1. Louden, Programming Languages
For Java (and most other langauges we will cover), you are given a ticket.
We can’t possibly cover everything you need to know, but we have allowed
you admitance to the “show”.
You know the debugger. You can run hello world. You understand key
differences. You know where the documenation is. What you do with your
admission ticket is up to you!
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2. Chapter 1 - Introduction
4/17/2017
Chapter 1 - Introduction
Programming Languages:
Principles and Practice, 2nd Ed.
© Kenneth C. Louden, Adapted by Vicki Allan 2006

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Course Motivation
Why are programming languages the way they are?
How are particular language features implemented/supported?
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4. Course Motivation cont…
understand the underlying ideas of the main programming paradigms
know more about the huge variety of programming languages
understand how the syntax and semantics of languages can be defined precisely.
know how important features are supported
have a deeper understanding of the history and rationale behind languages like C++.
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5. Relationship between thought and language.
The Sapir-Whorf hypothesis in linguistics states that the structure of one's mother-tongue influences the way one's mind perceives the world. It has found at best very limited experimental support, at least in its strong form.
One study has shown that subjects in memory tests are more likely to remember a given color if their mother language includes a word for that color.
Example – if you had no identified concept of recursion, how would that affect the ability to reason about it?
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6. Why study programming languages?
Increased capacity to express ideas
improved background for choosing language
increased ability to learn new languages
Better understanding of significance of implementation
Ability to design new languages - or user interface
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Example
Beginning students – always wanted to know specific answers:Can I do X? What happens if I do Y?
Often hadn’t tried the specific test, but could reason about it from general knowledge of implementation.
Ex: What happens if I try to return a reference to a local variable?
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The human-computer semantic gap
Human:
Interested in modelling the real world
More interested in what computer should do than how
Computer:
Only data it can manipulate is sequences of zeros and ones.
Understands low-level “how” instructions.
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9. What are programming languages…
High-level languages bridge the human-computer semantic gap by providing a higher level notation that can still be executed by computer
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10. What is a Programming Language?
Definition: A programming language is a notational system for describing computation in machine-readable and human-readable form.
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Computation:
Described by a Turing Machine - a very simple computer that can carry out all known computations (albeit not very efficiently).
A programming language is Turing complete if it can be used to describe any computation performed by a Turing Machine.
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12. Turing Machine – 1936 Alan Turing
based on the idea of a person executing a well-defined procedure by changing the contents of an unlimited paper tape, divided into squares that can contain one of a finite set of symbols.
"If your state is 42 and the symbol you see is a '0' then replace this with a '1', move one symbol to the right, and assume state 17 as your new state."
A Turing machine is equivalent to a pushdown automaton made more powerful by relaxing the last-in-first-out requirement of its stack.
More precisely, a Turing machine consists of:
A tape which is divided into cells, one next to the other. Each cell contains a symbol from some finite alphabet. The alphabet contains a special blank symbol (here written as '0') and one or more other symbols. The tape is assumed to be arbitrarily extendible to the left and to the right, i.e., the Turing machine is always supplied with as much tape as it needs for its computation. Cells that have not been written to before are assumed to be filled with the blank symbol.
A head that can read and write symbols on the tape and move left and right.
A state register that stores the state of the Turing machine.
An action table (or transition function) that tells the machine what symbol to write, how to move the head ('L' for one step left, and 'R' for one step right) and what its new state will be, given the symbol it has just read on the tape and the state it is currently in. If there is no entry in the table for the current combination of symbol and state then the machine will halt.
Note that every part of the machine is finite; it is the potentially unlimited amount of tape that gives it an unbounded amount of storage space.
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13. What is needed for Turing completeness?
Virtually nothing:
A programming language is Turing complete provided it has
integer variables and
arithmetic and
sequentially executes statements, which include assignment, selection (if) and loop (while) statements.
Even if statements are unnecessary
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14. Machine-readability:
Also not a huge requirement: Basically, the existence of a (more or less) linear-time translation algorithm.
Usually boils down to: The syntax must be given by a context-free grammar.
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Human-readability:
This is the real issue!
Virtually all the complex details of a programming language are there to (supposedly) enhance human readability.
Still not very well understood.
Is strongly dependent on good choice of abstractions.
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16. What about human “writability??”
Aren’t programming languages there to promote the writing of programs, not the reading of them?
Nonsense! Writability is a hacker’s goal: Perl is very writable, but try to read it!
Readability is the real goal: many people are going to have to read your program after you have written it.
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Abstractions:
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18. Computational Paradigms
Programming languages began by imitating the operations of a computer.
It is not surprising that the kind of computer for which they were written had significant effect on their design.
variables representing memory
assignment to change values
sequential execution of statements
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Language Paradigms:
Imperative (procedural): traditional sequential programming (passive data, active control). Characterized by variables, assignment, and loops.
Object-oriented: data-centric, data controls its own use, action by request to data objects. Characterized by messages, instance variables, and protection. Extension of imperative paradigm.
Functional: passive data, but no sequential control; all action by function evaluation (“call”), particularly recursion. No local variables! ~ to mathematics
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20. Language Paradigms (cont.):
Logic: Assertions are the basic data; logic inference the basic control. Again, no sequential operation. ~ to mathematics
ex: I am your sister if I am female and we have common parents.
Parallel: well, maybe not really a paradigm, but some think so. Again, no sequential operation.
“Declarative”: Logic and functional paradigms share this property: state “what” needs computing, not “how” (sequence).
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21. Languages and paradigms
Imperative: C, Pascal, core Ada, FORTRAN
Functional: Lisp (Scheme), ML, Haskell
Object-oriented: C++, Java, Smalltalk
Logic: Prolog
Parallel: Java (threads), Ada (tasks)
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Perl
The overall structure of Perl derives broadly from the programming language C. Perl is a procedural programming language, with variables, expressions, assignment statements, brace-delimited code blocks, control structures, and subroutines.
Like the Unix shells, Perl has many built-in functions for common tasks, like sorting, and for accessing system facilities.
Perl takes lists from Lisp, associative arrays from awk, and regular expressions from sed. These simplify and facilitate all manner of parsing, text handling, and data management tasks.
Perl has many and varied applications.
It has been used since the early days of the Web to write CGI scripts, and is an integral component of the popular LAMP (Linux / Apache / MySQL / (Perl / PHP / Python)) platform for web development.
Perl is often used as a "glue language", tying together systems and interfaces that were not specifically designed to interoperate.
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23. Are functional languages Turing-complete?
Previous theorem on Turing-completeness depends on the existence of variables and loops.
Functional programs do not have variables or loops. Can all computation be expressed?
Yes!:
A programming language is Turing complete if it has integer values, arithmetic functions on those values, and if it has a mechanism for defining new functions using existing functions, selection, and recursion.
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24. Paradigm use is rarely “pure”:
The C program (in text)defined gcd function in a purely functional style, even though C is mainly imperative.
The Java program used some imperative code to compute the gcd, and was not completely object-oriented (integers aren’t objects).
The Scheme code used sequencing to do I/O, an imperative feature.
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25. Examples of languages that are pure (mostly):
Imperative: (old) FORTRAN
Functional: Haskell
Object-oriented: Smalltalk
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Language definition
Syntax: the structure of a program. Usually given a formal (i.e., mathematical) definition using a context-free language. (Lexical structure - the structure of the words or tokens - uses regular expressions.)
Semantics: the actual result of execution. Usually described in English, but can be done mathematically.
Semantics can have a static component: type checking, definition checking, other consistency checks prior to execution.
What are dynamic components of semantics?
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Language translation
inputs
compiler
outputs
source
executable
run
Compiler: two-step process that (1) translates source code into target code; then (2) the user executes the target code.
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Language translation
inputs
outputs
source
run
Interpreter: one-step process in which the source code is executed directly.
Hybrids are also possible (Java).
inputs
Java
compiler
machine
dependent
interpreter
source
bytecode
outputs
Bytecode is composed of instructions that have been brought to the
lowest level possible without making them machine dependent.
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Compilation, Interpretation, and Hybrid systems
Consider this piece of code:
public class Test {    public static void main(String args[])    {       int i;       i = 2;       i = i + 7;    } }
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If we were to compile it, we would change it to machine instructions that would only work for one architecture. 
If we were to interpret it, our interpreter would have to be able to understand the high level code AND would repeatedly parse it (if the code was in a loop).
When we use the hybrid approach of Java, we produce the file Test.class, which is a binary file that's not readable by most humans. We can convert the file to a readable form with the javap tool as shown here:
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C:\ > javap -c Test Compiled from Test.java public class Test extends java.lang.Object {    public Test(); // a default constructor created    public static void main(java.lang.String[]); }
Method Test()    0 aload_0    1 invokespecial #3    4 return
Method void main(java.lang.String[])    0 iconst_2 // Put integer 2 on stack    1 istore_1 // Store the top stack value at location 1    2 iload_1  // Put the value at location 1 on stack    3 bipush 7 // Put the value 7 on the stack    5 iadd     // Add two top stack values together    6 istore_1 // The sum, on top of stack, stored at location 1    7 return   // Finished processing
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Although the bytecode cannot access registers or directly reference memory locations and must obey various other restrictions, the actual JVM (java virtual machine) program can use internally whatever techniques are convenient to use for a particular platform. As long as the Java bytecode sees only a JVM specification compliant system, the JVM programmer has broad discretion for its implementation
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33. Language Implementation Methods
Compilation:
lexical analysis: characters grouped into logical chunks (keywords, constants, etc)
syntax analysis: figure out what it means - usually use parse trees (grammars to define). Like diagraming sentences.
small dogs and cats – what is meant?
optimization - to make smaller or faster
linking: supplying missing addresses to system code
load module: user code augmented with system code
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The tall boy ran fast. 
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"Maria gave Joe the rice"
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36. Language Implementation Methods (cont)
Pure Interpretation:
no translation phase - fetch, decode, and execute the source code (not a machine code)
Advantages/Disadvantages
easy to write debugger - as source lines are unchanged
execution is times slower; statement decoding is bottleneck
better for simple structure - as not so slow to decode
natural for some kinds of features - like dynamic binding of type.
Ex: a = a+b If a may be integer, string, or a set, how can we know what code to generate?
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37. What is meant by dynamic binding?
Girls choice dance:
Will you go with Sofie? (early binding)
Will you go with Sofie/Ann/Betty (whoever shows up at your door)? (delayed binding)
No specific partner assigned, but will change throughout the night. (changing binding)
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38. Hybrid Implementation System
Compile into intermediate code. Java - compiles to bytecode. Then bytecode is interpreted.
JVM (Java Virtual Machine) is byte code interpreted and run time system.
Java is delivered partially compiled. Developers compile it into byte codes, downloaded across the network. Then those byte codes are interpreted by the browser.
Architecture-neutral: Sun designed the language so that it is also partially interpreted.
Creating Bytecode does about 80% of the compilation work. However, one set of Bytecode can run on any Java-enabled computer.
The last 20% is performed at runtime by the Java environment provided by the machine specific browser.
Eliminates the version mismatch problems: All external program references are resolved when the application is executed.
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Compilation Steps
Figure out the full name of the class to be invoked
Determine which method signature to use If there is more than one matching signature, the one that is most specific is chosen.
Example: doit(Object o) or doit(ColoredPoint p) or doit (Point p)
A method is applicable if
the number of parameters matches
the type of each actual argument can be converted to the type of the corresponding parameter.
A method is accessible if the access modifier allows access.
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Error classification
Lexical: character-level error, such as illegal character (hard to distinguish from syntax).
Syntax: error in structure (e.g., missing semicolon or keyword).
Static semantic: non-syntax error prior to execution (e.g., undefined vars, type errors).
Dynamic semantic: non-syntax error during execution (e.g., division by 0).
Logic: programmer error, program not at fault.
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41. Notes on error reporting
A compiler will report lexical, syntax, and static semantic errors. It cannot report dynamic semantic errors.
An interpreter will often only report lexical and syntax errors when loading the program. Static semantic errors may not be reported until just prior to execution. Indeed, most interpreted languages (e.g. Lisp, Smalltalk) do not define any static semantic errors.
No translator can report a logic error.
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Sample Errors (Java):
public int gcd ( int v# ) // lexical
{ int z = value // syntax - missing ;
y = v; // static semantic - y undefined
while ( y >= 0 ) // dynamic semantic -
// division by zero
{ int t = y; y = z % y; z = t; }
return y; // logic - should return z
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Language design
Good, consistent set of abstractions.
Tie-in to other technology:
C : Unix
Java : Internet
C++ : most efficient OO language
Now also:
Ease of interface with other languages and systems
Good libraries
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