cs784(TK)1 Semantics of Procedures and Scopes

Kinds of Scope Static or Lexical scope –determined by structure of program –Scheme, C++, Java, and many compiled languages Dynamic scope –determined by path of execution –Lisp dialects, Perl, and many interpreted languages Global Scope –File scope Local Scope –Block Body of a procedure Body of a loop Scope alters the meaning cs784(pm)2

cs784(TK)3 proc main() int x := 5; proc q() { x := x + 1;} proc r() {int x := 0; q(); } { r(); print(x); }. Scoping : Free (non-local) Variables Static scoping x -> x output: 6 Dynamic scoping x -> x output: 5

cs784(TK)4 let x = 15 in let f = proc () x in ( let x = 8 in (f) ) + (f) Static Scoping x -> x 30 Dynamic Scoping x -> x 23 x -> x

Informal Semantics of Procedures Procedure Definition –Store formal parameters and body Procedure Invocation –Evaluate body in an environment that binds formals to actual argument values Interpretation of free-variables –Use env. at proc. creation (static scoping) –Use env. at proc. call (dynamic scoping) cs784(TK)5

Procedures: Syntaxes Concrete Syntax ::= … | ( + ) | proc ::= () | ( {, }* ) Abstract Syntax proc-exp (ids body) app-exp (rator rands) cs784(TK)6

7 Encoding Procedures Static scoping (define-datatype procval procval? (closure (ids (list-of symbol?) (body expression?) (env environment?))) Closure retains the bindings of the free variables at procedure creation time. Dynamic scoping (define-datatype procval procval? (procv (ids (list-of symbol?) (body expression?)))

cs784(TK)8 Processing Procedure Definitions Static scoping (define (eval-expression exp env) (cases expression exp (proc-exp (ids body) (closure ids body env)) )) Dynamic scoping (define (eval-expression exp env) (cases expression exp (proc-exp (ids body) (procv ids body)) ))

cs784(TK)9 Processing Procedure Invocations: Static scoping (define (eval-expression exp env) (cases expression exp... (app-exp (rator rands) (let ((proc (eval-expression rator env)) (args (eval-rands rands env))) (if (procval? proc) (apply-procval proc args) (eopl:error 'eval-expression “Applying non-procedure ~s" proc) ) ))

cs784(TK)10 Processing Procedure Invocations Dynamic scoping (define (eval-expression exp env) (cases expression exp (app-exp (rator rands)... (let ((proc (eval-expression rator env)) (args (eval-rands rands env))) (if (procval? proc) (apply-procval proc args env) (eopl:error 'eval-expression "Applying non-procedure ~s" proc) ) ))

cs784(TK)11 Processing Procedure Invocations Static scoping (define (apply-procval proc args) (cases procval proc (closure (ids body s-env) (eval-expression body (extend-env ids args s-env)) ))) Dynamic scoping (define (apply-procval proc args c-env) (cases procval proc (procv (ids body) (eval-expression body (extend-env ids args c-env)) )))

cs784(TK)12 Processing Procedure Invocations >(run "let x = 1 in let f = proc () x in (f)") 1 >(run "let x = 1 in let f = proc (x) x in (f 2)") 2 Static scoping >(run "let x = 1 in let f = proc () x x = 5 in(f)") 1 Dynamic scoping >(run "let x = 1 in letf = proc () x x = 5 in(f)") 5

cs784(TK)13 Alternative Description: Static Scoping (define (closure ids body env) (lambda (args) (eval-expression body (extend-env ids args env)) ) ) (define (apply-procval proc args) (proc args) )

Implementing Dynamic Scoping The value of variable x is the last value bound to x. => stack discipline Deep binding Use a global stack for all variables Shallow binding Use a separate stack for each variable Implementing recursion is trivial. Meaning of a call depends on the context. cs784(TK)14

Lexical Scope Pros and Cons Scope of names is known to the compiler. Permits type checking by compiler Easily check for uninitialized variables Easier to analyze program correctness Recursion is harder to implement cs784(pm)15

Dynamic Scope Pros and Cons Meaning of variables known only at run-time. –Cannot perform type checking before execution –Programs are flexible, but harder to understand Easy to implement recursion Renaming in the caller can effect the semantics of a program –Locality of formals destroyed. –if renamed parameter now captures a free variable in the called function. –Recall the interpretation of a free variable in a function body can change based on the context (environment) of a call. cs784(pm)16

Application of Dynamic Scoping Exception Handling –provide a separate construct in statically scoped language Setting Local Formatting Parameters Input-Output Redirection –avoids passing parameters explicitly through “intermediate” procedures cs784(TK)17

cs784(TK)18 Difference between Scheme and ML > (define x 1) > (define (f) x) > (f) 1 > (define x 2) > (f) 2 > (define (g) x) > x 2 > (g) 2 - val x = 1; - fun f () = x; val f = fn : unit -> int - f (); val it = 1 : int - val x = 2; - f () ; val it = 1 : int - fun g () = x; val g = fn : unit -> int - g (); val it = 2: int