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authorLudovic Courtès <ludo@gnu.org>2014-07-12 17:16:36 +0200
committerLudovic Courtès <ludo@gnu.org>2014-07-12 23:17:53 +0200
commitf62435e2868f5db15cc2f31300630c8ec873dd58 (patch)
tree4c48f36fafd1c02b5d65fb92fd257e3bd0c78dc0
parentc2150d9acece1dcaf54b3183254db4f83a992523 (diff)
downloadpatches-f62435e2868f5db15cc2f31300630c8ec873dd58.tar
patches-f62435e2868f5db15cc2f31300630c8ec873dd58.tar.gz
monads: Fix 'mapm' so that effects happen from left to right.
* guix/monads.scm (mapm): Don't reverse LST, so that items are processed from left to right. Bind the result of 'foldm' and reverse it. * tests/monads.scm ("sequence"): Change 'frob' so it performs its side effect within an 'mlet' body. Adjust call accordingly.
-rw-r--r--guix/monads.scm16
-rw-r--r--tests/monads.scm14
2 files changed, 17 insertions, 13 deletions
diff --git a/guix/monads.scm b/guix/monads.scm
index ec2b7f8b3b..c2c6f1a03d 100644
--- a/guix/monads.scm
+++ b/guix/monads.scm
@@ -209,13 +209,15 @@ monadic value seeded by INIT."
(define (mapm monad mproc lst)
"Map MPROC over LST, a list of monadic values in MONAD, and return a monadic
-list."
- (foldm monad
- (lambda (item result)
- (mlet monad ((item (mproc item)))
- (return (cons item result))))
- '()
- (reverse lst)))
+list. LST items are bound from left to right, so effects in MONAD are known
+to happen in that order."
+ (mlet monad ((result (foldm monad
+ (lambda (item result)
+ (mlet monad ((item (mproc item)))
+ (return (cons item result))))
+ '()
+ lst)))
+ (return (reverse result))))
(define-inlinable (sequence monad lst)
"Turn the list of monadic values LST into a monadic list of values, by
diff --git a/tests/monads.scm b/tests/monads.scm
index 82f4b9989c..ac19d33f93 100644
--- a/tests/monads.scm
+++ b/tests/monads.scm
@@ -166,14 +166,16 @@
(let* ((input (iota 100))
(order '()))
(define (frob i)
- ;; The side effect here is used to keep track of the order in
- ;; which monadic values are bound.
- (set! order (cons i order))
- i)
+ (mlet monad ((foo (return 'foo)))
+ ;; The side effect here is used to keep track of the order in
+ ;; which monadic values are bound. Perform the side effect
+ ;; within a '>>=' so that it is performed when the return
+ ;; value is actually bound.
+ (set! order (cons i order))
+ (return i)))
(and (equal? input
- (run (sequence monad
- (map (lift1 frob monad) input))))
+ (run (sequence monad (map frob input))))
;; Make sure this is from left to right.
(equal? order (reverse input)))))