Intro to CS4400/CS5400

• General plan for how the course will go.

• Administrative stuff. (Mostly going over the web pages.)

  https://pl.barzilay.org/

Intro to Programming Languages

PLAI §1

• •

  Why should we care about programming languages? (Any examples of big projects without a little language?)

• •

  What defines a language?

  • syntax
  • semantics
  • libraries (runtime)
  • idioms (community)
• •

  How important is each of these?

  • •

    libraries give you the run-time support, not an important part of the language itself. (BTW, the line between “a library” and “part of the language” is less obvious than it seems.)

  • •

    idioms originate from both language design and culture. They are often misleading. For example, JavaScript programmers will often write:

    function explorer_move() { doThis(); }
    function mozilla_move() { doThat(); }
    if (isExplorer)
      document.onmousemove = explorer_move;
    else
      document.onmousemove = mozilla_move;

    or

    if (isExplorer)
      document.onmousemove = function() { doThis(); };
    else
      document.onmousemove = function() { doThat(); };

    or

    document.onmousemove =
      isExplorer ? function() { ... }
                : function() { ... };

    or

    document.onmousemove =
      isExplorer ? () => { doThis(); } : () => { doThat(); };

    or

    document.onmousemove = isExplorer ? doThis : doThat;

    How many JavaScript programmers will know what this does:

    function foo(n) {
      return function(m) { return m+n; };
    }

    or this:

    n => m => m+n;
    (x,y) => s => s(x,y);

    or, what seems fishy in this? —

    const foo = (x,y) => bar(x,y)

    Yet another example:

    let x = "";
    while (foo())
      x += whatever();

    How would you expect this code perform? How do you think it does in the reality of many uses of JS by people who are not really programmers?

  • •

    Compare:

    • • a[25]+5 (Java: exception)
    • • (+ (vector-ref a 25) 5) (Racket: exception)
    • • a[25]+5 (JavaScript: exception (or NaN))
    • • a[25]+5 (Python: exception)
    • • $a[25]+5 (Perl: 5)
    • • a[25]+5 (C: BOOM)
    • 🡆 syntax is mostly in the cosmetics department; semantics is the real thing.
  • •

    Another example:

    • • a + 1 > a (Python: always true)
    • • (> (+ a 1) a) (Racket: always true)
    • • a + 1 > a (C: sometimes true)
• •

  How should we talk about semantics?

  • A few well-known formalisms for semantics.

  • We will use programs to explain semantics: the best explanation is a program.

  • Ignore possible philosophical issues with circularity (but be aware of them). (Actually, they are solved: Scheme has a formal explanation that can be taken as a translation from Scheme to logic, which means that things that we write can be translated to logic.)

  • We will use Racket for many reasons (syntax, functional, practical, simple, formal, statically typed, environment).