Why ('f 1) does deliver nil?

[Johannes]

... instead of an exception?

Can anyone explain it to me?

Johannes

[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

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[Faiz Ahmed Mushtak Halde]

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

[Johannes]

Thank you for the explanation; until now I associated this behavior only to keywords not not symbols.

Johannes

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