在Common Lisp中使用 List Comprehension

在 Python 和 Haskell中有非常简洁强大的 List Comprehension 用法：

>>> [(x, y) for x in range (1, 4) for y in ["A", "B"]]
[(1, 'A'), (1, 'B'), (2, 'A'), (2, 'B'), (3, 'A'), (3, 'B')]

在 Common Lisp 中可以用 incf-cl 库来实现， 通过 quicklisp 安装很方便：

* (ql:quickload "incf-cl")
To load "incf-cl":
  Load 1 ASDF system:
    cl-ppcre
  Install 1 Quicklisp release:
    incf-cl
; Fetching #<URL "http://beta.quicklisp.org/archive/incf-cl/2010-10-06/incf-cl-20101006-git.tgz">
; 14.63KB
==================================================
14,981 bytes in 0.01 seconds (1044.99KB/sec)
; Loading "incf-cl"
..................................................
[package incf-cl].....
("incf-cl")
* (use-package :incf-cl)
T
* (lc (cons x y) (<- x (range 1 3)) (<- y '(a b)))
((1 . A) (1 . B) (2 . A) (2 . B) (3 . A) (3 . B))

incf-cl 是模仿的Haskell的语法：

Prelude> [(x, y) | x <- [1..3], y <- ["A", "B"]]
[(1,"A"),(1,"B"),(2,"A"),(2,"B"),(3,"A"),(3,"B")]