I'm using a hash-table as a cache, and had a problem with ensure-gethash. Some example code / output:
CL-USER> (let ((h (make-hash-table))) (alexandria:ensure-gethash :x h (progn (format T "expensive computation~%") 1)) (alexandria:ensure-gethash :x h (progn (format T "expensive computation~%") 1))) expensive computation expensive computation 1 T CL-USER>
This patch (created using git format-patch origin/master) converts ENSURE-GETHASH to a macro that only evaluates the default if we get to the setf branch. Has a check to create and call a lambda if the default value looks complicated (using CONSTANTP).
After applying my patch, here's the output: CL-USER> (let ((h (make-hash-table))) (alexandria:ensure-gethash :x h (progn (format T "expensive computation~%") 1)) (alexandria:ensure-gethash :x h (progn (format T "expensive computation~%") 1))) expensive computation 1 T CL-USER>
Thoughts?
Thanks,
On Wed, Dec 21, 2011 at 7:30 PM, Ryan Davis ryan@acceleration.net wrote:
This patch (created using git format-patch origin/master) converts ENSURE-GETHASH to a macro that only evaluates the default if we get to the setf branch. Has a check to create and call a lambda if the default value looks complicated (using CONSTANTP).
[...]
Thoughts?
I'd like to see this in alexandria, as it is a very common idiom for me.
-Hans
On 21 December 2011 20:30, Ryan Davis ryan@acceleration.net wrote:
I'm using a hash-table as a cache, and had a problem with ensure-gethash. Some example code / output:
I must be missing something. Why the CONSTANTP and LAMBDA instead of
(defmacro ensure-gethash (key hash-table &optional default) "Like GETHASH, but if KEY is not found in the HASH-TABLE saves the DEFAULT under key before returning it. Secondary return value is true if key was already in the table." `(multiple-value-bind (value ok) (gethash ,key ,hash-table) (if ok (values value ok) (values (setf (gethash ,key ,hash-table) ,default) nil))))
?
Cheers,
-- nikodemus
No, you're not missing anything, your version is better. I just didn't think that through. I was combining a similar function from a personal utils library, and my macro used lambdas to pass the default value to a driver function. I also recently learned about constantp and just got excited :).
Thanks,
Ryan Davis Acceleration.net Director of Programming Services 2831 NW 41st street, suite B Gainesville, FL 32606
Office: 352-335-6500 x 124 Fax: 352-335-6506
On Friday, December 30, 2011 12:41:12 PM, Nikodemus Siivola wrote:
On 21 December 2011 20:30, Ryan Davis ryan@acceleration.net wrote:
I'm using a hash-table as a cache, and had a problem with ensure-gethash. Some example code / output:
I must be missing something. Why the CONSTANTP and LAMBDA instead of
(defmacro ensure-gethash (key hash-table &optional default) "Like GETHASH, but if KEY is not found in the HASH-TABLE saves the DEFAULT under key before returning it. Secondary return value is true if key was already in the table." `(multiple-value-bind (value ok) (gethash ,key ,hash-table) (if ok (values value ok) (values (setf (gethash ,key ,hash-table) ,default) nil))))
?
Cheers,
-- nikodemus
On Tue, Jan 3, 2012 at 3:25 PM, Ryan Davis ryan@acceleration.net wrote:
I also recently learned about constantp and just got excited :).
Talking about constantp: it takes as arguments: form &optional environment.
Constantness can depend on the environment, e.g. if the form is (m) and its definition is like:
(defmacro m (&environment env) (if (foo-p env) :constant-form (non-constant-form)))
Therefore you should always pass on environment within macros, so instead of:
(defmacro ensure-gethash (key hash-table &optional default) `(... ,(if (constantp default) ..)))
use:
(defmacro ensure-gethash (key hash-table &optional default &environment env) `(... ,(if (constantp default env) ..)))
- Willem
On 3 January 2012 16:25, Ryan Davis ryan@acceleration.net wrote:
No, you're not missing anything, your version is better.
Ok, pushed.
Cheers,
-- nikodemus
alexandria-devel@common-lisp.net
-
Hans Hübner -
Nikodemus Siivola -
Ryan Davis -
Willem Broekema