Re: putting 2-element colls into a map: works with vectors, but not with lists?
https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/APersistentMap.java#L24 The implementation assumes you're attempting to conj one of the following 3 things into the hash map: 1. A MapEntry object 2. A vector of the format [key value] 3. A seq of MapEntry objects (commonly created via http://docs.oracle.com/javase/6/docs/api/java/util/HashMap.html#entrySet() and the like) On Monday, June 24, 2013 11:57:46 AM UTC-7, Michael Gardner wrote: On Jun 24, 2013, at 11:14 , Sean Corfield seanco...@gmail.comjavascript: wrote: On Mon, Jun 24, 2013 at 8:49 AM, John Gabriele jmg...@gmail.comjavascript: wrote: Why does `into` fail when the 2-element collections are lists and not vectors? : Because the implementation special cases vectors :) It's the one place where a two element vector is treated like a Map$Entry so that you are not forced to somehow create Map$Entry instances for the key/value pairs. Huh. I had assumed this would work with any 2-element collection, like destructuring. Why not? Performance reasons? -- -- 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: putting 2-element colls into a map: works with vectors, but not with lists?
On Jun 25, 2013, at 15:03 , Michael-Keith Bernard (SegFaultAX) mkbernard@gmail.com wrote: https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/APersistentMap.java#L24 The implementation assumes you're attempting to conj one of the following 3 things into the hash map: • A MapEntry object • A vector of the format [key value] • A seq of MapEntry objects (commonly created via http://docs.oracle.com/javase/6/docs/api/java/util/HashMap.html#entrySet() and the like) If I read you correctly, you're saying the reason 'into et al. can't accept arbitrary two-element collections for adding to a map is that APersistentMap's .cons method does something special with seqable things that aren't vectors. Right? Any insight into why Rich would have written .cons that way, instead of putting that seq-of-MapEntry functionality into a separate method? -- -- 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.
putting 2-element colls into a map: works with vectors, but not with lists?
Why does `into` fail when the 2-element collections are lists and not
vectors? :
~~~
user= (into {} [[:a 1] [:b 2]])
{:a 1, :b 2}
user= (into {} ['(:a 1) '(:b 2)])
ClassCastException clojure.lang.Keyword cannot be cast to
java.util.Map$Entry clojure.lang.ATransientMap.conj (ATransientMap.java:44)
~~~
or even:
~~~
user= (conj {} [:a 1])
{:a 1}
user= (conj {} '(:a 1))
ClassCastException clojure.lang.Keyword cannot be cast to
java.util.Map$Entry clojure.lang.APersistentMap.cons
(APersistentMap.java:42)
~~~
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Re: putting 2-element colls into a map: works with vectors, but not with lists?
On Mon, Jun 24, 2013 at 8:49 AM, John Gabriele jmg3...@gmail.com wrote:
Why does `into` fail when the 2-element collections are lists and not
vectors? :
Because the implementation special cases vectors :)
It's the one place where a two element vector is treated like a
Map$Entry so that you are not forced to somehow create Map$Entry
instances for the key/value pairs.
There are some other quirks around that special casing - for example:
user= (conj {} {:a 1 :b 2})
{:b 2, :a 1}
user= (conj {} '([:a 1] [:b 2]))
ClassCastException clojure.lang.PersistentVector cannot be cast to
java.util.Map$Entry clojure.lang.APersistentMap.cons
(APersistentMap.java:42)
So conj on a map acts like a map-preserving concat since the second
argument can be a map, not just a new key/value pair to add, yet you
cannot conj a sequence of two-element vectors onto a map.
You can see here that maps contain MapEntry elements, not actual two
element vectors:
user= (first {:a 1})
[:a 1]
user= (type (first {:a 1}))
clojure.lang.MapEntry
user= (type [:a 1])
clojure.lang.PersistentVector
Things can get even stranger if you start doing interop between
Clojure and other languages that have slightly different map
implementations...
--
Sean A Corfield -- (904) 302-SEAN
An Architect's View -- http://corfield.org/
World Singles, LLC. -- http://worldsingles.com/
Perfection is the enemy of the good.
-- Gustave Flaubert, French realist novelist (1821-1880)
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Re: putting 2-element colls into a map: works with vectors, but not with lists?
On Monday, June 24, 2013 12:14:56 PM UTC-4, Sean Corfield wrote:
On Mon, Jun 24, 2013 at 8:49 AM, John Gabriele
jmg...@gmail.comjavascript:
wrote:
Why does `into` fail when the 2-element collections are lists and not
vectors? :
Because the implementation special cases vectors :)
It's the one place where a two element vector is treated like a
Map$Entry so that you are not forced to somehow create Map$Entry
instances for the key/value pairs.
There are some other quirks around that special casing - for example:
{snip}
Thanks, Sean!
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Re: putting 2-element colls into a map: works with vectors, but not with lists?
On Jun 24, 2013, at 11:14 , Sean Corfield seancorfi...@gmail.com wrote: On Mon, Jun 24, 2013 at 8:49 AM, John Gabriele jmg3...@gmail.com wrote: Why does `into` fail when the 2-element collections are lists and not vectors? : Because the implementation special cases vectors :) It's the one place where a two element vector is treated like a Map$Entry so that you are not forced to somehow create Map$Entry instances for the key/value pairs. Huh. I had assumed this would work with any 2-element collection, like destructuring. Why not? Performance reasons? -- -- 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.
