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Programming Languages: Design, Specification, and Implementation

Programming Languages: Design, Specification, and Implementation. G22.2210-001 Rob Strom September 21, 2006. Administrative. Alternative mailing address for me: robstrom@us.ibm.com Everyone should subscribe to the class mailing list: http://www.cs.nyu.edu/mailman/listinfo/g22_2110_001_fa06

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Programming Languages: Design, Specification, and Implementation

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  1. Programming Languages:Design, Specification, and Implementation G22.2210-001 Rob Strom September 21, 2006

  2. Administrative • Alternative mailing address for me: robstrom@us.ibm.com • Everyone should subscribe to the class mailing list: http://www.cs.nyu.edu/mailman/listinfo/g22_2110_001_fa06 • Readings: Remainder of chapter 3 of Gelernter and Jagannathan; chapter 7; MIT Scheme documentation • Gelernter textbook: we will notify about availability of photocopied notes • Homework due end of class 4 (Scheme): • Convert list of atoms to concrete tree according to the simple grammar • Convert concrete tree to abstract tree

  3. Invariants 1 <= N BigA = max(A(1..1))  1 <= N BIGA = A(1) 5 DO 20 I = 2, N 30 IF (BIGA-A(I)) 10,21,21 10 BIGA = A(I) 21 CONTINUE 20 CONTINUE PRINT … BigA = max(A(1..I-1))  I-1 <= N BigA = max(A(1..I-1))  I <= N BigA = max(A(1..I-1))  A(I) > BigA  I <= N A(I) = max(1..I))  I <= N BigA = max(1..I))  I <= N BigA = max(A(1..I-1))  BigA >= A(I)  I <= N BigA = max(1..I))  I <= N BigA = max(1..I-1))  I-1 <= N BigA = max(A(1..I-1))  I-1 <= N  I > N BigA = max(A(1..N))

  4. FORTRAN: Everything static • Dimensions of arrays (although not necessarily visible to subroutines) • Number of instances of storage blocks • All programs both static and global (except statement functions) • All I/O devices static and global

  5. Introduces: Types with operator overloading Arrays Separate compilation 2 levels of name space: Global and Local Static name spaces Procedures and Functions: call by reference Specialized I/O Pragma (Frequency Statement) Differences w.r.t. modern languages: No Lexical level (tokens) No 1st class char/string No 1st class procedure No recursion No dynamic data No nesting No closures/objects Gotchas: Near unrestricted GOTO Name aliasing No checks for Type errors Overwriting constants Overwriting array bounds FORTRAN: Summary

  6. Algol 60: Static and Runtime Naming; Scope PROCEDURE MAIN; … x := read(); BEGIN INTEGER ARRAY FOO[1:X]; … j := 20; blat(j, FOO(j)); … PROCEDURE blat(x,y); BEGIN x := 1000; y:= 1000 END … INTEGER PROCEDURE random; BEGIN OWN INTEGER seed; random := seed := some new value … END …

  7. New features • Call by name • Dynamic array bounds • Recursive creation of local stack variables • Own variables • Nested declarations, scopes • Inner procedures

  8. Call by name • Requires access to the caller’s environment from within the environment where the name is used • Equivalent to a “closure” with no parameters

  9. Beyond: Algol 68; PL/I; C • Fully dynamic storage – e.g. PL/I’s storage classes: • STATIC – like FORTRAN (local & external) • AUTOMATIC – like Algol 60 non-own • CONTROLLED – dynamically allocated • X BASED(Y) – dynamically allocated with pointer • Exception checking and Exception Handlers

  10. Memory Heap (PL/I) • DCL 1 ARecordTemplate BASED(P), • 2 AField FIXED, • 2 AnotherField FLOAT; • DCL Q Pointer; /* Untyped  */ • ALLOCATE ARecordTemplate; • Q=P; • Q->AField = 2;

  11. A: PROCEDURE (…) DCL X FIXED INIT(3); … AddX: PROCEDURE (Y) RETURNS(FIXED); DCL Y FIXED; RETURN(X+Y); END AddX; Z = AddX(5); CALL AFunc(AddX); … AFunc: PROCEDURE(F); DCL F ENTRY; DCL X FIXED INIT(4); DCL Z FIXED; … Z = F(5); … END AFunc; Closures (PL/I):

  12. Closures • AddX is a closure, that is: • It is a function whose execution has access to an external environment (the variable X) • If invoked from AFunc, it uses: • AddX’s environment’s value of X (static binding) • Not the most recent value of X on the stack, the one defined inside AFunc (dynamic binding) • LISP was the earliest language to do closures, and it did them the other way (dynamic)

  13. New things that can go wrong • Memory fills with unaccessible cells • Type-mismatches on references and entry variables • Dangling references via pointers • Dangling closures • For an amusing critique of C vs PL/I see: • http://www.uni-muenster.de/ZIV/Mitarbeiter/EberhardSturm/PL1andC.html

  14. Scheme (define isort ( lambda (l) (letrec ( ; defines a list of bindings (here just 1) (insert ( ; inserts item x in sorted order to list l lambda (x l) (if (null? l) (list x) (if (<= x (car l)) (cons x l) (cons (car l) (insert x (cdr l)))) )))) ; the below is executed in the context of the bindings (if (null? l) nil (insert (car l) (isort (cdr l)))) ) ))

  15. Essential features in Scheme • 1st class procedures • Dynamically created procedures • Based on lambda calculus over atoms and pairs; by convention, lists are pairs <head, rest> • Continuations • Automatic garbage collection • Applicative style: binding, no update, no side effects (but there are exceptions to this) • Static scoping, but no static typing! • Simple syntax • (afunction arg1 arg2 …) ; function application • Special forms, e.g. (if … )

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