My Patented Lambda Substitute
From: David Steuber (david_at_david-steuber.com)
Date: 01/24/05
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Date: 23 Jan 2005 23:14:44 -0500
OK, I haven't actually applied for a patent...
I was looking for a way to understand lambda better. After looking
through the CLHS and still puzzling a bit over the (lambda ...) =>
#'(lambda ...) expansion, I came up with this:
CL-USER> (defmacro my-lambda (args &body forms)
(let ((anon (gensym)))
`(flet ((,anon ,args ,@forms)) (function ,anon))))
MY-LAMBDA
CL-USER> (mapcar (my-lambda (x) (* 2 x)) '(1 2 3 4 5))
(2 4 6 8 10)
CL-USER> (mapcar (my-lambda (x) (* x x)) '(1 2 3 4 5))
(1 4 9 16 25)
CL-USER> (mapcar #'(my-lambda (x) (* x x)) '(1 2 3 4 5))
; Evaluation aborted
The key wasn't so much lambda the macro vs lambda the symbol. It
really turned out to be the fact that function is a special form.
When function is passed a list who's car is lambda, it returns an
anonymous function that takes arguments matching the lambda-list.
The real lesson is that I don't need lambda to make an anonymous
function or to create a closure but I do need function. Lambda
expands into function. To me, lambda used to look like a recursive
macro.
How's my understanding?
-- An ideal world is left as an excercise to the reader. --- Paul Graham, On Lisp 8.1
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