Programming Languages Design Specification and Implementation G22 2210 001 Rob Strom December 12 2006 Administrative Fill out evaluations return them to Room 405 All written assignments due by noon today December 12 so that the TA s can grade them Anyone who cannot do this MUST see me personally I will be here after class today The final will be held on Thursday December 21st in this room Open book any kinds of written or printed notes or books permitted No computers The final exam Not deep Not like programming projects Anyone who understands all the classes and could do homework problems should get full marks Short answers What does this program output Where is the bug in this program Fix it What is the most general type of this ML function Why is this feature useful Why is it potentially dangerous How would you implement this feature Short discussions sentence paragraph Things expected to be understood for the final Languages in General What benefits do high level languages achieve What are the criteria for good decomposition into modules What is the difference between an implementation and a specification What is performance transparency How is it good and how is it bad What are name spaces What are defining and applied occurrences of names Illustrate in various languages Be able to say for a given language what the scope of a name in it is Language Syntax Context free grammars and BNF what are Tokens Terminals Non terminals Syntax rules Syntax tree strings that are the units of parsing identifiers literals punctuation nodes in the grammar that accept tokens and do not reduce nodes in the grammar that have rules reducing them to other nodes definitions of how a grammar node reduces to another node the result of parsing grouping all nodes according to the reductions specified in the syntax rules Given a BNF grammar and a string be able to say whether the language accepts the string and if it does what the concrete syntax tree is Be able to define an abstract representation of a program By eliminating redundant productions and unnecessary punctuation There exists a string of tokens that has more than one legal syntax tree What does it mean for a context free grammar to be ambiguous Regular expressions Be able to state whether a RE accepts a string or how to modify an RE to accept a certain kind of string The first high level programming language What are Fortran s types Fixed float arrays Variables are local to the procedure common blocks and subprogram names are global all are static What are Fortran s name spaces How does Fortran pass arguments By reference Every program either a raises a compile time error b compiles and produces an answer c compiles and produces a valid run time error Fortran is not secure because of lack of bounds checking and lack of type checking of COMMON and EQUIVALENCE that can cause storing values of the wrong type and because by reference passing can cause bizarre behavior such as overriding of constants What is a secure language In what ways is Fortran not secure What can interfere with invariant checking in Fortran How are later languages better worse GOTO statements make it hard to judge all the ways control can reach a program point If I call a procedure it might not only update my parameters by also any COMMON storage If I am a called procedure it is possible that two of my parameters are aliased Later languages have structured control flow declare constants in out inout But they have pointers multi tasking inner procedures which add additional opportunities for killing invariants Imperative languages What are Stack Heap Static Storage Static Bounds Dynamic Bounds What s the difference between an array with dynamic bounds and a varying sized array What s the difference between a local variable and an own variable Understand the difference between Call by reference Call by value Call by name Call by value result Imperative languages continued What are invariants How can I analyze a program at compile time to check them What are global variables Why are they harmful What s a better way What are nested blocks and procedures What benefit do they provide What risks do they have What does it mean for a language to support first class procedure variables What is an example of how it s useful What is a closure What is an example of using a closure Under what circumstances can using a closure in some imperative languages be unsafe How is it represented at runtime What s the difference between C struct types and higher level language records Give examples of each What are exceptions Why are they preferable to using return codes Applicative languages Lisp Scheme ML What is an applicative language What is a lambda expression What is the difference between dynamic versus lexical scope of binding Be able to understand a case where the two would lead to different results Why did the applicative community realize that lexical scoping was better What are let and letrec Be able to translate a simple imperative program to applicative Use lambda binding instead of assignments Use recursion instead of loops Be able to translate a simple program into continuation passing style e g to avoid multiple outputs or to avoid exceptions Be able to simulate objects with closures Be able to manipulate LISP s favorite data type the list Understand functionals like mapcar What are macros and what is referential transparency What is eval When might you want to use it What is garbage collection Why is it needed How is it done Why does it work in Lisp or Scheme Why does it work in Java Why can t it work in C What is parametric polymorphism Let polymorphism Be able to give the most general type for an ML function Simulating objects with closures Simple case an object with one method define make closure lambda lst letrec l lst getBefore lambda l e cond eq cadr l e car l getBefore cdr l e lambda e getBefore l e Fancier case use continuations define make closure lambda lst cont letrec l lst getBefore lambda l e cond eq cadr l e car l getBefore cdr l e theMethod lambda e getBefore l e anotherMethod lambda cont theMethod anotherMethod Homework ML Type Inference Problem Type inference fun zip f nil nil nil zip f h t i s f h i zip f t s What does it do Takes two lists of equal length and applies a binary operator to the corresponding members of each to produce a new list What is its most general type a b c a list b list c list Consider these two expressions 1 fun f g g 7 g false 2 let