`(nth nil 10)` returns `nil` when it should throw

Question

The doc string for nth reads:

clojure.core/nth
([coll index] [coll index not-found])
Returns the
value at the index. get returns nil if index out of bounds, nth
throws an exception unless not-found is supplied. nth also works
for strings, Java arrays, regex Matchers and Lists, and, in O(n)
time, for sequences.

The behavior agrees when the coll is [] or '() or other empty collections. However:

user=> (nth [] 10) Execution error (IndexOutOfBoundsException) at user/eval280 (REPL:1). null user=> (nth nil 10) nil

Trying to get the 10th item of [] throws but trying to get the 10th item of nil returns nil.

Answer 1

This is the expected behvaior (older ticket where this was considered is at https://clojure.atlassian.net/browse/CLJ-2029). nth, like get or contains? has polymorphic behavior on nil, and that is as intended. The docstring could probably be clarified.

Comments

OK. I think the docstring should definitely be clarified.

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Having said that:

```
user=> (nth (rest [1]) 10)
Execution error (IndexOutOfBoundsException) at user/eval286 (REPL:1).
null
user=> (nth (next [1]) 10)
nil
```

That there is a huge difference in the behavior of `nth` after taking the `rest` of a sequence and the `next` of a sequence is odd.

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yes. there is a huge difference between nothing and something