Sunday, December 21, 2008

Principle of least surprise my ass

Let's start off by saying I'm not anti-Ruby. I like Ruby. Ruby is cool. Matz is cool. But, a while back I was wondering, What is a Ruby code block? My feeble curiosity has been revealed for the half-assed dilettantery it is by Paul Cantrell. Mr. Cantrell chases this question down, grabs it by the scruff of the neck, and wrings it out like a bulldog with a new toy. He also rocks on the piano, by the way.

So in fact, there are no less than seven -- count 'em, SEVEN -- different closure-like constructs in Ruby:
  1. block (implicitly passed, called with yield)
  2. block (&b => f(&b) => yield)
  3. block (&b => b.call)
  4. Proc.new
  5. proc
  6. lambda
  7. method

This is quite a dizzing array of syntactic options, with subtle semantics differences that are not at all obvious, and riddled with minor special cases. It's like a big bear trap from programmers who expect the language to just work. Why are things this way? Because Ruby is:

  1. designed by implementation, and
  2. defined by implementation.

Again, neither I nor Mr. P.C. are bashing Ruby. He shows how to pull off some tasty functional goodness like transparent lazy lists later in the article. Thanks to railspikes for the link.

2 comments:

  1. That doesn't look like seven different closure constructs to me. It looks like three different ways to pass and/or call a block, two different ways to create a Proc, then lambda and Method - four different constructs. I didn't know the differences between any them until I read this article, but now it seems fairly clear.

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  2. Discussed on hacker news:

    http://news.ycombinator.com/item?id=1241780

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