\m (p, e) -> unify p e >>= reconcile m) Map.empty (zip ps es)
unify _ _ = Nothing
reconcile :: Map.Map Name Expr -> Map.Map Name Expr -> Maybe (Map.Map Name Expr)
reconcile m1 m2 = foldM func m1 (Map.assocs m2)
where func m (k, v) = case Map.lookup k m of
Nothing -> Just (Map.insert k v m)
Just v' | v == v' -> Just m
| otherwise -> Nothing
normalise :: Expr -> Expr
normalise (App e []) = normalise e
normalise (Lam [] e) = normalise e
normalise e = recurse id normalise e
recurse :: (Pat -> Pat) -> (Expr -> Expr) -> Expr -> Expr
recurse _ f (App e as) = App (f e) (map f as)
recurse _ _ (Ref n) = Ref n
recurse _ _ (Num k) = Num k
recurse _ f (Tup es) = Tup (map f es)
recurse _ f (Lam ns e) = Lam ns (f e)
recurse fp f (Case e as) = Case (f e) (map (\(p, e') -> (fp p, f e')) as)
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