Midterm Exam Announcements

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1 Miderm Exam Noe: This was a challenging exam. CSCI 4: Principles o Programming Languages Lecure 1: Excepions Insrucor: Dan Barowy Miderm Exam Scores needs improvemen { no so good (come see me) good (seriously!) grea exceedingly grea Miderm Exam Announcemens Miderm exam grades are no necessarily a relecion o your inal grade; homework is more imporan! HW5 soluions I you are worried, come see me!

2 Announcemens Announcemens Typo on HW6: i you wan a new parner, noiy HW6 ou oday, due nex Wednesday, April 11. me (via ) by Wed, April 4 wih your parner s name Announcemens Grades or HW programming porion, HW4, HW5 will be back soon. Reresher: Firs-class uncions A language wih irs-class uncions reas uncions no dierenly han any oher value: You can assign uncions o variables: val = n x => x + 1 You can pass uncions as argumens: un g h = h You can reurn uncions: un k x = n () => x + Firs-class uncion suppor complicaes implemenaion o lexical scope.

3 Firs Class Funcions To implemen suppor or irs class uncions, we need wo addiional daa srucures: Access links Closures The implemenaion diiculy o mainaining lexical scope or irs class uncion is called he unarg problem. Access link An access link is a poiner rom he curren acivaion record o he acivaion record o he closes lexical scope. In oher words, he access link in he acivaion rame or a uncion poins o where was deined. Why do we need access links? So ha he language can deermine he values o ree variables in a uncion. Closure Example A closure is a uple ha represens a uncion value. One uple value poins o a uncion s code and he oher value poins o he acivaion record o he poin o deiniion o he uncion (i.e., closes lexical scope). un y = x * y un g h = le val x = 7 in (h ) + x

4 Desugared Example le in le = n y => x * 4 in le n h => le val x = 7 in (h ) + x in end end end Blocks Deine Acivaion Records un y = x * y un g h = le val x = 7 in (h ) + x Blocks Deine Acivaion Records un y = x * y un g h = le val x = 7 in (h ) + x Blocks Deine Acivaion Records un y = x * y un g h = le val x = 7 in (h ) + x = =

5 Blocks Deine Acivaion Records un y = x * y un g h = le val x = 7 in (h ) + x Blocks Deine Acivaion Records un y = x * y un g h = le val x = 7 in (h ) + x h = x = 7 g g = = h Blocks Deine Acivaion Records un y = x * y un g h = le val x = 7 in (h ) + x y = x = AL.x h = x = 7 Blocks Deine Acivaion Records un y = x * y un g h = le val x = 7 in (h ) + x y = x = AL.x h x =? h = x = 7 g g = =

6 Blocks Deine Acivaion Records un y = x * y un g h = le val x = 7 in (h ) + x y = x = AL.x h h = x = 7 = g Acivaion Records in Funcional Langs le val le val x = 1 un () = x + 1 in end in g() end How is his uncion evaluaed? Do we have a problem when we call g()? Upward unargs le val le val x = 1 un () = x + 1 in end in g() end 1. Push le block or g ono call sack. We don ye know g s value. Upward unargs le val le val x = 1 un () = x + 1 in end in g() end 1. Push le block or g ono call sack. We don ye know g s value. 2. Push le block or x and. x = 1 = CL =

7 Upward unargs le val le val x = 1 un () = x + 1 in end in g() end 1. Push le block or g ono call sack. We don ye know g s value. 2. Push le block or x and.. Reurn. We have a problem! x = 1 = CL = Upward unargs le val le val x = 1 un () = x + 1 in end in g() end 1. Push le block or g ono call sack. We don ye know g s value. 2. Push le block or x and.. Reurn. We have a problem! 4. The ix: delay deallocaing record unil we are done using i. Insead o using sack, jus heap allocae rames and use garbage collecor!??? heap-allocaed records x = 1 = CL = g() Upward unargs CL = le val le val x = 1 un () = x + 1 in end in g() end 1. Push le block or g ono call sack. We don ye know g s value. 2. Push le block or x and.. Reurn. We have a problem! 4. The ix: delay deallocaing record unil we are done using i. Insead o using sack, jus heap allocae rames and use garbage collecor! Saey SML is a sae language. Wha does ha mean? I means ha execuion behavior is deermined solely by he program, no: a. he implemenaion o he language, or b. he design o he hardware heap-allocaed records 5. Now we can call g() and i will work correcly.

8 Saey How is saey achieved? Type checking rules ou maniesly incorrec consrucs. hello - world However, ype checking canno rule ou all errors. un sum (xs: in lis) = oldl (n (x,acc) => x + acc) 0 xs un mean (xs: in lis) = (sum xs) div (Lis.lengh xs) For hese kinds o errors, we use excepions. Excepions In ML (and in Java), excepions have hree pars: a. Excepion declaraion: excepion MyExcepion o sring b. Excepion use: raise MyExcepion Don send me back o school! c. Excepion handling: handle MyExcepion ms> msg ^? Fine. Here s your uiion bill. Pay i yoursel. Excepions A real example More generally a. Excepion declaraion: excepion <excepion name> [o <ype>] b. Excepion use: raise <excepion name> [expr] c.excepion handling: handle <paern> un sum (xs: in lis) = oldl (n (x,acc) => x + acc) 0 xs un mean (xs: in lis) = (sum xs) div (Lis.lengh xs) - mean [] handle Div => 0; val i = 0 : in

9 A real example excepion ZeroLengh un sum (xs: in lis) = oldl (n (x,acc) => x + acc) 0 xs un mean (xs: in lis) = i Lis.lengh xs = 0 hen raise ZeroLengh else (sum xs) div (Lis.lengh xs) Excepions aren jus or errors Excepions are acually a special orm o goo. You can use hem o reurn daa o any callinuncion on he sack. - mean [] handle Div => 0 ZeroLengh => 1 (* or un *) val i = 1 : in Excepions or eiciency daaype ree = Lea o in Node o ree * ree un (Lea x) = x (Node(x,y)) = x * y val = Node(Node(Lea 1, Lea 2), Lea ) Excepions or eiciency Wha i val = Node(Node(Lea 0, Lea 2), Lea ) - ; val i = 0 : in Somewha ineicien, isn i? - ; val i = 6 : in

10 Excepions or eiciency excepion Zero un (Lea x) = i x = 0 hen raise Zero else x (Node(x,y)) = x * y val = Node(Node(Lea 0, Lea 2), Lea ) - handle Zero => 0; val i = 0 : in Excepions are dynamically scoped Remember: variable bindings are saically (lexically) scoped. Excepions are dynamically scoped. un (Lea x) = i x = 0 hen raise Zero else x (Node(x,y)) = x * y Remember ha I said raise is like goo? Where would his raise go o? We haven even used ye! Excepions are dynamically scoped val = Node(Node(Lea 2, Lea 0), Lea ) handle Zero => 0; Excepions are dynamically scoped val = Node(Node(Lea 2, Lea 0), Lea ) handle Zero => 0;

11 Excepions are dynamically scoped val = Node(Node(Lea 2, Lea 0), Lea ) handle Zero => 0; Excepions are dynamically scoped val = Node(Node(Lea 2, Lea 0), Lea ) handle Zero => 0; 2 * Excepions are dynamically scoped val = Node(Node(Lea 2, Lea 0), Lea ) handle Zero => 0; Pop ( unwind ) he sack unil handler is ound. Excepions are dynamically scoped val = Node(Node(Lea 2, Lea 0), Lea ) handle Zero => 0; Pop ( unwind ) he sack unil handler is ound. raise Zero 2 * 2 * handler is here

12 Excepions are dynamically scoped val = Node(Node(Lea 2, Lea 0), Lea ) handle Zero => 0; Pop ( unwind ) he sack unil handler is ound. Excepions are dynamically scoped val = Node(Node(Lea 2, Lea 0), Lea ) handle Zero => 0; Pop ( unwind ) he sack unil handler is ound. handler is here handler is here Excepions are dynamically scoped val = Node(Node(Lea 2, Lea 0), Lea ) handle Zero => 0; Pop ( unwind ) he sack unil handler is ound. Aciviy Wha is he value o he ollowing expression? excepion X (le un (y) = raise X and g(h) = h(1) handle X => 2 in g() handle X => 4 end) handle X => 6 handler is here now handle excepion