Principles of Programming LanguagesSubroutines vs CoroutinesSubroutines vs Coroutines (cont.)Simple CoroutinesRecursive CoroutinesRecursive Coroutines—the problemRecursive Coroutines—solutionsSL5SL5 PrimitivesSL5 Primitives (cont.)Slide 11Procedure Call/Return—special caseProcedure Call/Return (cont.)SIMULA 67SIMULA 67 (cont.)Slide 16SIMULA 67 PrimitivesSIMULA67 Primitives (cont.)Slide 19SIMULA67 ExampleExample: picture at pc Example (cont.): Static LinksExample: 2 coroutinesExample (cont.)Slide 25Slide 26C SC 520 Principles of Programming LanguagesPrinciples of ProgrammingLanguagesLecture 09CoroutinesC SC 520 Principles of Programming Languages2Subroutines vs Coroutines•Subroutine call/returncall A A Bcall B returncall B Breturn returnseparateactivationentryentryentryC SC 520 Principles of Programming Languages3Subroutines vs Coroutines (cont.)•Coroutine resume/resumeresume A A Bresume Bresume Aresume Bresume Aresume ?/return•Non-nested lifetimes  abandon stack•Activation lifetimes potentially unlimited if no “return”sameactivationentryentry?C SC 520 Principles of Programming Languages4Simple Coroutines•No recursion•Only one activation of each coroutine at any time resume B: *a = pc + 2jrst @b(control returns here)resume BABa  b resumption pointpc activesuspendedC SC 520 Principles of Programming Languages5Recursive Coroutines•Initial resume (call) of X:create activation for Xresume execution at entry point•resume Y : suspend current activationResume which activation of Y?•resume ?  returnanonymous resume“terminated” activation•Call  create & resumeC SC 520 Principles of Programming Languages6Recursive Coroutines—the problemproc x { proc y { proc z{⋮ ⋮ ⋮⋮ ⋮ ⋮ call y call z resume y⋮ ⋮ ⋮⋮ ⋮ ⋮ resume z resume x return ⋮⋮ return} } }caller of z?resumer of z?C SC 520 Principles of Programming Languages7Recursive Coroutines—solutions•SIMULA 67return (“detach” in Simula 67) in z resumes “caller of z”•SL5return resumes in latest activation that resumed zWith bindings at activation creationC SC 520 Principles of Programming Languages8SL5•Activations (called “environments”) are first class objects•p := procedure ( . . . ) . . . endCreates a procedure (“template”) and assigns it to p•e := create pUses template to create activation (including variable storage, continuation point &c.)•e := e with ( . . . )Transmits arguments (through “transmitters” for each argument)•resume eSuspends current activation and resume in eSuspender becomes latest resumer of e•return [to e]Suspends current activation Returns control to most recent resumer [to e ]•No deallocation of ARs—they are garbage collectedC SC 520 Principles of Programming Languages9SL5 Primitives•Let c = currently executing AR•e := create pe = allocate(p)e.cont = entrypoint(p)e.creator = ce.resumer = cfor each X nonlocal in p do {t = cwhile t != nil doif X public in t thene.X.lval = t.X.lvalelse t = t.creatorif t == nil then error(X)}C SC 520 Principles of Programming Languages10SL5 Primitives (cont.)•e := e with (a , a , …, a₁ ₂ n)e.par[1]= transmitter₁(a₁). . .•resume ec.cont = resumepoint// e.creator untouchede.resumer = cc = egoto c.contresumepoint:C SC 520 Principles of Programming Languages11SL5 Primitives (cont.)•returnc.cont = resumepointc = c.resumergoto c.contresumepoint:•return to ec.cont = resumepoint// no alteration of e.resumerc = egoto c.contresumepoint:C SC 520 Principles of Programming Languages12Procedure Call/Return—special casef(a , a , …, a₁ ₂ n)  resume (create f with (a , a , …, ₁ ₂an))return  return•Binding is dynamic e= allocate(f) e.cont = entrypoint(f) e.creator = c // “access link”e.resumer = c // dynamic link// bind nonlocals using creator chaine.par[1]= transmitter₁(a₁). . .e := create fwith(a ,₁…,an)C SC 520 Principles of Programming Languages13Procedure Call/Return (cont.) e.cont = resumepoint e.resumer = c // redundantc = egoto c.cont // entrypoint(f)resumepoint:. . .e.cont = resumepoint //never usedc = c.resumer // follow dlgoto c.cont // entrypoint(f)resumepoint:. . .resume ereturnC SC 520 Principles of Programming Languages14SIMULA 67•Can create class instances (= objects) subordinate to block (AR) in which created•All objects are “attached” to some AR during execution•When suspended, AR is “detached”•class p(...);declarations;begin ... end p;Defines class template with formal parameters•e :- new p(...);Creates an object (AR) of class p: [ref(p) e;]Transmits argumentsCommences execution in AR of eAR e is “attached” to the suspended (creating) ARC SC 520 Principles of Programming Languages15SIMULA 67 (cont.)•detach;Suspend current activationResume in AR to which current is “attached”Current AR marked “detached”Approximately a “return”end  detach (blocks detach when exited)•call(e)If e is detached, mark AR e as “attached” to caller (current AR)Suspend caller (current AR)Resume in AR e•resume(e)If e is detached, suspend current AR and resume execution in AR ee is “attached” to AR to which current AR is “attached”—resume passes its attachment to eC SC 520 Principles of Programming Languages16SIMULA 67 (cont.) cfe efee:-new Classdetachf:-new Classdetachcall(e)resume(f)detachcall(e)detachsame AR esame AR ftimeC SC 520 Principles of Programming Languages17SIMULA 67 Primitives•Let c = currently executing AR•e :- new p( . . . );e = allocate(p){ transmit parameters (CBN, CBV in Simula67)}e.cont = entrypoint(p)e.attached = c // attacher of e is c{ using c.sl and snl(p) and c’s snl, calculate AR in which p was defined (created) & put ptr into t}c.sl = tc.cont = resumepointc.attached = nilc = egoto c.contresumepoint:C SC 520 Principles of Programming Languages18SIMULA67 Primitives (cont.)•detach; c.cont = resumepointif c.attached == nil then error()else { t = c.attachedc.attached = nilc = t // back to attachergoto c.cont}resumepoint:C SC 520 Principles of Programming Languages19SIMULA67 Primitives (cont.)•call(e) —no parametersif e.attached != nil then error()e.attached = c // e attached to callerc.cont = resumepointc.attached = nilc = egoto c.contresumepoint:•resume(e)if e.attached != nil then error()e.attached =