wip: stack frames

doc/liveness.org

@@ -0,0 +1,88 @@
+* example
+
+#+begin_src scheme
+  (letrec ((fac (λ (n)
+                (if (zero? n)
+                    1
+                    (* n (fac (- n 1)))))))
+  (fac 3))
+#+end_src
+
+#+begin_src scheme
+  (λ (ktail0)
+  (letrec ((fac
+            (λ (n ktail1)
+              (zero?
+               n
+               (κ (x0)
+                  (if x0
+                      (continue ktail1 1)
+                      (- n 1
+                         (κ (x1)
+                            (fac x1
+                                 (κ (x2)
+                                    (* n x2 ktail1)))))))))))
+    (fac 3)))
+#+end_src
+
+#+begin_example
+n ktail1
+| |
+| | x0
+| | |
+| | ^
+| |
+| | x1
+| | |
+| | ^
+| | 
+| | x2
+| | |
+^ ^ ^
+#+end_example
+
+#+begin_src scheme
+  (define $fac-c0
+    (pop! %x0 0) ; [ x0 $fac-c0 n $fac ktail1 ]
+    (if %x0 ; [ $fac-c0 n $fac ktail1 ]
+        ;; every variable but `ktail1' is dead so we pop them all.
+        ;; this probably means that `if' should take two continuations
+        ;; rather than two blocks.
+        (then (pop! %_) ; [ $fac-c0 n $fac ktail1 ]
+              (pop! %_) ; [ n $fac ktail1 ]
+              (pop! %_) ; [ $fac ktail1 ]
+              (push! 1) ; [ ktail1 ]
+              (return 1)) ; [ 1 ktail1 ]
+        (else (load! %n 1) ; [ $fac-c0 n $fac ktail1 ]
+              (prim %x1 (- %n 1)) ; [ $fac-c0 n $fac ktail1 ]
+              (push! $fac-c1) ; [ $fac-c0 n $fac ktail1 ]
+              (push! $fac) ; [ $fac-c1 $fac-c0 n $fac ktail1 ]
+              (push! %x1) ; [ $fac $fac-c1 $fac-c0 n $fac ktail1 ]
+              (call 1) ; [ x1 $fac $fac-c1 $fac-c0 n $fac ktail1 ]
+              )))
+
+  (define $fac-c1
+    (pop! %x2) ; [ x2 $fac-c1 $fac-c0 n $fac ktail1 ]
+    (load! %n 3) ; [ $fac-c1 $fac-c0 n $fac ktail1 ]
+    (prim %x3 (* %n %x2))
+    (push! %x3) ; [ $fac-c1 $fac-c0 n $fac ktail1 ]
+    (return 1) ; [ x3 $fac-c1 $fac-c0 n $fac ktail1 ]
+    )
+
+  (define $fac
+    (load! %ktail1 2) ; [ n $fac ktail1 ]
+    (load! %n 0) ; [ n $fac ktail1 ]
+    (push! $fac-c0) ; [ n $fac ktail1 ]
+    (push! $zero?) ; [ $fac-c0 n $fac ktail1 ]
+    (push! %n) ; [ $zero? $fac-c0 n $fac ktail1 ]
+    (call 1) ; [ n $zero? $fac-c0 n $fac ktail1 ]
+    )
+
+  (define $start
+    (push! $fac) ; [ $start ktail0 ]
+    (push! 3) ; [ $fac $start ktail0 ]
+    (tail-call 1) ; [ 3 $fac $start ktail0 ]
+    ;; ↑ `tail-call' knows how to dispose of the caller's stack frame.
+    )
+#+end_src
+

doc/make-vm-stackier.org

@@ -0,0 +1,68 @@
+* rationale?
+
+previously, the VM's stack was used for storing local variables across blocks; a Scheme procedure was split into several low-level routines (one for the procedure itself and one for each continuation), and the stack was used as a communication channel for these separate routines. in contrast, registers were local to each routine. this aligns with Wasm's model of functions pretty well, with Wasm /locals/ acting as the VM's /registers/, and a global mutable stack serving as fallback.
+
+this worked quite well until it became time to implement ~call/cc~.
+
+we are considering making the following alterations to the VM:
+- explicitly segment the stack into frames.
+- passing procedures and return addresses on the stack.
+
+* scratchpad
+
+** Scheme source
+
+#+begin_src scheme
+(* 2 (call/cc
+      (λ (cc)
+        (begin (cc 6)
+               3))))
+#+end_src
+
+** CPS
+
+#+begin_src scheme
+(λ (ktail0)
+  (letrec ((with-cc
+            (λ (cc ktail1)
+              (letrec ((k0 (κ (_)
+                              (continue ktail1 3))))
+                (cc 6 k0)))))
+    (prim (call/cc with-cc)
+          (κ (x1)
+             (prim (* 2 x1)
+                   (κ (x2) (continue ktail0 x2)))))))
+#+end_src
+
+** stack VM
+
+#+begin_src scheme
+;; (call n) expects `n' values on the stack as arguments. then the
+;; procedure is expected at index `n', and the return continuation
+;; should be at `n+1'.
+
+(define $k0
+  (pop! %_) ; [ _ ret ]
+  (push! 3) ; [ ret ]
+  (tail-call 1) ; [ 3 ret ]
+  )
+
+(define $with-cc
+  (pop! %cc) ; [ cc ret ]
+  (push! $k0) ; [ ret ]
+  (push! %cc) ; [ $k0 ret ]
+  (push! 6) ; [ cc $k0 ret ]
+  ;; call a procedure with one argument.
+  (call 1) ; [ 6 cc $k0 ret ]
+  )
+
+(define $main
+  (pop! %ktail0) ; [ ret ]
+  (prim %x1 (call/cc $with-cc)) ; []
+  (prim %x2 (* 2 %x1)) ; []
+  (push! %ktail0) ; []
+  (push! %x2) ; [ ret ]
+  (tail-call 1) ; [ %x2 ret ]
+  )
+#+end_src
+

src/Gyehoek/CPS/Stackify.hs

@@ -63,31 +63,22 @@ stackify g (ExpIf c t f) = do
   f' <- buildBlock <$> stackify g f
   pure . Tail $ Stk.If c' t' f'
 
-stackify g (ExpApply f xs ktail) = pure $
-    Code [ Stk.Push (Stk.ValReg l) | l <- ls ] $
-    Tail (Stk.TailCall (stackifyVal g f) (k : (stackifyVal g <$> xs)))
-  where
-    k = var g ktail
-    ls = fold $ (k ^? #ValImm . #ImmLabel)
-      >>= \klbl -> g ^. #liveness . at klbl
+-- stackify g (ExpApply f xs ktail) = pure $
+--     Code [ Stk.Push (Stk.ValReg l) | l <- ls ] $
+--     Tail (Stk.TailCall (stackifyVal g f) (k : (stackifyVal g <$> xs)))
+--   where
+--     k = var g ktail
+--     ls = fold $ (k ^? #ValImm . #ImmLabel)
+--       >>= \klbl -> g ^. #liveness . at klbl
 
-stackify g e@(ExpContinue k xs) = do
-  pure $
-    Code [ Stk.Push (Stk.ValReg l) | l <- ls ] $
-    Tail (Stk.TailCall k' (stackifyVal g <$> xs))
-  where
-    k' = stackifyVal g k
-    ls = fold $ (k' ^? #ValImm . #ImmLabel)
-      >>= \klbl -> g ^. #liveness . at klbl
-
--- stackify g (ExpPrim (PrimCallCC withcc) cc) = do
---   cc_l <- gensym' "cc"
---   rcc_l <- gensym' "reified-cc"
---   stackifyKappa g cc_l cc \g' rt -> do
---     emitRoutine rt
---     pure $
---       Code [ Stk.Prim rcc_l PrimCaptureCC ] $
---       Tail (Stk.TailCall (stackifyVal g' withcc) [Stk.ValLabel rcc_l])
+-- stackify g e@(ExpContinue k xs) = do
+--   pure $
+--     Code [ Stk.Push (Stk.ValReg l) | l <- ls ] $
+--     Tail (Stk.TailCall k' (stackifyVal g <$> xs))
+--   where
+--     k' = stackifyVal g k
+--     ls = fold $ (k' ^? #ValImm . #ImmLabel)
+--       >>= \klbl -> g ^. #liveness . at klbl
 
 stackify g (ExpPrim p (MkKappa [x] e)) = do
   e' <- stackify (g & #bound . at x ?~ Stk.ValReg x) e
@@ -105,16 +96,17 @@ stackifyKappa
   => Env -> Name -> Kappa
   -> (Env -> Stk.Routine -> Eff es r)
   -> Eff es r
-stackifyKappa g name kap@(MkKappa xs m) w = do
-  let vs = (name, Stk.ValLabel name) : (bindReg <$> xs)
-  let ls = live g kap
-  m' <- stackify (g & #bound <>~ H.fromList (vs ++ (bindReg <$> ls))) m
-  let g' = g & #bound . at name ?~ Stk.ValLabel name
-             & #liveness . at name ?~ live g kap
-  let rt = Stk.MkRoutine name xs . buildBlock $
-       -- pop in the opposite order we push
-       Code [Stk.Pop x | x <- reverse ls] m'
-  w g' rt
+stackifyKappa g name kap@(MkKappa xs m) w = _
+-- stackifyKappa g name kap@(MkKappa xs m) w = do
+--   let vs = (name, Stk.ValLabel name) : (bindReg <$> xs)
+--   let ls = live g kap
+--   m' <- stackify (g & #bound <>~ H.fromList (vs ++ (bindReg <$> ls))) m
+--   let g' = g & #bound . at name ?~ Stk.ValLabel name
+--              & #liveness . at name ?~ live g kap
+--   let rt = Stk.MkRoutine name xs . buildBlock $
+--        -- pop in the opposite order we push
+--        Code [Stk.Pop x | x <- reverse ls] m'
+--   w g' rt
 
 stackifyLambda
   :: (Stackify :> es, GenSym :> es)
@@ -125,7 +117,7 @@ stackifyLambda g name (MkLambda xs k m) w = do
   let vs = [ (x, Stk.ValReg x) | x <- k:xs ]
   m' <- stackify (g & #bound <>~ H.fromList vs) m
   let g' = g & #bound . at name ?~ Stk.ValLabel name
-  w g' $ Stk.MkRoutine name (k:xs) (buildBlock m')
+  w g' $ Stk.MkRoutine name (buildBlock m')
 
 stackifyVal :: Env -> Val -> Stk.Val
 stackifyVal g = \case

src/Gyehoek/Stack/Syntax.hs

@@ -45,7 +45,6 @@ instance IsList Program where
 
 data Routine = MkRoutine
   { label :: Name
-  , params :: List Name
   , start :: Block
   }
   deriving stock (Show, Generic, Data)
@@ -59,10 +58,12 @@ data Block = MkBlock
   deriving anyclass (NFData)
 
 data Tail
-  = TailCall Val (List Val)
+  -- | call the procedure at stack index `n` supplied with `n`
+  -- arguments on top of the stack, then return by calling the
+  -- continuation at stack index `n+1`.
+  = TailCall Int
   | If Val Block Block
-  -- | Invoke a reified continuation.
-  | InvokeC Val
+  | Return
   deriving stock (Show, Generic, Data)
   deriving anyclass (NFData)
 
@@ -105,11 +106,12 @@ instance S.DataIso Block where
 
 instance S.DatumIso Tail where
   datumIso = S.match
-    $ S.With (S.headTagged1' "tail-call" S.datumIso S.datumIso >>>)
+    $ S.With (S.headTagged1 "tail-call" S.datumIso >>>)
     $ S.With (if_ >>>)
-    $ S.With (S.headTagged1 "invoke/c" S.datumIso >>>)
+    $ S.With (S.headTagged0 "return" >>>)
     $ S.End
     where
+      -- if_ = S.ifLike "if" (S.datumIso @Val) S.datumIso S.datumIso
       if_ = S.ifLike "if" (S.datumIso @Val) (branch "then") (branch "else")
       branch :: Text -> S.DatumGrammar Block
       branch s =
@@ -127,7 +129,7 @@ instance S.DatumIso Routine where
   datumIso = S.with \rout ->
       S.listWithIndentation (S.NSpecial 1)
        ( S.el (S.decorate S.SynBuiltin >>> S.sym "define")
-       >>> S.el (S.list $ S.el labelName >>> S.rest regName)
+       >>> S.el labelName
        >>> S.restData (S.dataIso @Block)
        )
       >>> rout

src/Gyehoek/Stack/VM.hs

@@ -12,7 +12,7 @@ module Gyehoek.Stack.VM
 import Gyehoek.Stack.Syntax
 import Control.Lens
 import qualified Data.HashMap.Strict as H
-import Data.List (unfoldr, intersperse)
+import Data.List (unfoldr, intersperse, compareLength)
 import Gyehoek.Prelude
 import qualified Data.List.NonEmpty as NE
 import Lucid
@@ -36,8 +36,18 @@ data DebugVM = MkDebugVM
   }
   deriving (Show, Generic)
 
+data Frame = MkFrame
+  { returnAddress :: Name
+  , procedure :: Name
+  , locals :: List Obj
+  }
+  deriving (Show, Generic)
+
+newtype Stack = MkStack (NonEmpty Frame)
+  deriving (Show, Generic)
+
 data VM = MkVM
-  { stack :: List Obj
+  { frames :: NonEmpty Frame
   , code :: List Instr
   , tail :: Tail
   , registers :: HashMap Name Obj
@@ -47,6 +57,14 @@ data VM = MkVM
   }
   deriving (Show, Generic)
 
+_NonEmpty :: Iso (NonEmpty a) (NonEmpty b) (a, List a) (b, List b)
+_NonEmpty = iso
+  (\(x:|xs) -> (x,xs))
+  (\(x,xs) -> x:|xs)
+
+stack :: Lens' VM (List Obj)
+stack = #frames . _NonEmpty . _1 . #locals
+
 data Env = MkEnv
   { labels :: HashMap Name Routine
   }
@@ -62,7 +80,7 @@ vmerror = throwError . VMError
 
 stepI :: Jalmot :> es => Env -> VM -> Instr -> Eff es VM
 
-stepI e vm (Push v) = traverseOf #stack push vm
+stepI e vm (Push v) = traverseOf stack push vm
   where push xs = (:) <$> evalVal e vm v <*> pure xs
 
 stepI e vm (Prim r p) = traverse (evalVal e vm) p >>= \case
@@ -99,27 +117,52 @@ stepI e vm (Prim r p) = traverse (evalVal e vm) p >>= \case
       ret $ ObjImm (ImmInt (op x y))
     arith_binop _ x y = vmerror [i|bad arith: #{x}, #{y}|]
 
-stepI e vm (Pop r) = case vm ^. #stack of
+stepI e vm (Pop r) = case vm ^. stack of
   [] -> vmerror "empty stack"
   (x:xs) -> pure $ vm & #registers . at r ?~ x
-               & #stack .~ xs
+               & stack .~ xs
 
 stepI e vm ins = vmerror [i|unimplemented instruction: #{ins}|]
 
 stepT :: Jalmot :> es => Env -> VM -> Tail -> Eff es VM
 
-stepT g vm (TailCall f xs) = do
-  xs' <- traverse (evalVal g vm) xs
-  evalToLabel g vm f >>= \case
-    "halt" -> pure $ vm & #result ?~ xs'
-    l -> do
-      rt <- case g ^. #labels . at l of
-          Nothing -> vmerror [i|undefined label: #{l}|]
-          Just x -> pure x
-      pure $ vm & #code .~ rt.start.code
+stepT g vm Return = vmerror "aajak"
+
+stepT g vm tc@(TailCall nargs) =
+  case setupCall nargs (vm ^. stack) of
+    Nothing -> vmerror [i|bad stack at #{tc}|]
+    Just (xs,f,rest) ->
+      case f of
+        "halt" -> pure $ vm & #result ?~ xs
+        l -> do
+          rt <- case g ^. #labels . at l of
+            Nothing -> vmerror [i|undefined label: #{l}|]
+            Just x -> pure x
+          let newFrame = MkFrame
+                { returnAddress =
+                  vm ^. #frames . _NonEmpty . _1 . #returnAddress
+                , procedure = f
+                , locals = xs
+                }
+          pure $ vm
+            & #code .~ rt.start.code
             & #tail .~ rt.start.tail
-            & #registers .~ H.fromList (rt.params `zip` xs')
+            & stack .~ rest
+            & #frames %~ NE.cons newFrame
+            -- it is not essential we clear the registers, but it'll
+            -- make bugs more obvious.
+            & #registers .~ mempty
             & #debug . #currentRoutine .~ rt.label
+  -- evalToLabel g vm f >>= \case
+  --   "halt" -> pure $ vm & #result ?~ xs'
+  --   l -> do
+  --     rt <- case g ^. #labels . at l of
+  --         Nothing -> vmerror [i|undefined label: #{l}|]
+  --         Just x -> pure x
+  --     pure $ vm & #code .~ rt.start.code
+  --           & #tail .~ rt.start.tail
+  --           & #registers .~ H.fromList (rt.params `zip` xs')
+  --           & #debug . #currentRoutine .~ rt.label
 
 stepT g vm (If c t f) = do
   branch <- evalVal g vm c <&> \case
@@ -140,11 +183,39 @@ evalVal e vm = \case
     Just x -> pure x
     Nothing -> vmerror [i|undefined register: #{r}|]
 
+splitAtExact :: Int -> List a -> Maybe (List a, List a)
+splitAtExact n xs = case compareLength xs n of
+  (EQ;GT) -> Just $ splitAt n xs
+  LT -> Nothing
+
+setupCall :: Int -> List Obj -> Maybe (List Obj, Name, Name, List Obj)
+setupCall n stk = do
+  (xs,stk') <- splitAtExact (n+2) stk
+  case splitAtExact n xs of
+    Just (args, [p,ret]) -> case (p,ret) of
+      (ObjImm (ImmLabel p'),ObjImm (ImmLabel ret')) -> Just (args,p',ret',stk')
+      _ -> Nothing
+    _ -> error "unreachable"
+
+setupTailCall :: Int -> List Obj -> Maybe (List Obj, Name, List Obj)
+setupTailCall n stk = do
+  (xs,stk') <- splitAtExact (n+1) stk
+  case xs ^? _Snoc of
+    Just (args, [p,ret]) -> case (p,ret) of
+      (ObjImm (ImmLabel p'),ObjImm (ImmLabel ret')) -> Just (args,p',stk')
+      _ -> Nothing
+    _ -> error "unreachable"
+
 initialVM :: VM
 initialVM = MkVM
-  { stack = []
+  { frames = NE.singleton $ MkFrame
+    { returnAddress = "halt"
+    , procedure = "<nowhere at all>"
+    , locals = [ ObjImm (ImmLabel "start")
+               ]
+    }
+  , tail = TailCall 0
   , code = []
-  , tail = TailCall (ValLabel "start") [ValLabel "halt"]
   , registers = mempty
   , stdout = ""
   , result = Nothing
@@ -288,10 +359,15 @@ ppVM vm = do
     td_ do
       code_ curi
     td_ do
-      let xs = code_ . ppDatum <$> (vm ^. #stack)
+      let xs = code_ . ppDatum <$> (vm ^. stack)
       sequence_ $ intersperse " | " xs
   where
     curi = vm ^?! failing (#code . _head . to ppDatum) (#tail . to ppDatum)
 
 ppDatum :: S.DatumIso a => a -> Html ()
 ppDatum = htmlDatum . runJalmotUnsafe . S.toDatum S.datumIso
+
+blah = [stkP|
+        (define $start
+          (return))
+      |]

test/Gyehoek/Test/Stack/VM.hs

@@ -13,64 +13,70 @@ evalsTo :: List Obj -> Program -> Assertion
 evalsTo rs p = runJalmotUnsafe (Sut.eval p) @?= rs
 
 test_root = testGroup "stack machine"
-  [ testCase "lit int" do
+  [ testCase "immediate halt" do
       evalsTo [ObjImm (ImmInt 3)] [stkP|
-        (define ($start %ktail)
-          (tail-call %ktail 3))
+        (define $start
+          (tail-call 0))
       |]
-  , testCase "return constant" do
-      evalsTo [ObjImm (ImmInt 123)] [stkP|
-        (define ($start %ktail)
-          (tail-call $silly %ktail))
-        (define ($silly %ktail)
-          (tail-call %ktail 123))
+  , testCase "lit int" do
+      evalsTo [ObjImm (ImmInt 3)] [stkP|
+        (define $start
+          (push! 3)
+          (tail-call 1))
       |]
-  , testCase "identity continuation" do
-      evalsTo [ObjImm (ImmInt 45)] [stkP|
-        (define ($start %ktail)
-          (push! %ktail)
-          (tail-call $id 45))
-        (define ($id %x)
-          (pop! %ktail)
-          (tail-call %ktail %x))
-      |]
-  , testCase "identity function" do
-      evalsTo [ObjImm (ImmInt 45)] [stkP|
-        (define ($start %ktail)
-          (tail-call $id 45 %ktail))
-        (define ($id %x %ktail)
-          (tail-call %ktail %x))
-      |]
-  , testCase "square" do
-      evalsTo [ObjImm (ImmInt 16)] [stkP|
-        (define ($start %ktail)
-          (tail-call $square 4 %ktail))
-        (define ($square %x %ktail)
-          (prim %x2 (* %x %x))
-          (tail-call %ktail %x2))
-      |]
-  , testCase "factorial" do
-      let hsfac (n :: Int) = foldr (*) (1) [1..n]
-      let fac (n :: Int) = [stkP|
-            (define ($fac %n %ktail)
-              (prim %x0 (zero? %n))
-              (if %x0
-                (then (tail-call %ktail 1))
-                (else (push! %n)
-                      (push! %ktail)
-                      (prim %x1 (- %n 1))
-                      (tail-call $fac %x1 $fac-k0))))
-            (define ($fac-k0 %x2)
-              (pop! %ktail)
-              (pop! %n)
-              (prim %x3 (* %x2 %n))
-              (tail-call %ktail %x3))
-            (define ($start %ktail)
-              (tail-call $fac #{n} %ktail))
-          |]
-      evalsTo [ObjImm (ImmInt 1)] $ fac 0
-      evalsTo [ObjImm (ImmInt 1)] $ fac 1
-      evalsTo [ObjImm (ImmInt 720)] $ fac 6
-      -- 20 is the greatest `n` for which n! ≤ maxBount @Int
-      evalsTo [ObjImm (ImmInt 2432902008176640000)] $ fac 20
+  -- , testCase "return constant" do
+  --     evalsTo [ObjImm (ImmInt 123)] [stkP|
+  --       (define ($start %ktail)
+  --         (tail-call $silly %ktail))
+  --       (define ($silly %ktail)
+  --         (tail-call %ktail 123))
+  --     |]
+  -- , testCase "identity continuation" do
+  --     evalsTo [ObjImm (ImmInt 45)] [stkP|
+  --       (define ($start %ktail)
+  --         (push! %ktail)
+  --         (tail-call $id 45))
+  --       (define ($id %x)
+  --         (pop! %ktail)
+  --         (tail-call %ktail %x))
+  --     |]
+  -- , testCase "identity function" do
+  --     evalsTo [ObjImm (ImmInt 45)] [stkP|
+  --       (define ($start %ktail)
+  --         (tail-call $id 45 %ktail))
+  --       (define ($id %x %ktail)
+  --         (tail-call %ktail %x))
+  --     |]
+  -- , testCase "square" do
+  --     evalsTo [ObjImm (ImmInt 16)] [stkP|
+  --       (define ($start %ktail)
+  --         (tail-call $square 4 %ktail))
+  --       (define ($square %x %ktail)
+  --         (prim %x2 (* %x %x))
+  --         (tail-call %ktail %x2))
+  --     |]
+  -- , testCase "factorial" do
+  --     let hsfac (n :: Int) = foldr (*) (1) [1..n]
+  --     let fac (n :: Int) = [stkP|
+  --           (define ($fac %n %ktail)
+  --             (prim %x0 (zero? %n))
+  --             (if %x0
+  --               (then (tail-call %ktail 1))
+  --               (else (push! %n)
+  --                     (push! %ktail)
+  --                     (prim %x1 (- %n 1))
+  --                     (tail-call $fac %x1 $fac-k0))))
+  --           (define ($fac-k0 %x2)
+  --             (pop! %ktail)
+  --             (pop! %n)
+  --             (prim %x3 (* %x2 %n))
+  --             (tail-call %ktail %x3))
+  --           (define ($start %ktail)
+  --             (tail-call $fac #{n} %ktail))
+  --         |]
+  --     evalsTo [ObjImm (ImmInt 1)] $ fac 0
+  --     evalsTo [ObjImm (ImmInt 1)] $ fac 1
+  --     evalsTo [ObjImm (ImmInt 720)] $ fac 6
+  --     -- 20 is the greatest `n` for which n! ≤ maxBount @Int
+  --     evalsTo [ObjImm (ImmInt 2432902008176640000)] $ fac 20
   ]