Hello Chickeneers,
find attached a suggested addition to miscmacros: the thrush operator
family. Apparently it stems from Raymond Smullyan's book on
combinatorial logic "To Mock a Mockingbird", which I haven't read
(yet). I've first learned about it through Clojure and have found it
quite nice for increasing the readability of chained calls. See Debasish
Ghosh's post [1] about it for some background.
As a practical example, consider this code:
(write (fold + 0
(map (lambda (i) (* 2 i))
(iota 10))))
Using the thrush operator it can be written like this:
(->> (iota 10)
(map (lambda (i) (* 2 i)))
(fold + 0)
(write))
which basically expands to the expression above, i.e. it inserts each
expression as the last argument of the following expression. I find it
much easier to read though since the order of evaluation is the order of
appearance in the code. I like to think of it like `let*' as opposed to
nested `lambda':
((lambda (x) ((lambda (y) ((lambda (z) (+ x y z)) 3)) 2)) 1)
versus:
(let* ((x 1) (y 2) (z 3)) (+ x y z))
Much easier to track what's going on there as stuff that belongs
together is visually close to each other.
There's also `->' which inserts each expression as the first argument of
the following one. The difference:
(->> 10 (- 3)) ; => -7
(-> 10 (- 3)) ; => 7
For kicks and fun I also added starred versions (`->*' and `->>*') which
are multi-value aware. Not sure how useful that is and I didn't want to
add it to the default macros to avoid the overhead. But it's good to
have them just for completeness sake!
I guess that's about it. I'm not sure if adding it to miscmacros is a
great idea, might as well just create a `thrush' egg. OTOH we have the
similar `doto' macro in there already. Comments?
Moritz
[1] http://debasishg.blogspot.com/2010/04/thrush-in-clojure.html
Index: miscmacros.scm
===================================================================
--- miscmacros.scm (revision 25904)
+++ miscmacros.scm (working copy)
@@ -9,13 +9,14 @@
define-optionals define-parameter define-enum
ignore-values ignore-errors
ecase
- define-syntax-rule)
+ define-syntax-rule
+ -> ->* ->> ->>*)
(import scheme)
;; No effect -- caller must import these manually.
(import (only chicken
when unless handle-exceptions let-optionals make-parameter
- add1 sub1))
+ add1 sub1 define-for-syntax))
;;; Modify locations, T-like:
@@ -303,4 +304,55 @@
clauses ...
(else (error "no valid case" val))))
+(define-for-syntax (expand-thrush x weave)
+ (let loop ((y (cdr x)) (form (car x)))
+ (if (null? y)
+ form
+ (let ((z (car y)))
+ (loop (cdr y)
+ (weave z form))))))
+
+(define-syntax ->
+ (ir-macro-transformer
+ (lambda (x i c)
+ (expand-thrush
+ (cdr x)
+ (lambda (z form)
+ (cons (car z)
+ (cons form (cdr z))))))))
+
+(define-syntax ->*
+ (ir-macro-transformer
+ (lambda (x i c)
+ (expand-thrush
+ (cdr x)
+ (lambda (z form)
+ `(receive args ,form
+ (apply
+ ,(car z)
+ (append args (list . ,(cdr z))))))))))
+
+(define-syntax ->>
+ (ir-macro-transformer
+ (lambda (x i c)
+ (expand-thrush
+ (cdr x)
+ (lambda (z form)
+ (append z (list form)))))))
+
+(define-syntax ->>*
+ (ir-macro-transformer
+ (lambda (x i c)
+ (expand-thrush
+ (cdr x)
+ (lambda (z form)
+ `(receive args ,form
+ (apply
+ ,(car z)
+ (append (list . ,(cdr z)) args))))))))
+
)
+
+
+
+
Index: tests/run.scm
===================================================================
--- tests/run.scm (revision 0)
+++ tests/run.scm (working copy)
@@ -0,0 +1,22 @@
+(use test srfi-1 miscmacros)
+
+(test 1 (-> 99 (/ 11) (/ 9)))
+
+(test '(1 2 3 4)
+ (->* (values 1 2)
+ (list 3)
+ (append '(4))))
+
+(test 7 (-> 10 (- 3)))
+(test -7 (->> 10 (- 3)))
+
+(test 9 (->> 1 (+ 2) (* 3)))
+
+(test 9 (->> '(1 2 3)
+ (map add1)
+ (fold + 0)))
+
+(test '((foo . 100) (bar . 200))
+ (->>* (values '(foo bar) '(100 200))
+ (map cons)))
+
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