I'm trying to create a bunch of cookie-cutter functions and stick 'em in a hash. So far, I've got a macro that expands into such a function:
(defmacro make-canned-format-macro (template field-names)
`(function (lambda ,field-names
(apply #'format `(nil ,,template ,,@field-names)))))
(defparameter *cookie-cutter-functions* (make-hash-table))
(setf (gethash 'zoom-zoom *cookie-cutter-functions*)
(make-canned-format-macro "~A-powered ~A" (fuel device)))
(setf (gethash 'delicious *cookie-cutter-functions*)
(make-canned-format-macro "~A ice cream" (flavor)))
(setf (gethash 'movie-ad *cookie-cutter-functions*)
(make-canned-format-macro "~A: A ~A and ~A film" (title star co-star)))
That repetitive setf, gethash, make-canned-format-macro pattern is awfully boilerplate-y, so I tried converting it to a loop:
(loop
for template in '(('zoom-zoom "~A-powered ~A" (fuel device))
('delicious "~A ice cream" (flavor))
('thematic "~A: A ~A and ~A film" (title star co-star)))
do (let ((func-name (car template))
(format-string (cadr template))
(params (caddr template)))
(setf (gethash func-name *cookie-cutter-functions*)
(make-canned-format-macro format-string params))))
Unfortunately, this blows up because make-canned-format-macro is operating on the value PARAMS instead of the value OF params, because it's getting macroexpanded at compile time, not evaluated at runtime. But as I learned when I asked this question, make-canned-format-macro won't work as a function, because it needs to construct the lambda form at compile time. (At least, I think that's what I learned from that? Please tell me I'm wrong about this point! I'd love to have my function-factory be a function, not a macro!)
My current thought is to write a turn-this-list-of-templates-into-make-canned-format-macro-forms macro instead of a loop. Is that the right thing to do (or at least a non-insane thing to do), or is there a better way?
Since you know the arguments at compile/macro-expansion time, you won't need apply:
CL-USER 35 > (defmacro make-canned-format-macro (template field-names)
`(function (lambda ,field-names
(format nil ,template ,@field-names))))
MAKE-CANNED-FORMAT-MACRO
CL-USER 36 > (macroexpand-1 '(make-canned-format-macro "~A-powered ~A" (fuel device)))
(FUNCTION (LAMBDA (FUEL DEVICE) (FORMAT NIL "~A-powered ~A" FUEL DEVICE)))
T
There is also no need to double quote things in a list:
'('(a))
Code like that is very unusual.
Code generation at runtime
The name -macro makes no sense, since it makes a function.
The function needs to generate executable code: either use EVAL or use COMPILE.
CL-USER 56 > (defun make-canned-format-function (template field-names)
(compile nil `(lambda ,field-names
(format nil ,template ,@field-names))))
MAKE-CANNED-FORMAT-FUNCTION
CL-USER 57 > (loop
for (func-name format-string params)
in '((zoom-zoom "~A-powered ~A" (fuel device))
(delicious "~A ice cream" (flavor))
(thematic "~A: A ~A and ~A film" (title star co-star)))
do (setf (gethash func-name *cookie-cutter-functions*)
(make-canned-format-function format-string params)))
NIL
Construction via macros
CL-USER 77 > (defun make-canned-format-function-code (template fields)
`(lambda ,fields
(format nil ,template ,@fields)))
MAKE-CANNED-FORMAT-FUNCTION-CODE
CL-USER 78 > (defmacro def-canned-format-functions (ht description)
`(progn ,@(loop
for (func-name format-string params) in description
collect `(setf (gethash ',func-name ,ht)
,(make-canned-format-function-code format-string params)))))
DEF-CANNED-FORMAT-FUNCTIONS
CL-USER 79 > (pprint
(macroexpand-1
'(def-canned-format-functions
*foo*
((zoom-zoom "~A-powered ~A" (fuel device))
(delicious "~A ice cream" (flavor))
(thematic "~A: A ~A and ~A film" (title star co-star))))))
(PROGN
(SETF (GETHASH 'ZOOM-ZOOM *FOO*)
(LAMBDA (FUEL DEVICE)
(FORMAT NIL "~A-powered ~A" FUEL DEVICE)))
(SETF (GETHASH 'DELICIOUS *FOO*)
(LAMBDA (FLAVOR)
(FORMAT NIL "~A ice cream" FLAVOR)))
(SETF (GETHASH 'THEMATIC *FOO*)
(LAMBDA (TITLE STAR CO-STAR)
(FORMAT NIL "~A: A ~A and ~A film" TITLE STAR CO-STAR))))
In your code you would write at top-level:
(def-canned-format-functions
*foo*
((zoom-zoom "~A-powered ~A" (fuel device))
(delicious "~A ice cream" (flavor))
(thematic "~A: A ~A and ~A film" (title star co-star))))