clojure.core/reduce calls (f) if given a reducible coll and no init value
Hi all,
in the following, set is clojure.set, and r is clojure.core reducers.
Why is
user (reduce set/intersection (map set [[1 2] [3 1] [1 3]]))
#{1}
but
user (reduce set/intersection (r/map set [[1 2] [3 1] [1 3]]))
ArityException Wrong number of args (0) passed to: set$intersection
clojure.lang.AFn.throwArity (AFn.java:437)
The clojure.core/reduce docs state that when there's no init value, the
reduce function is called with the first two elements of the coll.
The behavior of calling the reduce function with no arguments to create
an init value is specified for r/reduce, but I'm not using that.
Is that a bug, or should I not expect that clojure.core/reduce works
with reducible collections? (I've not followed which reducer changes
were made in clojure 1.5.1.)
Bye,
Tassilo
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Re: clojure.core/reduce calls (f) if given a reducible coll and no init value
The reason you can't get this to work is that r/map returns a reducible
not a coll for reduce to operate on. I'm not sure of a solution because
I'm not familiar with core.reducers. But reading this:
http://clojure.com/blog/2012/05/08/reducers-a-library-and-model-for-collection-processing.html
informed
me of your problems. Sorry I can only point you in the right direction
rather than provide a solution.
On Friday, January 24, 2014 5:26:11 AM UTC-5, Tassilo Horn wrote:
Hi all,
in the following, set is clojure.set, and r is clojure.core reducers.
Why is
user (reduce set/intersection (map set [[1 2] [3 1] [1 3]]))
#{1}
but
user (reduce set/intersection (r/map set [[1 2] [3 1] [1 3]]))
ArityException Wrong number of args (0) passed to: set$intersection
clojure.lang.AFn.throwArity (AFn.java:437)
The clojure.core/reduce docs state that when there's no init value, the
reduce function is called with the first two elements of the coll.
The behavior of calling the reduce function with no arguments to create
an init value is specified for r/reduce, but I'm not using that.
Is that a bug, or should I not expect that clojure.core/reduce works
with reducible collections? (I've not followed which reducer changes
were made in clojure 1.5.1.)
Bye,
Tassilo
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Re: clojure.core/reduce calls (f) if given a reducible coll and no init value
Jarrod Swart jcsw...@gmail.com writes: The reason you can't get this to work is that r/map returns a reducible not a coll for reduce to operate on. Ah, indeed. I couldn't see the forest for the trees. I'm not sure of a solution because I'm not familiar with core.reducers. This works: (reduce set/intersection (r/foldcat (r/map set [[1 2] [3 1] [1 3]]))) Bye, Tassilo -- -- You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups Clojure group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
Re: clojure.core/reduce calls (f) if given a reducible coll and no init value
Ah cool, thanks for posting your solution! On Friday, January 24, 2014 3:29:49 PM UTC-5, Tassilo Horn wrote: Jarrod Swart jcs...@gmail.com javascript: writes: The reason you can't get this to work is that r/map returns a reducible not a coll for reduce to operate on. Ah, indeed. I couldn't see the forest for the trees. I'm not sure of a solution because I'm not familiar with core.reducers. This works: (reduce set/intersection (r/foldcat (r/map set [[1 2] [3 1] [1 3]]))) Bye, Tassilo -- -- You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups Clojure group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
Re: clojure.core/reduce calls (f) if given a reducible coll and no init value
An interesting question this raises is if there is any sensible way to define (intersection). It would need to behave as an identity element for intersection, so would need to behave as a set (so, (set? (intersection)) = truthy) that contained everything (so, (contains? (intersection) foo) = foo no matter what foo is; (partial contains? (intersection)) = identity). The problem would be what to do with seq? Ideally an infinite seq that will produce any particular value after finite time would be produced, but there's no way to sensibly produce any particular value given the wide variety of constructor semantics, builders, factory methods, things not known to this particular runtime instance but that conceptually exist somewhere, etc.; of course, the seq return is a dummy of sorts anyway since you couldn't really use it sensibly to it might as well just return (range). Printing should likely be overridden to just print (intersection) rather than b0rk the REPL with a neverending stream of integers (or whatever). But then it also subtly violates another property of Clojure set objects: if (= a b), (not (identical? a b)), and (identical? (a-set a) a), then (identical? (a-set b) a) and thus (not (identical? (a-set b) b)). The latter is true under the hypothesis for every real set but would be false for (intersection). Perhaps this is why (intersection) is not supported at this time, even though (union) returns an empty set object, the identity element for the union operation. On Fri, Jan 24, 2014 at 3:34 PM, Jarrod Swart jcsw...@gmail.com wrote: Ah cool, thanks for posting your solution! On Friday, January 24, 2014 3:29:49 PM UTC-5, Tassilo Horn wrote: Jarrod Swart jcs...@gmail.com writes: The reason you can't get this to work is that r/map returns a reducible not a coll for reduce to operate on. Ah, indeed. I couldn't see the forest for the trees. I'm not sure of a solution because I'm not familiar with core.reducers. This works: (reduce set/intersection (r/foldcat (r/map set [[1 2] [3 1] [1 3]]))) Bye, Tassilo -- -- You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups Clojure group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out. -- -- You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups Clojure group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
Re: clojure.core/reduce calls (f) if given a reducible coll and no init value
If I understand you correctly I am in agreement. I don't think you could take this problem to clojure.core.reducers/reduce or fold because the problem is inherently sequential is it not? The reduction is basically (intersection (intersection (intersection A B) C) D). I was curious of this myself, how do I abstract out the order of the (reduce set/intersection ...). I couldn't think of one. Breaking this problem out into 'parallel' units of reduction isn't possible because the problem is dependent on order. Which reducers can't have, or so I think after what I have read today. On Friday, January 24, 2014 3:56:23 PM UTC-5, Cedric Greevey wrote: An interesting question this raises is if there is any sensible way to define (intersection). It would need to behave as an identity element for intersection, so would need to behave as a set (so, (set? (intersection)) = truthy) that contained everything (so, (contains? (intersection) foo) = foo no matter what foo is; (partial contains? (intersection)) = identity). The problem would be what to do with seq? Ideally an infinite seq that will produce any particular value after finite time would be produced, but there's no way to sensibly produce any particular value given the wide variety of constructor semantics, builders, factory methods, things not known to this particular runtime instance but that conceptually exist somewhere, etc.; of course, the seq return is a dummy of sorts anyway since you couldn't really use it sensibly to it might as well just return (range). Printing should likely be overridden to just print (intersection) rather than b0rk the REPL with a neverending stream of integers (or whatever). But then it also subtly violates another property of Clojure set objects: if (= a b), (not (identical? a b)), and (identical? (a-set a) a), then (identical? (a-set b) a) and thus (not (identical? (a-set b) b)). The latter is true under the hypothesis for every real set but would be false for (intersection). Perhaps this is why (intersection) is not supported at this time, even though (union) returns an empty set object, the identity element for the union operation. On Fri, Jan 24, 2014 at 3:34 PM, Jarrod Swart jcs...@gmail.comjavascript: wrote: Ah cool, thanks for posting your solution! On Friday, January 24, 2014 3:29:49 PM UTC-5, Tassilo Horn wrote: Jarrod Swart jcs...@gmail.com writes: The reason you can't get this to work is that r/map returns a reducible not a coll for reduce to operate on. Ah, indeed. I couldn't see the forest for the trees. I'm not sure of a solution because I'm not familiar with core.reducers. This works: (reduce set/intersection (r/foldcat (r/map set [[1 2] [3 1] [1 3]]))) Bye, Tassilo -- -- You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clo...@googlegroups.comjavascript: Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+u...@googlegroups.com javascript: For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups Clojure group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+u...@googlegroups.com javascript:. For more options, visit https://groups.google.com/groups/opt_out. -- -- You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups Clojure group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
Re: clojure.core/reduce calls (f) if given a reducible coll and no init value
On Fri, Jan 24, 2014 at 12:56 PM, Cedric Greevey cgree...@gmail.com wrote: An interesting question this raises is if there is any sensible way to define (intersection). It would need to behave as an identity element for intersection, so would need to behave as a set (so, (set? (intersection)) = truthy) that contained everything (so, (contains? (intersection) foo) = foo no matter what foo is; (partial contains? (intersection)) = identity). The problem would be what to do with seq? Ideally an infinite seq that will produce any particular value after finite time would be produced, but there's no way to sensibly produce any particular value given the wide variety of constructor semantics, builders, factory methods, things not known to this particular runtime instance but that conceptually exist somewhere, etc.; of course, the seq return is a dummy of sorts anyway since you couldn't really use it sensibly to it might as well just return (range). Printing should likely be overridden to just print (intersection) rather than b0rk the REPL with a neverending stream of integers (or whatever). Why should printing be overridden? If I print (range) I don't get (range). Also, contains? returns true if the first argument contains the second, not the second; (partial contains? (intersection)) = (constantly true). But then it also subtly violates another property of Clojure set objects: if (= a b), (not (identical? a b)), and (identical? (a-set a) a), then (identical? (a-set b) a) and thus (not (identical? (a-set b) b)). The latter is true under the hypothesis for every real set but would be false for (intersection). Is this a real property of Clojure's sets or an artifact of their present implementation? Is it something that anything impleneting IPersistentSet has to promise to uphold? -- Ben Wolfson Human kind has used its intelligence to vary the flavour of drinks, which may be sweet, aromatic, fermented or spirit-based. ... Family and social life also offer numerous other occasions to consume drinks for pleasure. [Larousse, Drink entry] -- -- You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups Clojure group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
Re: clojure.core/reduce calls (f) if given a reducible coll and no init value
Intersection is associative and commutative: (intersection A B) = (intersection B A) and (intersection A (intersection B C)) = (intersection (intersection A B) C) = the elements common to all three sets. So it's actually perfectly well-founded for use with reducers, at least in principle, and intersecting A B C D can be parallelized sensibly by parallel intersecting A B and C D and then intersecting the two resulting sets. On Fri, Jan 24, 2014 at 6:43 PM, Jarrod Swart jcsw...@gmail.com wrote: If I understand you correctly I am in agreement. I don't think you could take this problem to clojure.core.reducers/reduce or fold because the problem is inherently sequential is it not? The reduction is basically (intersection (intersection (intersection A B) C) D). I was curious of this myself, how do I abstract out the order of the (reduce set/intersection ...). I couldn't think of one. Breaking this problem out into 'parallel' units of reduction isn't possible because the problem is dependent on order. Which reducers can't have, or so I think after what I have read today. On Friday, January 24, 2014 3:56:23 PM UTC-5, Cedric Greevey wrote: An interesting question this raises is if there is any sensible way to define (intersection). It would need to behave as an identity element for intersection, so would need to behave as a set (so, (set? (intersection)) = truthy) that contained everything (so, (contains? (intersection) foo) = foo no matter what foo is; (partial contains? (intersection)) = identity). The problem would be what to do with seq? Ideally an infinite seq that will produce any particular value after finite time would be produced, but there's no way to sensibly produce any particular value given the wide variety of constructor semantics, builders, factory methods, things not known to this particular runtime instance but that conceptually exist somewhere, etc.; of course, the seq return is a dummy of sorts anyway since you couldn't really use it sensibly to it might as well just return (range). Printing should likely be overridden to just print (intersection) rather than b0rk the REPL with a neverending stream of integers (or whatever). But then it also subtly violates another property of Clojure set objects: if (= a b), (not (identical? a b)), and (identical? (a-set a) a), then (identical? (a-set b) a) and thus (not (identical? (a-set b) b)). The latter is true under the hypothesis for every real set but would be false for (intersection). Perhaps this is why (intersection) is not supported at this time, even though (union) returns an empty set object, the identity element for the union operation. On Fri, Jan 24, 2014 at 3:34 PM, Jarrod Swart jcs...@gmail.com wrote: Ah cool, thanks for posting your solution! On Friday, January 24, 2014 3:29:49 PM UTC-5, Tassilo Horn wrote: Jarrod Swart jcs...@gmail.com writes: The reason you can't get this to work is that r/map returns a reducible not a coll for reduce to operate on. Ah, indeed. I couldn't see the forest for the trees. I'm not sure of a solution because I'm not familiar with core.reducers. This works: (reduce set/intersection (r/foldcat (r/map set [[1 2] [3 1] [1 3]]))) Bye, Tassilo -- -- You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clo...@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+u...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups Clojure group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+u...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out. -- -- You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups Clojure group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out. -- -- You received this message because you are subscribed to the Google Groups Clojure group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to
Re: clojure.core/reduce calls (f) if given a reducible coll and no init value
Good points. But the identity thing is still what gets me. What is the
identity of an intersection?
Like you said it can't be #{}. If you seed an intersection with #{} you
get #{}, so you can't intersect from the empty set. The identity for an
intersection is whatever the common element is, but how would you know that?
On Friday, January 24, 2014 7:03:40 PM UTC-5, Cedric Greevey wrote:
Intersection is associative and commutative: (intersection A B) =
(intersection B A) and (intersection A (intersection B C)) = (intersection
(intersection A B) C) = the elements common to all three sets. So it's
actually perfectly well-founded for use with reducers, at least in
principle, and intersecting A B C D can be parallelized sensibly by
parallel intersecting A B and C D and then intersecting the two resulting
sets.
On Fri, Jan 24, 2014 at 6:43 PM, Jarrod Swart jcs...@gmail.comjavascript:
wrote:
If I understand you correctly I am in agreement. I don't think you could
take this problem to clojure.core.reducers/reduce or fold because the
problem is inherently sequential is it not?
The reduction is basically (intersection (intersection (intersection A B)
C) D).
I was curious of this myself, how do I abstract out the order of the
(reduce set/intersection ...). I couldn't think of one.
Breaking this problem out into 'parallel' units of reduction isn't
possible because the problem is dependent on order. Which reducers can't
have, or so I think after what I have read today.
On Friday, January 24, 2014 3:56:23 PM UTC-5, Cedric Greevey wrote:
An interesting question this raises is if there is any sensible way to
define (intersection). It would need to behave as an identity element for
intersection, so would need to behave as a set (so, (set? (intersection))
= truthy) that contained everything (so, (contains? (intersection) foo) =
foo no matter what foo is; (partial contains? (intersection)) = identity).
The problem would be what to do with seq? Ideally an infinite seq that will
produce any particular value after finite time would be produced, but
there's no way to sensibly produce any particular value given the wide
variety of constructor semantics, builders, factory methods, things not
known to this particular runtime instance but that conceptually exist
somewhere, etc.; of course, the seq return is a dummy of sorts anyway since
you couldn't really use it sensibly to it might as well just return
(range). Printing should likely be overridden to just print
(intersection) rather than b0rk the REPL with a neverending stream of
integers (or whatever).
But then it also subtly violates another property of Clojure set
objects: if (= a b), (not (identical? a b)), and (identical? (a-set a) a),
then (identical? (a-set b) a) and thus (not (identical? (a-set b) b)). The
latter is true under the hypothesis for every real set but would be false
for (intersection).
Perhaps this is why (intersection) is not supported at this time, even
though (union) returns an empty set object, the identity element for the
union operation.
On Fri, Jan 24, 2014 at 3:34 PM, Jarrod Swart jcs...@gmail.com wrote:
Ah cool, thanks for posting your solution!
On Friday, January 24, 2014 3:29:49 PM UTC-5, Tassilo Horn wrote:
Jarrod Swart jcs...@gmail.com writes:
The reason you can't get this to work is that r/map returns a
reducible
not a coll for reduce to operate on.
Ah, indeed. I couldn't see the forest for the trees.
I'm not sure of a solution because I'm not familiar with
core.reducers.
This works:
(reduce set/intersection (r/foldcat (r/map set [[1 2] [3 1] [1
3]])))
Bye,
Tassilo
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Re: clojure.core/reduce calls (f) if given a reducible coll and no init value
No, the identity for intersection is a set that has everything, as
(intersection A Everything) = A no matter what A is.
On Fri, Jan 24, 2014 at 7:38 PM, Jarrod Swart jcsw...@gmail.com wrote:
Good points. But the identity thing is still what gets me. What is the
identity of an intersection?
Like you said it can't be #{}. If you seed an intersection with #{} you
get #{}, so you can't intersect from the empty set. The identity for an
intersection is whatever the common element is, but how would you know that?
On Friday, January 24, 2014 7:03:40 PM UTC-5, Cedric Greevey wrote:
Intersection is associative and commutative: (intersection A B) =
(intersection B A) and (intersection A (intersection B C)) = (intersection
(intersection A B) C) = the elements common to all three sets. So it's
actually perfectly well-founded for use with reducers, at least in
principle, and intersecting A B C D can be parallelized sensibly by
parallel intersecting A B and C D and then intersecting the two resulting
sets.
On Fri, Jan 24, 2014 at 6:43 PM, Jarrod Swart jcs...@gmail.com wrote:
If I understand you correctly I am in agreement. I don't think you
could take this problem to clojure.core.reducers/reduce or fold because the
problem is inherently sequential is it not?
The reduction is basically (intersection (intersection (intersection A
B) C) D).
I was curious of this myself, how do I abstract out the order of the
(reduce set/intersection ...). I couldn't think of one.
Breaking this problem out into 'parallel' units of reduction isn't
possible because the problem is dependent on order. Which reducers can't
have, or so I think after what I have read today.
On Friday, January 24, 2014 3:56:23 PM UTC-5, Cedric Greevey wrote:
An interesting question this raises is if there is any sensible way to
define (intersection). It would need to behave as an identity element for
intersection, so would need to behave as a set (so, (set? (intersection))
= truthy) that contained everything (so, (contains? (intersection) foo) =
foo no matter what foo is; (partial contains? (intersection)) = identity).
The problem would be what to do with seq? Ideally an infinite seq that will
produce any particular value after finite time would be produced, but
there's no way to sensibly produce any particular value given the wide
variety of constructor semantics, builders, factory methods, things not
known to this particular runtime instance but that conceptually exist
somewhere, etc.; of course, the seq return is a dummy of sorts anyway since
you couldn't really use it sensibly to it might as well just return
(range). Printing should likely be overridden to just print
(intersection) rather than b0rk the REPL with a neverending stream of
integers (or whatever).
But then it also subtly violates another property of Clojure set
objects: if (= a b), (not (identical? a b)), and (identical? (a-set a) a),
then (identical? (a-set b) a) and thus (not (identical? (a-set b) b)). The
latter is true under the hypothesis for every real set but would be false
for (intersection).
Perhaps this is why (intersection) is not supported at this time, even
though (union) returns an empty set object, the identity element for the
union operation.
On Fri, Jan 24, 2014 at 3:34 PM, Jarrod Swart jcs...@gmail.comwrote:
Ah cool, thanks for posting your solution!
On Friday, January 24, 2014 3:29:49 PM UTC-5, Tassilo Horn wrote:
Jarrod Swart jcs...@gmail.com writes:
The reason you can't get this to work is that r/map returns a
reducible
not a coll for reduce to operate on.
Ah, indeed. I couldn't see the forest for the trees.
I'm not sure of a solution because I'm not familiar with
core.reducers.
This works:
(reduce set/intersection (r/foldcat (r/map set [[1 2] [3 1] [1
3]])))
Bye,
Tassilo
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Re: clojure.core/reduce calls (f) if given a reducible coll and no init value
Okay, I see now.
Thanks for the Socratic dialogue, at the onset of the day I knew nothing
about core.reducers. I feel fairly conversational now!
On Friday, January 24, 2014 7:44:03 PM UTC-5, Cedric Greevey wrote:
No, the identity for intersection is a set that has everything, as
(intersection A Everything) = A no matter what A is.
On Fri, Jan 24, 2014 at 7:38 PM, Jarrod Swart jcs...@gmail.comjavascript:
wrote:
Good points. But the identity thing is still what gets me. What is
the identity of an intersection?
Like you said it can't be #{}. If you seed an intersection with #{} you
get #{}, so you can't intersect from the empty set. The identity for an
intersection is whatever the common element is, but how would you know that?
On Friday, January 24, 2014 7:03:40 PM UTC-5, Cedric Greevey wrote:
Intersection is associative and commutative: (intersection A B) =
(intersection B A) and (intersection A (intersection B C)) = (intersection
(intersection A B) C) = the elements common to all three sets. So it's
actually perfectly well-founded for use with reducers, at least in
principle, and intersecting A B C D can be parallelized sensibly by
parallel intersecting A B and C D and then intersecting the two resulting
sets.
On Fri, Jan 24, 2014 at 6:43 PM, Jarrod Swart jcs...@gmail.com wrote:
If I understand you correctly I am in agreement. I don't think you
could take this problem to clojure.core.reducers/reduce or fold because
the
problem is inherently sequential is it not?
The reduction is basically (intersection (intersection (intersection A
B) C) D).
I was curious of this myself, how do I abstract out the order of the
(reduce set/intersection ...). I couldn't think of one.
Breaking this problem out into 'parallel' units of reduction isn't
possible because the problem is dependent on order. Which reducers can't
have, or so I think after what I have read today.
On Friday, January 24, 2014 3:56:23 PM UTC-5, Cedric Greevey wrote:
An interesting question this raises is if there is any sensible way to
define (intersection). It would need to behave as an identity element for
intersection, so would need to behave as a set (so, (set? (intersection))
= truthy) that contained everything (so, (contains? (intersection) foo)
=
foo no matter what foo is; (partial contains? (intersection)) =
identity).
The problem would be what to do with seq? Ideally an infinite seq that
will
produce any particular value after finite time would be produced, but
there's no way to sensibly produce any particular value given the wide
variety of constructor semantics, builders, factory methods, things not
known to this particular runtime instance but that conceptually exist
somewhere, etc.; of course, the seq return is a dummy of sorts anyway
since
you couldn't really use it sensibly to it might as well just return
(range). Printing should likely be overridden to just print
(intersection) rather than b0rk the REPL with a neverending stream of
integers (or whatever).
But then it also subtly violates another property of Clojure set
objects: if (= a b), (not (identical? a b)), and (identical? (a-set a)
a),
then (identical? (a-set b) a) and thus (not (identical? (a-set b) b)).
The
latter is true under the hypothesis for every real set but would be
false
for (intersection).
Perhaps this is why (intersection) is not supported at this time, even
though (union) returns an empty set object, the identity element for the
union operation.
On Fri, Jan 24, 2014 at 3:34 PM, Jarrod Swart jcs...@gmail.comwrote:
Ah cool, thanks for posting your solution!
On Friday, January 24, 2014 3:29:49 PM UTC-5, Tassilo Horn wrote:
Jarrod Swart jcs...@gmail.com writes:
The reason you can't get this to work is that r/map returns a
reducible
not a coll for reduce to operate on.
Ah, indeed. I couldn't see the forest for the trees.
I'm not sure of a solution because I'm not familiar with
core.reducers.
This works:
(reduce set/intersection (r/foldcat (r/map set [[1 2] [3 1] [1
3]])))
Bye,
Tassilo
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