Re: Why ('f 1) does deliver nil?
Thank you for the explanation; until now I associated this behavior only to
keywords not not symbols.
Johannes
Am Samstag, 26. Oktober 2019 19:12:11 UTC+2 schrieb Andy Fingerhut:
>
> Both symbols and keywords, when placed inside of an expression in the
> first position, e.g. (:my-keyword my-map), or ('some-symbol my-map), behave
> as functions that "look themselves up" in the map that is the first
> argument, and if that symbol or keyword is a key in that map, the
> associated value is returned from the expression. This is most often used
> for keywords, but it does also work for symbols, which is what is happening
> in your expression. The number `1` is not a map, and both this kind of
> lookup expression, as well as the function `get`, have always returned nil
> when the thing-to-be-looked-up-in is not a map or set (or anything else
> that implements the appropriate lookup interface methods being used in the
> Java implementation under the covers there).
>
> Some other examples of similar behavior:
>
> $ clj
> Clojure 1.10.1
> user=> ('f 1)
> nil
> user=> ('f 1 :not-found)
> :not-found
> user=> ('f {'f 17 :bar 8})
> 17
> user=> ('f {:bar 8} :not-found-value)
> :not-found-value
> user=> (get {'f 17 :bar 8} 'f)
> 17
> user=> (get 1 'f)
> nil
> user=> (get 1 'f :not-found)
> :not-found
>
> Andy
>
>
>
> On Sat, Oct 26, 2019 at 10:05 AM Johannes > wrote:
>
>> ... instead of an exception?
>>
>> Can anyone explain it to me?
>>
>> Johannes
>>
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>>
>>
>> .
>>
>
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Re: Why ('f 1) does deliver nil?
Well, Symbols implement the IFn and if you look at the invoke method, the
arg passed to it is considered to be a map (I guess more like an
environment it expects in which it will try to look for the symbol)
Anyways, since you've passed in a number, the look up in RT.getFrom leads
to a null hence the nil
('f1 {'f1 3}) returns 3 FYI and ('f 1 'not-found) returns 'not-found
On Saturday, October 26, 2019 at 10:35:25 PM UTC+5:30, Johannes wrote:
>
> ... instead of an exception?
>
> Can anyone explain it to me?
>
> Johannes
>
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Re: Why ('f 1) does deliver nil?
Both symbols and keywords, when placed inside of an expression in the first
position, e.g. (:my-keyword my-map), or ('some-symbol my-map), behave as
functions that "look themselves up" in the map that is the first argument,
and if that symbol or keyword is a key in that map, the associated value is
returned from the expression. This is most often used for keywords, but it
does also work for symbols, which is what is happening in your expression.
The number `1` is not a map, and both this kind of lookup expression, as
well as the function `get`, have always returned nil when the
thing-to-be-looked-up-in is not a map or set (or anything else that
implements the appropriate lookup interface methods being used in the Java
implementation under the covers there).
Some other examples of similar behavior:
$ clj
Clojure 1.10.1
user=> ('f 1)
nil
user=> ('f 1 :not-found)
:not-found
user=> ('f {'f 17 :bar 8})
17
user=> ('f {:bar 8} :not-found-value)
:not-found-value
user=> (get {'f 17 :bar 8} 'f)
17
user=> (get 1 'f)
nil
user=> (get 1 'f :not-found)
:not-found
Andy
On Sat, Oct 26, 2019 at 10:05 AM Johannes wrote:
> ... instead of an exception?
>
> Can anyone explain it to me?
>
> Johannes
>
> --
> You received this message because you are subscribed to the Google
> Groups "Clojure" group.
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> Note that posts from new members are moderated - please be patient with
> your first post.
> To unsubscribe from this group, send email to
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> http://groups.google.com/group/clojure?hl=en
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>
> .
>
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Why ('f 1) does deliver nil?
... instead of an exception? Can anyone explain it to me? Johannes -- 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. To view this discussion on the web visit https://groups.google.com/d/msgid/clojure/f2007954-107c-4918-bf0a-6bfd8040ba39%40googlegroups.com.
Why ('f 1) does deliver nil?
... instead of an exception? Can anyone explain it to me? Johannes -- 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. To view this discussion on the web visit https://groups.google.com/d/msgid/clojure/e572919f-3a44-4f1b-8120-06ccc98c2be0%40googlegroups.com.
