Random Hacks: Tag Macros

Ruby-style metaprogramming in JavaScript (plus a port of RSpec)

Eric Kidd — Sun, 01 Jul 2007 19:00:00 -0400

Programming in Ruby makes me happy. It’s a lovable language, with a pleasantly quirky syntax and lots of expressive power.

Programming in JavaScript, on the other hand, frustrates me to no end. JavaScript could be a reasonable language, but it has all sorts of ugly corner cases, and it forces me to roll everything from scratch.

I’ve been trying to make JavaScript a bit more like Ruby. In particular, I want to support Ruby-style metaprogramming in JavaScript. This would make it possible to port over many advanced Ruby libraries.

You can check out the interactive specification, or look at some examples below. If the specification gives you any errors, please post them in the comment thread, and let me know what browser you’re running!

How to make Data.Set a monad

Eric Kidd — Thu, 15 Mar 2007 22:29:00 -0400

…and how to fake Lisp macros with Template Haskell

(I wrote this article in response to a comment by sigfpe. You may find it pretty dry reading, unless you want to build domain-specific languages in Haskell. Proceed at your own risk.)

Haskell’s built-in Monad type has some serious limitations. We can fix those limitations using a number of advanced Haskell techniques, including Template Haskell, Haskell’s closest equivalent to Lisp macros.

We can illustrate the limitations of Monad with an example from math. In set theory, we can define a set by specifying how to compute each element:

{ xy : x ∈ {1,2,4}, y ∈ {1,2,4} }

We can read this as, “the set of all xy, where x is one of {1,2,4}, and y is one of {1,2,4}.” To calculate the answer, we first multiply together all the possible combinations:

1×1=1, 1×2=2, 1×4=4, 2×1=2, 2×2=4, 2×4=8, 4×1=4, 4×2=8, 4×4=16

We then collect up the answers, and—because we’re working with sets–we throw away the duplicates:

{1,2,4,8,16}

Can we do the same thing in Haskell? Well, using Haskell’s list monad, we can write:

listExample = do
  x <- [1,2,4]
  y <- [1,2,4]
  return (x*y)

But when we run this, Haskell gives us lots of duplicate values:

> listExample
[1,2,4,2,4,8,4,8,16]

Our problem: We’re using lists (which can contain duplicate values) to represent sets (which can’t). Can we fix this by switching to Haskell’s Data.Set?

import qualified Data.Set as S

-- This doesn't work.
setExample = do
  x <- S.fromList [1,2,4]
  y <- S.fromList [1,2,4]
  return (x*y)

Unfortunately, this code fails spectacularly. A Haskell monad is required to work for any types a and b:

class Monad m where
  return :: a -> m a
  fail :: String -> m a
  (>>=) :: m a -> (a -> m b) -> m b

But Data.Set only works for some types. Specifically, it requires that values of type a can be ordered:

data (Ord a) => Set a = ...

As it turns out, we can make Data.Set into a monad. But be warned: The solution involves some pretty ugly Haskell abuse.

Why Ruby is an acceptable LISP

Eric Kidd — Sat, 03 Dec 2005 11:30:00 -0500

Years ago, I looked at Ruby and decided to ignore it. Ruby wasn’t as popular as Python, and it wasn’t as powerful as LISP. So why should I bother?

Of course, we could turn those criteria around. What if Ruby were more popular than LISP, and more powerful than Python? Would that be enough to make Ruby interesting?

Before answering this question, we should decide what makes LISP so powerful. Paul Graham has written eloquently about LISP’s virtues. But, for the sake of argument, I’d like to boil them down to two things:

LISP is a dense functional language.
LISP has programmatic macros.

As it turns out, Ruby compares well as a functional language, and it fakes macros better than I’d thought.

Why Hygienic Macros Rock

Eric — Fri, 13 Sep 2002 00:00:00 -0400

I've recently been reading a lot of excellent essays on programming language design by Paul Graham. Paul and I agree about a number of things: (1) LISP is beautiful and powerful family of languages, even by modern standards, (2) all existing dialects of LISP are lacking a certain something, and (3) programmatic macros are a Good Idea.