ven vendethiel

Reader Macros in Common Lisp

This post also appears on lisper.in.

Reader macros are perhaps not as famous as ordinary macros. While macros are a great way to create your own DSL, reader macros provide even greater flexibility by allowing you to create entirely new syntax on top of Lisp.

Paul Graham explains them very well in [On Lisp][] (Chapter 17, Read-Macros):

The three big moments in a Lisp expression's life are read-time, compile-time, and runtime. Functions are in control at runtime. Macros give us a chance to perform transformations on programs at compile-time. ...read-macros... do their work at read-time.

Hi, looking for scalac flags?

This gist has been upgraded to a blog post here.

The Learning Haskell guide has been moved to a git repo to enable outside contribution

Find it here: https://github.com/bitemyapp/learnhaskell

Kernel keywords

apply/2
apply/3
cond/1
destructure/2
exit/1
if/2
in/2
match?/2

First and foremost, let's take a look at the following pieces of code. The first one is something you should be rather familiar with, and the second one is also a somewhat familiar idiom these days (at least in languages with higher-order functions):

// Example 1:
30 + 12

// Example 2:
xs.map(f)

	module LoobVerifiedMonoid

	data Loob = Eurt \| Eslaf

	instance Semigroup Loob where
	Eurt <+> _ = Eurt
	Eslaf <+> r = r

	instance Monoid Loob where
	neutral = Eslaf

	import scalaz._

	// A right Kan extension of `F` along itself, constrained by `C`.
	trait RCodensity[C[_], F[_], A] {
	def apply[R:C](k: A => F[R]): F[R]
	}

	object RCodensity {
	implicit def codensityMonad[C[_], F[_]]: Monad[({type f[x] = RCodensity[C,F,x]})#f] =
	new Monad[({type f[x] = RCodensity[C,F,x]})#f] {

	sub trait_mod:<is>(&c, :$curried!) {
	my $arity = &c.arity;
	&c.wrap(-> \|args {
	args.list.elems == $arity
	?? callsame()
	!! &c.assuming(\|args)
	});
	}

	sub foo($a, $b) is curried {

	module DefaultSuperclass

	data Identity' i = Id' i

	class Functor' (f : Type -> Type) where
	map' : (a -> b) -> f a -> f b

	class Functor' f => Applicative' (f : Type -> Type) where
	instance Functor' f where
	map' = ap' . pure'