Symbol/keyword implements IFn for lookup but a non-collection argument produces non-intuitive results

Question

('+ 1 2) ;; return 2 because it is treated as (get 1 '+ 2)

Whilst this is "consistent" once you know the lookup behavior, it's confusing for Clojure newbies and it seems to be a non-useful behavior.

Proposal: modify Keyword.invoke() and Symbol.invoke() to restrict first Object argument to instanceof ILookup, Map or IPersistentSet (or null) so that the "not found" behavior doesn't produce non-intuitive behavior.

This should track CLJ-1107.

Comments

Ooh, I ran into this the other day, caught me off guard. An error message like the proposed patch would have been very helpful.

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I would be very concerned that making this existing, working, consistent behavior an error would break existing code (regardless of how strange that code might be considered).

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Having thought about it, you're right that this behavior is consistent and changing it would be not only a breaking change but a pretty deep reversal of expectations.

Answer 1

Comment made by: zjhmale

a patch to fix this issue

Answer 2

Comment made by: zjhmale

for now

user=> ('+ 1 2)
IllegalArgumentException class java.lang.Long is not lookupable: clojure.lang.Symbol.throwNotLookupable (Symbol.java:127)
user=> (:+ 1 2)
IllegalArgumentException class java.lang.Long is not lookupable: clojure.lang.Keyword.throwNotLookupable (Keyword.java:103)

and everything else works fine

Answer 3

Reference: https://clojure.atlassian.net/browse/CLJ-969 (reported by seancorfield)