CALCULATING AN EXCEPTIONAL MACHINE Graham Hutton and Joel Wright University of Nottingham

1 Exception zAn event that causes a computation to terminate in a non-standard way. Abstract Machine zA term-rewriting system for executing programs in a particular language.

2 This Talk zWe show how to calculate an abstract machine for a small language with exceptions; zThe key technique is defunctionalization, first introduced by John Reynolds in 1972; zSomewhat neglected in recent years, but now re-popularised by Olivier Danvy et al.

3 Arithmetic Expressions data Expr = Val Int | Add Expr Expr eval :: Expr  Int eval (Val n) = n eval (Add x y) = eval x + eval y Syntax: Semantics:

4 Step 1 - Add Continuations Make the evaluation order explicit, by rewriting the semantics in continuation-passing style. Definition: A continuation is a function that is applied to the result of another computation.

5 Example: Basic idea: Generalise the semantics to make the use of continuations explicit. eval x+ eval y continuationcomputation

6 Aim: define a new semantics and hence eval’ :: Expr  (Int  Int)  Int eval e = eval’ e ( n  n) eval’ e c = c (eval e) such that

7 eval’ (Add x y) c Case: e = Add x y c (eval (Add x y)) = c (eval x + eval y) = eval’ x ( n  eval’ y ( m  c (n+m)) = eval’ x ( n  ( m  c (n+m)) (eval y)) = eval’ x ( n  c (n + eval y)) = ( n  c (n + eval y)) (eval x) = c (eval (Add x y)) c (eval x + eval y) eval’ x ( n  ( m  c (n+m)) (eval y)) eval’ x ( n  c (n + eval y)) ( n  c (n + eval y)) (eval x)

8 eval’ :: Expr  Cont  Int eval’ (Val n) c = c n eval’ (Add x y) c = eval’ x ( n  eval’ y ( m  c (n+m))) New semantics: The evaluation order is now explicit.

9 Step 2 - Defunctionalize Make the semantics first-order again, by rewriting eval’ using the defunctionalization technique. Basic idea: Represent the continuations we actually need using a datatype.

10 Continuations: eval’ :: Expr  Cont  Int eval’ (Val n) c = c n eval’ (Add x y) c = eval’ x ( n  eval’ y ( m  c (n+m))) eval :: Expr  Int eval e = eval’ e ( n  n) ( n  n) ( m  c (n+m))( n  eval’ y ( m  c (n+m)))

11 Combinators: c2 :: Expr  Cont  Cont c2 y c = n  eval’ y (c3 n c) c1 :: Cont c1 = n  n c3 :: Int  Cont  Cont c3 n c = m  c (n+m)

12 Datatype: apply :: CONT  Cont apply C1 = c1 apply (C2 y c) = c2 y (apply c) apply (C3 n c) = c3 n (apply c) data CONT = C1 | C2 Expr CONT | C3 Int CONT Semantics:

13 Aim: define a function and hence eval’’ e c = eval’ e (apply c) eval e = eval’’ e C1 such that eval’’ :: Expr  CONT  Int

14 eval’’ (Val n) c = apply c n eval’’ (Add x y) c = eval’’ x (C2 y c) By calculation, we obtain: The semantics is now first-order again. apply C1 n = n apply (C2 y c) n = eval’’ y (C3 n c) apply (C3 n c) m = apply c (n+m)

15 Step 3 - Refactor zWhat have we actually produced? Question: zAn abstract machine, but this only becomes clear after we refactor the components. Answer:

16 data Cont = STOP | EVAL Expr Cont | ADD Int Cont run e = eval e STOP eval (Val n) c = exec c n eval (Add x y) c = eval x (EVAL y c) exec STOP n = n exec (EVAL y c) n = eval y (ADD n c) exec (ADD n c) m = exec c (n+m) Abstract machine:

17 Example: run (Add (Val 1) (Val 2)) eval (Add (Val 1) (Val 2)) STOP = eval (Val 1) (EVAL (Val 2) STOP) = exec 3 STOP = exec (ADD 1 STOP) 2 = eval (Val 2) (ADD 1 STOP) = exec (EVAL (Val 2) STOP) 1 = 3 =

18 eval :: Expr  Maybe Int eval (Val n) = Just n eval (Throw) = Nothing eval (Add x y) = eval x  eval y eval (Catch x y) = eval x  eval y Adding Exceptions data Expr = | Throw | Catch Expr Expr Syntax: Semantics:

19 Step 1 - Add Continuations Step 2 - Defunctionalize Step 3 - Refactor Make evaluation order explicit. Make first-order once again. Reveal the abstract machine.

20 Control stack: Evaluating an expression: data Cont = STOP | EVAL Expr Cont | ADD Int Cont | HAND Expr Cont eval :: Expr  Cont  Maybe Int eval (Val n) c = exec c n eval (Throw) c = unwind c eval (Add x y) c = eval x (EVAL y c) eval (Catch x y) c = eval x (HAND y c)

21 Executing the control stack: Unwinding the control stack: unwind :: Cont  Maybe Int unwind STOP = Nothing unwind (EVAL _ c) = unwind c unwind (ADD _ c) = unwind c unwind (HAND y c) = eval y c exec :: Cont  Int  Maybe Int exec STOP n = Just n exec (EVAL y c) n = eval y (ADD n c) exec (ADD n c) m = exec c (n+m) exec (HAND _ c) n = exec c n

22 Summary zPurely calculational development of an abstract machine for a language with exceptions; zKey ideas of marking/unmarking and unwinding the stack arise directly from the calculations; zTechniques have been used to systematically design many other machines - Danvy et al.

23 Further Work zExploiting monads and folds; zReasoning about efficiency; zGeneralising the language; zCalculating a compiler.