Big Ideas for CS 251Theory of Programming Languages

Principles of Programming Languages

CS251ProgrammingLanguagesFall2016,LynTurbakDepartmentofComputerScienceWellesleyCollege

Discussion: Programming Languages

•  What PLs have you used? •  Which PLs/PL features do you like/dislike. Why?

Your experience:

•  What is a PL? •  Why are new PLs created?

–  What are they used for? –  Why are there so many?

•  Why are certain PLs popular? •  What goes into the design of a PL?

More generally:

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PLismypassion!•  First PL project in 1982 as intern

at Xerox PARC

•  Created visual PL for 1986 MIT masters thesis

•  1994 MIT PhD on PL feature (synchronized lazy aggregates)

•  1996 – 2006: worked on types as member of Church project

•  1988 – 2008: Design Concepts in Programming Languages

•  2011 – current: lead TinkerBlocks research team at Wellesley

•  2012 – current: member of App Inventor development team

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GeneralPurposePLs

Python Fortran

C/C++

Java

RacketML

Haskell

CommonLisp

Perl

Ruby

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JavaScript

DomainSpecificPLs

IDL

CSS

PostScript�

HTML

OpenGL

LaTeX

Excel

Matlab R

Swift 1-5

ProgrammingLanguages:MechanicalView

Acomputerisamachine.OuraimistomakethemachineperformsomespecifiedacEons.WithsomemachineswemightexpressourintenEonsbydepressingkeys,pushingbuIons,rotaEngknobs,etc.Foracomputer,weconstructasequenceofinstrucEons(thisisa``program'')andpresentthissequencetothemachine.–LaurenceAtkinson,PascalProgramming 1-6

ProgrammingLanguages:LinguisEcView

Acomputerlanguage…isanovelformalmediumforexpressingideasaboutmethodology,notjustawaytogetacomputertoperformoperaEons.ProgramsarewriIenforpeopletoread,andonlyincidentallyformachinestoexecute.–HaroldAbelsonandGeraldJ.Sussman

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“Religious”ViewsTheuseofCOBOLcripplesthemind;itsteachingshould,therefore,beregardedasacriminaloffense.–EdsgerDijkstra

ItispracEcallyimpossibletoteachgoodprogrammingtostudentsthathavehadapriorexposuretoBASIC:aspotenEalprogrammerstheyarementallymuElatedbeyondhopeofregeneraEon.–EdsgerDijstra

You'reintroducingyourstudentstoprogramminginC?Youmightaswellgivethemafrontallobotomy!–Acolleagueofmine

ALISPprogrammerknowsthevalueofeverything,butthecostofnothing.-AlanPerlis

Ihavenevermetastudentwhocuttheirteethinanyoftheselanguagesanddidnotcomeawayprofoundlydamagedandunabletocope.ImeanthisreadstomeverysimilarlytoteachingsomeonetobeacarpenterbystarEngthemoffwithplasEctoytoolsandtellingthemtogosculptsandonthebeach.-AlfredThompson,onblockslanguages

Alanguagethatdoesn'taffectthewayyouthinkaboutprogramming,isnotworthknowing.-AlanPerlis 1-8

ProgrammingLanguageEssenEals

PrimiEves

MeansofCombinaEon

MeansofAbstracEon

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Thinkofthelanguagesyouknow.WhatmeansofabstracEondotheyhave?

PL Parts Syntax: form of a PL

•  What a P in a given L look like as symbols? •  Concrete syntax vs abstract syntax trees (ASTs)

Semantics: meaning of a PL •  Static Semantics: What can we tell about P before running it?

–  Scope rules: to which declaration does a variable reference refer?

–  Type rules: which programs are well-typed (and therefore legal)?

•  Dynamic Semantics: What is the behavior of P? What actions does it perform? What values does it produce? –  Evaluation rules: what is the result or effect of evaluating each language

fragment and how are these composed?

Pragmatics: implementation of a PL (and PL environment)

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•  How can we evaluate programs in the language on a computer? •  How can we optimize the performance of program execution?

Syntax (Form) vs. Semantics (Meaning)in Natural Language

Furiously sleep ideas green colorless.

Colorless green ideas sleep furiously.

Little white rabbits sleep soundly.

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ConcreteSyntax:AbsoluteValueFuncEonLogo: to abs :n ifelse :n < 0 [output (0 - :n)] [output :n] end

Javascript: function abs (n) {if (n < 0) return -n; else return n;}

Java: public static int abs (int n) {if (n < 0) return -n; else return n;}

Python: App Inventor: def abs(n): if n < 0: return -n else: return n

Scheme: (define abs (lambda (n) (if (< n 0) (- n) n)))

PostScript: /abs {dup 0 lt {0 swap sub} if} def

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AbstractSyntaxTree(AST):AbsoluteValueFuncEon

varref

return

n

return

intlit

0

relaEonalOperaEon

varref

n

condiEonalStatement

funcEonDeclaraEon

absn

testthen

bodyparamsfuncDo

nName

rand1

name

name

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arithmeEcOperaEon

value

subtract

varref

nname

value

intlit

0

lessThan

valuerand1

ThisASTabstractsovertheconcretesyntaxfortheLogo,JavaScript,andPythondefiniEons.TheotherdefiniEonswouldhavedifferentASTs.

SemanEcsExample1

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Whatisthemeaningofthefollowingexpression?

(1 + 11) * 10

Somepossibleanswers:•  120(regularintepretaEonofnumbers,operators)•  1000(binarynumbers,regularoperators)•  0(“+”means“minus”,“*”means“plus”)•  13(numberofcharactersinstring)•  5(numberofnodesinAST)•  3(numberofleavesinAST)

SemanEcsExample2

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Whatisprintedbythefollowingprogram?

a = 1;

b = a + 20;

print(b);

a = 300

print(b);

count = 0;

fun inc() { count = count + 1; return count; }

fun dbl(ignore, x) { return x + x; }

print(dbl(inc(), inc())

21214

21212

213203

213202

Herearesomepossibleanswers.WhatexecuEonmodelsgiverisetotheseanswers?

SemanEcsExample3

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Supposeaisanarray(orlist)containingthethreeintegervalues10,20,and30inthefollowinglanguages.Whatisthemeaningofthefollowingexpressions/statementsinvariouslanguages(thesyntaxmightdifferfromwhat’sshown).

a[1] a[3] a[2] = "foo" a[3] = 17

Java

C

Python

JavaScript

Pascal

AppInventor

SemanEcsExample3(Answers)

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Supposeaisanarray(orlist)containingthethreeintegervalues10,20,and30inthefollowinglanguages.Whatisthemeaningofthefollowingexpressions/statementsinvariouslanguages(thesyntaxmightdifferfromwhat’sshown).

a[1] a[3] a[2] = "foo" a[3] = 17

Java 20 dynamicindexoutofboundserror

staEctypeerror dynamicindexoutofboundserror

C 20 returnsvalueinmemoryslotalera[2]

staEctypeerror

Stores17inmemoryslotalera[2]

Python 20 dynamiclistindexoutofrangeerror

stores“foo”inthirdslotofa

dynamiclistindexoutofrangeerror

JavaScript 20 “undefined”value stores“foo”inthirdslotofa

Stores17ina[3]

Pascal 20 staEcindexoutofboundserror

staEctypeerror

staEcindexoutofboundserror

AppInventor 10 30 stores“foo”insecondslotofa

Stores17inthirdslotofa

Pragmatics: Raffle App In App Inventor

DesignerWindow BlocksEditor

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Toentertheraffle,textmenowwithanemptymessage:339-225-0287

hIp://ai2.appinventor.mit.edu

HowhardisthistodoinmoretradiEonaldevelopmentenvironmentsforAndroid/iOS?

PL Dimensions

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PLsdifferbasedondecisionslanguagedesignersmakeinmanydimensions.E.g.:

•  First-classvalues:whatvaluescanbenamed,passedasargumentstofuncEons,returnedasvaluesfromfuncEons,storedindatastructures.Whichofthesearefirst-classinyourfavoritePL:arrays,funcEons,variables?

•  Naming:Dovariables/parametersnameexpressions,thevaluesresulEngfromevaluaEngexpressions,ormutableslotsholdingthevaluesfromevaluaEngexpressions?Howarenamesdeclaredandreferenced?Whatdeterminestheirscope?

•  State:Whatismutableandimmutable;i.e.,whatenEEesinthelanguage(variables,datastructures,objects)canchangeoverEme.

•  Control:Whatconstructsarethereforcontrolflowinthelanguage,e.g.condiEonals,loops,non-localexits,excepEonhandling,conEnuaEons?

•  Data:Whatkindsofdatastructuresaresupportedinthelanguage,includingproducts(arrays,tuples,records,dicEonaries),sums(opEons,oneofs,variants),sum-of-products,andobjects.

•  Types:AreprogramsstaEcallyordynamicallytyped?Whattypesareexpressible?

ProgrammingParadigms

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•  Impera:ve(e.g.C,Python):ComputaEonisstep-by-stepexecuEononastatefulabstractmachineinvolvingmemoryslotsandmutabledatastructures.

•  Func:onal,func:on-oriented(e.gRacket,ML,Haskell):ComputaEonisexpressedbycomposingfuncEonsthatmanipulateimmutabledata.

•  Object-oriented(e.g.Simula,Smalltalk,Java):ComputaEonisexpressedintermsofstatefulobjectsthatcommunicatebypassingmessagestooneanother.

•  Logic-oriented(e.g.Prolog):ComputaEonisexpressedintermsofdeclaraEverelaEonships.

Note:InpracEce,mostPLsinvolvemulEpleparadigms.E.g.

•  PythonsupportsfuncEonalfeatures(map,filter,listcomprehensions)andobjects

•  RacketandMLhaveimperaEvefeatures.

quicksort :: Ord a => [a] -> [a]quicksort [] = []quicksort (p:xs) =

(quicksort lesser) ++ [p] ++ (quicksort greater) where lesser = filter (< p) xs greater = filter (>= p) xs

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ParadigmExample:Quicksortvoid qsort(int a[], int lo, int hi) { int h, l, p, t;

if (lo < hi) {

l = lo; h = hi; p = a[hi];

do { while ((l < h) && (a[l] <= p)) l = l+1;

while ((h > l) && (a[h] >= p)) h = h-1; if (l < h) {

t = a[l]; a[l] = a[h]; a[h] = t; }

} while (l < h);

a[hi] = a[l]; a[l] = p;

qsort( a, lo, l-1 ); qsort( a, l+1, hi );

}}

ImperaEveStyle(inC;Javawouldbesimilar)

FuncEonalStyle(inHaskell)

Pragmatics: Metaprogramming

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PLsareimplementedintermsofmetaprogams=programsthatmanipulateotherprograms.

Thismaysoundweird,butprogramsarejusttrees(ASTs),soametaprogramisjustaprogramthatmanipulatestrees(thinkamorecomplexversionofCS230binarytreeprograms).

ImplementaEonstrategies:•  Interpreta:on:interpretaprogramPinasourcelanguageSintermsofan

implementaEonlanguageI.

•  Transla:on(compila:on):translateaprogramPinasourcelanguageStoaprogramP’inatargetlanguageTusingatranslatorwriIeninimplementaEonlanguageI.

•  Embedding:expressprogramPinsourcelanguageSintermsofdatastructuresandfuncEonsinimplementaEonlanguageI.

Metaprogramming:InterpretaEon

InterpreterforlanguageLonmachineM

MachineMPrograminlanguageL

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Metaprogramming:TranslaEon

InterpreterforlanguageBonmachineM

MachineM

PrograminlanguageA AtoBtranslator

PrograminlanguageB

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Metaprogramming:Embedding

InterpreterforlanguageBonmachineM

MachineMPrograminlanguageAembeddedinlanguageB

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Metaprogramming:BootstrappingPuzzles

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HowcanwewriteaJava-to-x86compilerinJava?

We’lllearnhowtounderstandsuchpuzzles!

Metaprogramming:ProgrammingLanguageLayers

kernel

syntacEcsugar

primiEvevalues/datatypes

systemlibraries

userlibraries

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Why?Who?When?Where?DesignandApplicaEon

•  Historical context •  Motivating applications

–  Lisp: symbolic computation, logic, AI, experimental programming –  ML: theorem-proving, case analysis, type system –  C: Unix operating system –  Simula: simulation of physical phenomena, operations, objects –  Smalltalk: communicating objects, user-programmer,

pervasiveness •  Design goals, implementation constraints

–  performance, productivity, reliability, modularity, abstraction, extensibility, strong guarantees, …

•  Well-suited to what sorts of problems? 1-28

WhystudyPL?•  CrossroadsofCS

•  Approachproblemsasalanguagedesigner.

–  "Agoodprogramminglanguageisaconceptualuniverseforthinkingaboutprogramming"--AlanPerlis

–  Evaluate,compare,andchooselanguages

–  BecomebeIeratlearningnewlanguages

–  becomeabeIerproblem-solver

–  viewAPIdesignaslanguagedesign•  Ask:

–  WhyarePLsarethewaytheyare?

–  Howcouldthey(orcouldn'tthey)bebeIer?–  Whatisthecost-conveniencetrade-offforfeatureX?

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Administrivia

•  Schedule, psets, quizzes, lateness policy, etc.:see http://cs.wellesley.edu/~cs251/

•  PS1 is available; due next Friday •  office hours poll •  visit me in office hours next week! •  Install Dr. Racket for next time

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