2013-01-09
- Introduction to Programming Languages (CS4400/CS5400)

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>>> Introduction to Programming Languages (CS4400/CS5400)

* General plan for how the course will go.

* Administrative stuff.  (Most of the stuff from the web page.)
  >>> http://pl.barzilay.org/ <<<

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

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[[[ PLAI Chapter 1 ]]]

* What defines a language?
  - syntax
  - semantics
  - libraries
  - idioms

* 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:

      if (isExplorer)
        document.onmousemove = explorer_move;
      else
        document.onmousemove = mozilla_move;

    or

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

    or

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

    How many JavaScript programmers will know what this does:

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

  - Compare: a[25]+5                   (Java: exception)
             (+ (vector-ref a 25) 5)   (Racket: exception)
             a[25]+5                   (JavaScript: NaN or undefined)
             a[25]+5                   (Python: exception)
             $a[25]+5                  (Perl: 5)
             a[25]+5                   (C: <<<BOOM>>>)
             a[25]+5                   (ML: not an array ref at all)
    -> syntax is mostly in the cosmetics department.
    -> semantics is the real thing.

* 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).

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