FUNCTIONAL AND LOGIC PROGRAMS

Transcription

1 FUNCTIONAL AND LOGIC PROGRAMS

2 Contents Language Specific Compilation Context Handling Identification Scope Overloading Imported Scope Type Checking Type table Type equivalence Coercions Casts and conversions Object Oriented Language Issues Routines and Activation Code Generation and Control Flow

3 Contents Funtional Programming Introduction Basic Compilation Polymorphic type checking Compiling to register oriented architectures JavaCC

4 Language Specific Compilation Concept of compiler remains similar for any languages however its implementations differ from paradigm to paradigm(or language to language) This happens due to the syntactic and semantic differences. The main difference however in any paradigm lie in code generation : OO and structured oriented languages generate code at assembler or low level, whereas many compilers for functional, parallel and distributed languages generate code in C or C++.

5 Concerned with type checking it relates the type of a variable in a declaration to its use means in short semantic analysis. Context Handling It is also related to identification

6 Context Handling Identification Type Checking

7 Identification The process of finding the defining occurrence of a given applied occurrence of an identifier or operator Defining occurrence of the identifier is the place where it is declared. This gives us the information about the identifier : its initial value, whether it is a constant or a variable, a module or class etc. Applied occurrence of an identifier are the i.e use consumers of the information. Some languages like LISP or Prolog have no declaration of identifiers i.e no defining occurences. The identification process stores all the occurences of an identifier in the symbol table.

8 Identification int month[12];..1 month = 1; 2 do { cout<<month_name[month];.3 } while (month<=12); 4 In above code, 1 is defining occurrence 2,3,4 are applied occurrence of identifier month.

9 Identification In some languages like C, forward declaration is provided, which means having more than one defining occurrences. int x; int get( ) } { return x; int x = 5; main() { int x = 3;..1 cout<< Value of x = << get() ; } In above code, 1,3,4 are defining occurrence 2 is applied occurrence of identifier x. This is forward declaration.

10 Identification The identifiers can be searched in the symbol table depending upon their namespaces General namespace is the one in which identifiers like variable names, structure names, funtion names are stored Special namespace is the one for field selectors like struct members, class members etc

11 Identification struct one_int { } i;.... int i; i.i = 3;.. The first i before dot is searched in general namespace whereas second in special namespace.

12 Identification Labels, Module names can also live in special namespaces Namespaces depends upon the syntax of various languages In C there are three namespaces : One for enums, structs and unions, Second for Labels and the last containing variable names, function names, type names

13 Identification.Scope Some namespaces are scope- structured Scopes are arranged in the form of stack. For each scope there is an entry in the stack. Rules for creating stack of scopes A new Scope is pushed into the stack. Declared identifiers are entered in the top scope element For each applied occurrence, search scope elements from top to bottom Upon scope exit, remove the top scope element and all its declarations.

14 A scoped hash-based symbol table Name of identifier Pointer to declaration Other Information Level at which identifier is present Properti es of the identifie r Pointer to the next node

15 A scoped hash-based symbol table aap( int noot) { int mies, aap;... } noot decl 1 prop scope stack hash table bucket 0 bucket 1 bucket 2 aap decl 2 prop 0 prop bucket 3 mies level decl 2 prop

16 Identification.Overloading The ambiguity caused by overloading is resolved by considering the context in which the name to be identified occurs The overloaded identifiers come up with a set of definitions. These definitions are selected from the list of pointer to declaration field

17 Identification.Imported Scopes C++ scope resolution operator x:: Modula FROM module IMPORT... Solution: stack (or merge) the new scope

18 Type checking Operators and functions impose restrictions on the types of the arguments Types basic types structured types type names Some languages support forward references i.e referring to an identifier that is not yet declared Type information in a compiler must be implemented in such a way that all these checks must be performed conveniently

19 Forward Referencing Example Class A; Class B { A a; //forward reference.. } Class A {.. }

20 Type checking To resolve forward references, whenever it is met, it is added to the symbol table, marking as forward reference When type declaration for this forward reference is met, its symbol table entry is modified to represent the actual type instead of forward reference A check must be added for loose ends i.e any forward references not modified A check must also be there for circularity i.e. TYPE x = y; TYPE y = x;

21 Type checking... Type table All Type information for each type is stored in a type table The entry contains following : Its type constructor ( basic, record, array, pointer, and others); The size and alignment requirement of a variable of the type The types of components Various information is being recorded for types: For basic types : its precise type(integer, real etc.) For record type : The list of record fields, their names and types For Array : the number of dimension, index type(s) and element type For pointer : the referenced type

22 Type table Example TYPE a = b; TYPE b = Pointer to a; TYPE c = d; TYPE d = c;

23 Type table Example For, TYPE a = b; TYPE TABLE SYMBOL TABLE TYPE 0 : INTEGER integer : TYPE 0 TYPE 1 : ID_REF b a : TYPE 1 b : UNDEFINED TYPE

24 Type table Example TYPE a = b; TYPE b = Pointer to a; TYPE TABLE SYMBOL TABLE TYPE 0 : INTEGER integer : TYPE 0 TYPE 1 : ID_REF b a : TYPE 1 TYPE 2 : ID_REF a b : TYPE 3 TYPE 3 : Pointer to TYPE 2

25 Type table Example Finally we have, TYPE TABLE SYMBOL TABLE TYPE 0 : INTEGER integer : TYPE 0 TYPE 1 : ID_REF b a : TYPE 1 TYPE 2 : ID_REF a b : TYPE 3 TYPE 3 : Pointer to TYPE 2 c : TYPE4 TYPE 4 : ID_REF d d : TYPE 5 TYPE 5 : ID_REF c

26 Type table Example Finally we have, TYPE TABLE TYPE 0 : INTEGER TYPE 1 : TYPE 3 TYPE 2 : TYPE 1 TYPE 3 : Pointer to TYPE 2 TYPE 4 : TYPE 5 TYPE 5 : TYPE 4

27 Type checking... Type Equivalence Always while performing type checking of an expression or the formal and actual parameters ofa routine call check the equality of two types. When two types are equivalent value of these types are usually having same representations name equivalence [all types get a unique name] VAR a : ARRAY [Integer 1..10] OF Real; VAR b : ARRAY [Integer 1..10] OF Real;

28 Type checking... Type Equivalence structural equivalence [difficult to check] TYPE c = RECORD i : Integer; p : POINTER TO c; END RECORD; TYPE d = RECORD i : Integer; p : POINTER TO RECORD i : Integer; p : POINTER to c; END RECORD; END RECORD;

29 Type Checking : Coercions implicit data and type conversion to match operand (argument) type coercions complicate identification (ambiguity) VAR a : Real;... a := 5; two phase approach expand a type to a set by applying coercions reduce type sets based on constraints imposed by (overloaded) operators and language semantics

30 Variable: value or location?...kind checking two usages of variables rvalue: value lvalue: location VAR p : Real; VAR q : Real;... p := q; insert coercion to dereference variable checking rules: found lvalue expected rvalue lvalue - deref rvalue ERROR...lvalue required - (location of) p := deref (location of) q

31 Object Oriented Source language Issues Basic types of data Enumeration : copied,compared,incremented & decremented Pointer : Typed, Generic Structure and Union Array Object : Inheritance,, Polymorphism, Dynamic Binding, Method Overriding, Multiple Inheritance

32 Object Oriented Source language Issues : Pointer The run-time representation of pointer is an unsigned integer Operations include copy, assignment, comparison, increment, decrement, dereferencing Deferencing means obtaining the value of the data structure that the pointer refers to. : For Eg. ptr-> data or (*ptr).data. Pointers are usually of two types: Typed : Pointer to a specific data type. Eg. int *p; Generic : One that can be coerced to any other type For Eg. void *p;

33 Object Oriented Source language Issues : Pointer Issues Issue No Issue Solution 1 Pointer Never Intialized 2 NULL Pointer not assigned any value 3 Dangling Pointers : Arised when a pointer refers to a location and that location is freed Automatic initialization Dereferencing of null pointers must be done Use garbage collector

34 Object Oriented Source language Issues : Pointer Issues Scope of a pointer is that of the location into which it points. int x, *ptr, a; x = 5; ptr = &x; a= x; cout<< *ptr; cout<< a; { int y = 20; ptr = &y; a = y; cout<< *ptr; cout<< a; } cout<< *ptr; cout<< a; } o/p :

35 Object Oriented Source language Issues : Structure struct emp { int empid; //requires 4 bytes double salary; //requires 8 bytes }; We consider memory arrangement of the structure as follows : empid salary 4 bytes 8bytes 12 Bytes

36 Object Oriented Source language Issues : Structure But in real time, some gap must be inserted such that both structure members require equal amount of memory This memory is calculated as Least Common Multile(LCM) of the size of data members of the structure. Thus actually, the representation is as follows : empid salary Gap The size now is 16 bytes

37 Object Oriented Source language Issues : Arrays There s no disagreement about how plain arrays are stored in memory Any programming language that supports a plain (not associative) array type just stores the elements sequentially. Once a language supports multidimensional arrays, it needs to decide how to squeeze the 2D arrangement of data into a 1D arrangement in memory, typically as an 1D array. One classical use case for multidimensional arrays are matrices. Array copying can be done element by element or by copying block copy. Iilarly, comparison can be carried out. Arrays can be static as well as dynamic

38 Object Oriented Source language Issues : Arrays Given a Matrix, there s two canonical ways to store it in memory : i. Row-major ii. Column - major

39 Object Oriented Source language Issues : Arrays : Row Major Storage traverses the matrix by rows then within each row enumerates the columns. A would be stored in memory as a11, a12, a13, a21, a22, a23 The position of the element at row i, column j in the underlying 1D array is computed as : i*stride + j, where, stride is the number of elements stored per row, usually the width of the 2D array, but it can also be larger.

40 Object Oriented Source language Issues : Arrays : Column Major Storage traverses the matrix by columns, then enumerates the rows within each column. A would be stored in memory as a11, a21, a12, a22, a13,a23 The position of the element at row i, column j in the underlying 1D array is computed as : j*stride + i, where, stride is the number of elements stored per column, usually the width of the 2D array, but it can also be larger.

41 Routines and Activations : Activation record 41 When a function / subroutine is called, it will create an activation record. This record contains the location of local variable, return address etc.

42 Activation Record - Example foo()->bar()->baz(), determine the return address int foo() { int b; b = bar(); return b; } int bar() { int b = 0; b = baz(b); return b; } int baz(int b) { if (b < 1) return baz(b + 1); else return b; } 42

43 An example bar() 43

44 Activation Record temporaries: used in expression evaluation local data: field for local data saved machine status: holds info about machine status before procedure call access link : to access non local data control link :points to activation record of caller actual parameters: field to hold actual parameters Temporaries local data machine status Access links Control links Parameters Return value returned value: field for holding value to be returned 44

45 Routines Routines Classical Iterator Coroutines

46 Routines : Iterator Iterator is the one that can suspend itself temporarily and return to its parent without losing its activation record After suspension it can be resumed again at the point where it had left. This happens in C# with the help of keyword yield.

47 Routines : Coroutines Like Iterator it can suspend itself temporarily but control does not return to its parent but goes to another co-routine. This is called resume. This was offered by Simula C offers resume( )

48 Operations on Routines Define Call Return Pass

49 Advantages : INTRODUCTION TO FUNCTIONAL PROGRAMMING Have uniform view of programs Treat functions as data Automatic memory management Greatly flexible and has simple semantics Disadvantage : These are interpreted thus result in substantial loss in execution speed. Lecture 18

50 KEY PROPERTIES OF FUNCTIONAL PROGRAMMING LANGUAGES Lazy function evaluation First class objects All programs and procedures are functions Lack of variable and assignment Lack of loop and iteration Referential transparency Dynamic memory environment Garbage collection Side effect freedom Lecture 18

51 KEY FEATURES OF FUNCTIONAL PROGRAMMING LANGUAGE 1.Lazy function evaluation: Unnecessary function evaluation is done 2. First class objects : Functions are treated as objects 3. All programs and procedures are functions : Due to this feature the programs are considered as data and can be changed at run- time. This distinguishes 4. Lack of Variable Assignment 5. Lack of loops and iteration : replaced by recursive calls. Lecture 18

52 KEY FEATURES OF FUNCTIONAL PROGRAMMING LANGUAGE 6. Referential Transparency ( Side Effect Free) Property of function whereby its value depends upon the parameters, but not upon previous computations. 7.Dynamic Memory Management : Done automatically Maintaining free space Reclamation of storage 8.Garbage collection : Methods to collect and return unreferenced Lecture 18 storage

53 Factorial in Haskel vs. C fac 0 = 1 fac n = n * fac (n -1) int fac(int n) { int product = 1; while (n > 0) { product *= n ; n --; } return product; }

54 Offside rule Layout characters matter to parsing divide x 0 = inf divide x y = x / y Everything below and right of = in equations defines a new scope Applied recursively fac n = if (n ==0) then 1 else prod n (n-1) where prod acc n = if (n == 0) then acc else prod (acc * n) (n -1) Lexical analyzer maintains a stack

55 Lists Part of all functional programs since Lisp Empty list [] = Nil [1] [1, 2, 3, 4] [4, 3, 7, 7, 1] [ red, yellow, green ] [1.. 10] => arithmetic sequence Can be constructed using : infix operator [1, 2, 3] is equivalent to (1 : (2 : ( 3 : []))) range n m = if n > m then [ ] else ( n: range (n+1) m) Constructs arithmetic sequence [n m] dynamically

56 List Comprehension Inspired by set comprehension S = {n 2 n {1,, 100} odd n} Haskel code s = [n^2 n <- [1..100], odd n] n square such that n is an element of [1..100] and n is odd Qsort in Haskel qsort [] = [] qsort (x: xs) = qsort [y y <- xs, y < x] ++ [x] ++ qsort[y y <- xs, y >= x]

57 Pattern Matching Convenient way to define recursive functions A simple example fac 0 = 1 fac n = n * fac (n-1) Equivalent code fac n = if (n == 0) then 1 else n * fac (n -1) Another example length [ ] = 0 length (x: xs) = 1 + length xs Equivalent code length list = if (list == []) then 0 else let x = head list xs = tail list in 1 + length xs

58 Polymorphic Typing The basic types like int,char are said to be monomorphic types Polymorphic type means a variable can have many types. For eg. Empty list [ ] can have many types : list of characters, list of numbers etc. Benefits: Code reuse Guarantee consistency

59 Polymorphic Typing The compiler infers that in length [ ] = 0 length (x: xs) = 1 + length xs length has the type [a] -> int length :: [a] -> int Example expressions length [1, 2, 3] + length [ red, yellow, green ] length [1, 2, green ] // invalid list The user can optionally declare types Every expression has the most general type

60 Structure of a functional compiler High-level language Polymorphic type checking De-sugaring: 1. Pattern matching 2. List to pairs Optimizations 3. List comprehension 4. Lambda lifting Functional core Code generation C code Runtime system

61 Compiling Functional Languages Below is the compiler phase handles which aspect of Haskell: Compiler Phase Language Aspect Lexical Analyzer Parser Context Handling Run Time System Off side Rule List Notation List Comprehension Pattern Matching Polymorphic type checking Referential Transperency Higher Order Functions Lazy Evaluation

62 Polymorphic Type Checking These are rules that ML uses to infer the type correctness of polymorphic code known as : 1. All occurrences of the same identifier in a given scope must have the same type. 2. In an if/then/else expression, the condition must be of type bool, and the then and else clauses must be of the same type. 3. A programmer-defined function has type 'a -> 'b where 'a is the type of the function s parameter and 'b is the type of its result. (Functions have tuple arguments.) 4. When a function is applied, the type of the argument passed to the function must be the same as the parameter type in the function s definition and the type of the application is the same as the type of the result in the function s definition.

63 Polymorphic Type Checking The Hindley- Miler algorithm traverses a program s abstract syntax tree (AST)1, assigning type variables at each node. It then applies rules corresponding to syntactic constructs to attempt to resolve the assigned type variables against specific type information deduced from program contents and context.

64 Polymorphic Type Checking For example, consider checking the following Standard ML-ish function, which returns a string depending on the sign of its integer argument: fun sign 0 = "zero" sign n = if (> n) 0 then "positive" else "negative This function has type int -> string, i.e. it takes an integer argument and returns a string result

65 Polymorphic Type Checking Prior to type checking, it is assumed that: 0 : int i.e. 0 is an integer "negative" : string > : int -> int -> bool i.e. > compares two integers to return a boolean The rule for a function: Checks the first case: fun sign 0 = "zero" concludes 0 is int from assumptions checks the body i.e. deduces "zero" is string from assumptions concludes that the first case is int -> string

66 Polymorphic Type Checking Checks the second case: sign n = if (> n 0) then "positive" else "negative Assumes parameter n has type α checks the body i.e. checks the if expression: checks the condition i.e. checks the application of > n to 0 checks the application of > to n concludes that > is int -> int -> bool from assumptions assigns the type variable β to the result of the application concludes that n is α from assumptions unifies the anticipated type α -> β with the type of >, int -> int -> bool concludes that α is int and β is int -> bool

67 Polymorphic Type Checking assigns the type variable γ to the result of the application concludes that 0 is int from assumptions unifies the anticipated type int -> γ with the type of int -> bool concludes that γ is bool unifies the condition type bool with the required type for a condition, bool checks the then branch i.e. concludes that "positive" is string from assumptions checks the else branch i.e. concludes that "negative" is string from assumptions unifies the then and else branches, which must have the same type concludes that the if has type string concludes that the second case has type int -> string

68 Desugaring Translating high level construct into a simpler one means desugaring Translation of Pattern Matching Translation to List Translation to List Comprehension Desugaring

69 Translation to Lists List Notations has three operators in Functional Language : i), ii).. iii) : List of the form x : xs is transformed to (cons x xs) list[1,2,3] or list(1:3) is transformed to (cons 1(cons 2 ( cons 3 [ ] ) ) )

70 Translation OF Pattern Matching Pattern matching on constructors desugars to case statements

71 Translation OF Pattern Matching Pattern matching on numeric or string literals desugars to equality tests:

72 Translation OF List Comprehension List comprehensions are equivalent to do notation:

73 JAVA CC JAVA COMPILER COMPILER Parser and Lexical Analyzer Generator i.e it Produces Lexical Analyzers And Parsers in Java It generates Top Down Parser hence cannot parse left recursive grammar Lecture 18

74 JAVA CC JAVA COMPILER COMPILER Regular Expression Translation Grammar JAVA CC Java Code.jj file Lecture 18

75 Steps for Execution > JavaCC eg.jj //Generates eg. java > Javac eg.java //Generates eg. Class > Java eg Lecture 18

76 Java CC Specification : ClassName.jj Options { // Java CC Options } PARSER-BEGIN(Class-Name) // Code PARSER-END (Class-Name) SKIP : { } SKIP : { \n \r \r\n } TOKEN : { < PLUS : + > } TOKEN : { < NUMBER : ([ 0-9 ])+ > } void Start() : {} { <NUMBER> ( <PLUS> <NUMBER> )* <EOF> Parser Lexical Analyzer

77 JavaCC Compilation : javacc ClassName.jj Compiling this file generates Seven Files TokenMgrError is a simple error class; it is used for errors detected by the lexical analyser and is a subclass of Throwable. ParseException is another error class; it is used for errors detected by the parser and is a subclass of Exception and hence of Throwable. Token is a class representing tokens. SimpleCharStream is an adapter class that delivers characters to the lexical analyser. ClassNameConstants is an interface that defines a number of classes used in both the lexical analyser and the parser. ClassNameTokenManager is the lexical analyser ClassName is the Parser Lecture 18

78 Java CC Example PARSER_BEGIN(Calc0) // must define parser class public class Calc0 { public static void main (String args []) { Calc0 parser = new Calc0(System.in); for (;;) try { if (parser.expr() == -1) System.exit(0); } catch (Exception e) { e.printstacktrace(); System.exit(1); } } } PARSER_END(Calc0)

79 Java CC Example SKIP: { " " "\r" "\t" } // defines input to be ignored TOKEN: // defines token names { < EOL: "\n" > < CONSTANT: ( <DIGIT> )+ > // re: 1 or more < #DIGIT: ["0" - "9"] > // private re } int expr() : // expr: sum \n { } // -1 at eof, 0 at eol { sum() <EOL> { return 1; } <EOL> { return 0; } <EOF> { return -1; } }

80 Java CC Example // sum: product { + - product } void sum(): { } { product() ( ( "+" "-" ) product() )* } // product: term { *%/ term } void product():{} { term() (( "*" "%" "/" ) term() )* } // term: +term -term (sum) number void term(): {} { "+" term() "-" term() "(" sum() ")" <CONSTANT> }