- Concepts of Programming Languages

Course Overview

Instructor:

Autonomous and Intelligent Systems

  •   Want a computer to perform a task (transfer money, order goods)
  •   Interact with the physical world (drive a car, fly an airplane)
  • Requires clear instructions: a programming language
    • move left, right, up, down
    • do one step and then another step
    • repeat steps

PL Syntax, Semantics, Pragmatics

  • Syntax: structure of a programming language and rules for writing valid instructions (grammar)
  • Semantics: meaning of valid instructions
  • Pragmatics: how to use the language, best practices for writing code

PL Syntax and Semantics

Different Syntax, Same Semantics

Java

  1. final int x = y>0 ? 1 : -1;

Scala

  1. val x = if y>0 then 1 else -1

Same Syntax, Different Semantics

Java

  1. for  (var i = 0; i < 10; i++) {
  2.   var x = i;
  3. }
  4. // Here i and x are both undefined

JavaScript

  1. for (var i = 0; i < 10; i++) {
  2.   var x = i;
  3. }
  4. // Here i is 10 and x is 9

PL Concepts by Example

Java

  1. class Intro {
  2.   private static int isEven(int n) {
  3.     return n % 2 == 0 ? 1 : 0;
  4.   }
  5.   
  6.   public static int countEven(List<Integer> xs) {
  7.     int result = 0;
  8.     if (!xs.isEmpty()) {
  9.       result = 
  10.         isEven(xs.getFirst()) + 
  11.         countEven(xs.subList(1, xs.size()));
  12.     }
  13.     return result;
  14.   }
  16.   public static void main(String[] args) {
  17.     List<Integer> numbers = 
  18.       Arrays.stream(args).
  19.       map(str -> Integer.parseInt(str)).
  20.       toList();
  21.     System.out.println("You entered " + countEven(numbers) +
  22.       " even numbers");
  23.   }
  24. }

Concepts

  • Statements and expressions
  • Strict and nonstrict evaluation
  • Collection processing
  • Recursion
  • L-Values
  • Argument passing
  • Parametric polymorphism

PL Concepts by Example

C

  1. int isEven(int n) { return n % 2 == 0 ? 1 : 0; }
  2.   
  3. int countEven(int argc, const int* const xs) {
  4.   int result = 0;
  5.   if (argc > 0) {
  6.     result = 
  7.       isEven(xs[0]) + 
  8.       countEven(argc-1, &xs[1]);
  9.   }
  10.   return result;
  11. }
  13. int main(int argc, char **args) {  
  14.   int *vals = malloc((argc-1)*sizeof(int));
  15.   for (int i = 1; i < argc; ++i) {
  16.     char* end;
  17.     vals[i-1] = strtol(args[i], &end, 10);
  18.     // forgot to check whether all characters 
  19.     // are consumed (i.e., end == '\0')
  20.   }
  21.   int numEven = countEven(argc-1, vals);
  22.   printf("You entered %d even numbers\r\n", numEven);
  23.   // forgot to free the dynamically allocated memory
  24.   return 0;
  25. }

More Concepts

  • Safety and undefined behavior
  • Static and dynamic memory allocation
  • Scope and lifetime

PL Concepts by Example

Scala

  1. enum List:
  2.   case Empty
  3.   case Cons (head:Int, tail:List)
  4.   
  5.   def length (result:Int = 0): Int = this match 
  6.     case List.Empty      => result
  7.     case List.Cons(a,as) => length(as, result+1)
  9. object List:
  10.   def create(elems: Int*) = 
  11.     elems.foldRight(List.Empty)((e, l) => List.Cons(e, l))

More Concepts

  • Algebraic datatypes
  • Functional programming
  • Pattern matching
  • Tail recursion
  • Type inference
  • Currying

PL Pragmatics and Large Language Models

    1. Implement a Java program that counts 
    2. how many even even numbers are in 
    3. the command line inputs

    1. public static void main(String[] args) {
    2.   int evenCount = 0;
    3.   
    4.   for (String arg : args) {
    5.     try {
    6.       int number = Integer.parseInt(arg);
    7.       if (number % 2 == 0) {
    8.           evenCount++;
    9.       }
    10.     } catch (NumberFormatException e) {
    11.       System.out.println(
    12.         arg + " is not a valid integer.");
    13.     }
    14.   }
    15.   System.out.println(
    16.     "Count of even numbers: " + evenCount);
    17. }

    1. Implement a Scala program that counts 
    2. how many even even numbers are in 
    3. the command line inputs

    1. def main(args: Array[String]): Unit = {
    2.   val evenCount = args.foldLeft(0) { 
    3.     (count, arg) =>
    4.       try {
    5.         val number = arg.toInt
    6.         if number % 2 == 0 then count + 1
    7.         else count
    8.       } catch {
    9.         case e: NumberFormatException =>
    10.           println(
    11.             s"$arg is not a valid integer.")
    12.           count
    13.       }
    14.   }
    15.   println(
    16.     s"Count of even numbers: $evenCount")
    17. }
  • What are you doing to convince yourself that the code is correct?

Why is it Important to Understand PL Well?

  • Write high-quality code
  • Assess the correctness of (auto-generated) code
  • Assess the quality of (auto-generated) code
  • Assess the performance of (auto-generated) code
  • Generalize (auto-generated) code
  • Instruct tools to refactor and improve code
  • Read, maintain, and extend existing code

Programming Languages

  • 1970: assembly, FORTRAN, COBOL, Lisp
  • 1980: C, Pascal, BASIC, ML, Smalltalk
  • 1990: C++, Perl, Objective C, Erlang
  • 2000: Java, JavaScript, Python, Ruby, Lua
  • 2005: C#
  • 2010: Scala, F#, Clojure, Go
  • 2015: Rust, Swift, Kotlin, Elm, Elixir, TypeScript, PureScript
  • 2020: ReasonML, Crystal, Pony, Zig
  • Many more!

Programming Languages

In just a couple of minutes, we explored different implementations of the same feature in different languages!

  • Programming languages keep popping up
  • You will have to keep learning new languages
  • And keep up with changes to current languages
    • Java 8 and C++ 11 added Lambda expressions (nested, anonymous functions)
  • Lots of common concepts!

Learning Objectives

Learn PLs more easily by recognizing concepts

  • "it has conditionals, recursion, loops"
  • "it has closures"
  • "it has list comprehensions"
  • "it has dynamic dispatch"
  • Deeper understanding of PL concepts / paradigms
  • Impact of PL on program development, modularity, correctness, runtime efficiency, etc.
  • Alternative way of thinking to double-check generated code

Non-Goals

  • Learn a PL similar to one you already know
  • Instead, learn concepts to facilitate quick assimilation of new PLs as they appear
  • Learn an IDE
  • Instead, become familiar with a range of tools and build systems

Course Overview

This is a challenging course

  • deepen your understanding about programming while also learning a new language
  • study guides and extra credit will help you succeed!
  • take advantage of office hours!

What are different ways of expressing computations?

  • Programming paradigms and styles:
    • functional vs object-oriented
    • mutability vs immutability
    • iteration vs recursion
    • pattern matching vs visitor pattern
  • Concepts: lexical and dynamic scope; stack layout; inheritance and dynamic dispatch; nested structures (functions or objects); dynamic vs static type checking; subtyping; parametric polymorphism

Course Approach

  • Hands on: write many programs and experiments
  • Use AI coding support responsibly: write contracts and tests, develop language extensions to help analyze auto-generated code
  • Scala as main language:
    • carefully designed multi-paradigm language
    • textbook explains PL concepts in context
  • Also bits of: C, C++, C#, Java, JavaScript, ... chosen as exemplars of concepts

Discord

  • On Discord
  • Use appropriate language

Asking Questions

  • For non-personal messages use Discord
    • "I think there is a mistake with grading of Question 3 of Homework 2." - direct message/email to instructor
    • "I cannot run program foo. I have tried running it from the command line on OS X. See below for a transcript of what I typed and the error message I received. Could you please help?" - ask in Discord #general channel

Asking Questions

  • Include your actual (IRL) name in messages directly to the instructor
  • Include enough context to answer your question:
    • "program foo doesn't work." - starts discussion
    • "program foo fails with output pasted below" - good
    • "program foo fails with input and output pasted below" - better
    • "program foo fails with input and output pasted below; using OS X; I have run extra commands to show current working directory, the version of foo in use, and other relevant information" - best

Course Syllabus

  • Review the Syllabus linked from the course homepagecourse homepage
  • Programming in Scala, First Edition is available for free online
  • Get Programming in Scala 5th edition
  • Earlier editions use a different version of Scala than class; you can use them but may need to look up new syntax

Program a Robot in a Grid World

Grid World

Move Robot according to Instructions

  • Mark the robot's initial position on the grid
  • Mark a goal on the grid
  • Create instructions to express how the robot moves
  • Move the robot according to your instructions: fill in the sequential blue arrow instruction list

Program a Robot in a Grid World

Grid World

Move Robot along the Perimeter

  • Place the robot in a corner
  • Use your instructions to express your program
    • The loop has 4 slots for instructions that repeat forever
    • Place your elementary instructions into the slots
  • Move the robot according to your instructions

Computers that Program

  •   Teach a programming language to an LLM (ChatGPT)
    •   Syntax (grammar) of the language
      1. step ::= down n | up n | left n | right n | seq step then step (then step)* | repeat(step)
    •   Semantics of the language
  •   Ask LLM to write a program
    1. Use this language to move the robot in a 5 by 5 grid along the perimeter, starting in the top-left corner.
  • Our programming language has
    •   Parser to turn text into instructions
    •   Interpreter to execute the instructions