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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