CS 152: Programming Language Paradigms April 2 Class Meeting Department of Computer Science San Jose State University Spring 2014 Instructor: Ron Mak

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 2 Introduction to Compilers and Interpreters  In this class, you will use Java to write an interpreter for the Scheme language. You will be able to execute simple Scheme programs. _

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 3 A Compiler is a Translator  A compiler translates a program you’ve written... in a high-level language  C, C++, Java, Pascal, etc.... into a low-level language  assembly language or machine language... that a computer can understand and eventually execute. _

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 4 Conceptual Design (Version 1)  Parser Controls the translation process. Repeatedly asks the scanner for the next token.  Scanner Repeatedly reads characters from the source to construct tokens for the parser.  Token A source language element  identifier (name)  number  special symbol (+ - * / = etc.)  reserved word Also reads from the source  Source The source program

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 5 Token  A low-level element of the source language. AKA lexeme  Java language tokens identifiers  names of variables, types, procedures, functions, enumeration values, etc. numbers  integer and real (floating-point) reserved words  class interface if else for while etc. special symbols  + - * / = = > >>= <<=., : ( ) [ ] { } ' "

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 6 Parser  Controls the translation process. Repeatedly asks the scanner for the next token.  Knows the syntax (“grammar”) of the source language’s statements and expressions. Analyzes the sequence of tokens to determine what kind of statement or expression it is translating. Verifies that what it’s seeing is syntactically correct. Flags any syntax errors that it finds and attempts to recover from them.  What the parser does is called parsing. It parses the source program in order to translate it. AKA syntax analyzer _

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 7 Scanner  Whenever requested by the parser, the scanner reads characters sequentially from the source in order to construct the next token.  It knows the syntax of the source language’s tokens.  What the scanner does is called scanning. it scans the source program in order to extract tokens. AKA lexical analyzer _

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 8 Conceptual Design (Version 2)  We can architect a compiler with three major parts:

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 9 Major Parts of a Compiler  Front end Parser, Scanner, Source, Token  Intermediate tier Intermediate code (icode)  “Predigested” form of the source code that the back end can process efficiently.  Example: parse trees  AKA intermediate representation (IR) Symbol table (symtab)  Stores information about the symbols (such as the identifiers) contained in the source program.  Back end Code generator  Processes the icode and the symtab in order to generate the object code. Only the front end needs to be source language-specific. The intermediate tier and the back end can be language-independent!

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 10 What Else Can Compilers Do?  Allow you to program in a high-level language and think about your algorithms, not about machine architecture.  Provide language portability. You can run your C++ and Java programs on different machines because their compilers enforce language standards.  Can optimize and improve how your programs execute. Optimize the object code for speed. Optimize the object code for size. Optimize the object code for power consumption. _

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 11 What about Interpreters?  An interpreter executes a source program instead of generating object code. It executes a source program using the intermediate code and the symbol table.  It shares many of the components of a compiler. Instead of a code generator in the back end, it has an executor. _

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 12 Conceptual Design (Version 3)  A compiler and an interpreter can both use the same front end and intermediate tier.

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 13 Comparing Compilers and Interpreters  A compiler generates object code, but an interpreter does not. Executing the source program from object code can be several orders of magnitude faster than executing the program by interpreting the intermediate code and the symbol table. But an interpreter requires less effort to get a source program to execute = faster turnaround time.  An interpreter maintains control of the source program’s execution. Interpreters often come with interactive source-level debuggers that allow you to refer to source program elements, such as variable names. AKA symbolic debugger _

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 14 Compilers and Interpreters  Therefore... Interpreters are useful during program development. Compilers are useful to run released programs in a production environment. _

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 15 How to Scan for Pascal Tokens  Suppose the source line contains IF (index >= 10) THEN  The scanner skips over the leading blanks. The current character is I, so the next token must be a word.  The scanner extracts a word token by copying characters up to but not including the first character that is not valid for a word, which in this case is a blank. The blank becomes the current character. The scanner determines that the word is a reserved word.

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 16 How to Scan for Pascal Tokens, cont’d  The scanner skips over any blanks between tokens. The current character is (. The next token must be a special symbol.  After extracting the special symbol token, the current character is i. The next token must be a word.  After extracting the word token, the current character is a blank.

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 17 How to Scan for Pascal Tokens, cont’d  Skip the blank. The current character is >.  Extract the special symbol token. The current character is a blank.  Skip the blank. The current character is 1, so the next token must be a number.  After extracting the number token, the current character is ).

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 18 How to Scan for Pascal Tokens, cont’d  Extract the special symbol token. The current character is a blank.  Skip the blank. The current character is T, so the next token must be a word.  Extract the word token. Determine that it’s a reserved word.  The current character is \n, so the scanner is done with this line.

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 19 Basic Scanning Algorithm  Skip any blanks until the current character is nonblank. Treat each comment and the end-of-line character as a blank.  The current (nonblank) character determines what the next token is and becomes that token’s first character.  Extract the rest of the next token by copying successive characters up to but not including the first character that does not belong to that token.  Extracting a token consumes all the source characters that constitute the token. After extracting a token, the current character is the first character after the last character of that token. _

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 20 Scheme Syntax Diagrams

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 21 Scheme Syntax Diagrams, cont’d

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 22 Scheme Syntax Diagrams, cont’d

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 23 Scheme Syntax Diagrams, cont’d

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 24 Scheme Keywords (Partial List)  and  begin  cond  define  else  if  lambda  let  letrec  let*  not  or  quote

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 25 Scheme Intermediate Code  Scheme programs have a simple structure. Everything is a list.  The Scheme parser can translate a list into a binary tree. The left subtree is the car of the list. The right subtree is the cdr of the list. Each leaf node contains an element of the list.  Example: (1 2 3) 1 2 3

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 26 Scheme Intermediate Code, cont’d  Example: ((a b) c (d))  Do a preorder walk of the tree to recreate the list: Visit the root.  If the left subtree is not an element node, open a set of parentheses. Visit the left subtree.  If the left subtree is a leaf, print its element. Visit the right subtree. a b c d

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 27 The Symbol Table: Basic Concepts  Purpose To store information about certain tokens during the translation process (i.e., parsing and scanning).  What information to store? Anything that’s useful! For a symbol:  name  how it’s defined (as a variable, procedure name, etc.)  Basic operations Enter new information Look up existing information Update existing information

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 28 The Symbol Table: Conceptual Design  Each symbol table entry has the name of a symbol the symbol’s attributes  At the conceptual level, we don’t worry about implementation.

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 29 The Symbol Table: Implementation  Each symbol table entry includes the name of a symbol and its attributes.  To maintain maximum flexibility, implement the attributes as a hash table. Key: the attribute name Value: the attribute value

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 30 The Symbol Table: Implementation, cont’d  The symbol table itself can be a hash table. Key: the symbol name Value: the symbol table entry for the symbol  Therefore, we have a hash table of hash tables.

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 31 Assignment #5  Use Java to write a parser and a scanner for Scheme.  Your scanner should recognize Scheme tokens. Print each source input line after the scanner reads the line. For each token, your scanner should print the token string and the token type (identifier, number, keyword, special symbol, etc.).  One token per output line.  The scanner should ignore comments. _

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 32 Assignment #5, cont’d  The parser should enter each identifier into a symbol table. Don’t enter keywords. For now, the attributes of each symbol table entry can be null. For this assignment, you’ll have only one symbol table. _

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 33 Assignment #5, cont’d  Your parser should translate Scheme lists. Build the intermediate code (binary trees).  After an entire top-level list has been parsed, your backend takes over to perform the following: Walk the binary tree and print the list with the proper parentheses.  The list output does not need to be “pretty” but it must be correct.  For example, you can start each sublist on a new line after an indent. Print the contents of the symbol table in alphabetical order.  Hint: Use a Java TreeMap instead of a Hashtable or Hashmap for the symbol table.

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 34 Assignment #5, cont’d  Clear the intermediate code and the symbol table before parsing the next top-level list.  To ensure that your code is properly partitioned, put your classes into three packages: frontend, intermediate, and backend.  Your test input file is on the last slide.  a zip file The source directory containing all your Java source files. A text file of the output from your program. Subject: CS 152 Assignment #5, team-name

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 35 Assignment #5, cont’d  Due Wednesday, April 16 Do not wait until the last minute to do this assignment! _

SJSU Dept. of Computer Science Spring 2014: April 2 CS 152: Programming Language Paradigms © R. Mak 36 Assignment #5, cont’d ; Find the derivative of polynomial poly with respect to variable var. ; The polynomial must be in canonical infix form. (define deriv (lambda (poly var) (let* ((terms (terminize poly)) ; "terminize" the polynomial (deriv-term ; local procedure deriv-term (lambda (term) (cond ((null? term) '()) ((not (member? var term)) '(0)) ; deriv = 0 ((not (member? '^ term)) (upto var term)) ; deriv = coeff (else (deriv-term-expo term var)) ; handle exponent ))) (diff (map deriv-term terms))) ; map deriv-term over the terms (remove-trailing-plus (polyize diff)) ; finalize the answer ))) ; Convert an infix polynomial into a list of sublists, ; where each sublist is a term. (define terminize (lambda (poly) (cond ((null? poly) '()) (else (cons (upto '+ poly) (terminize (after '+ poly)))) ))) input.lisp