(flatten non-sequential) has a surprising result

[J. Pablo Fernández]

Hello Clojurists,

Today I was surprised by the result of (flatten 1) which is '(). I was expecting '(1) or an error. Talking in some other people in #clojure @ clojurians.net, not everybody agrees that '(1) is a good result but that '() is somewhat surprising. Would it be better if it raised an error when the attribute is not sequential?

[Mikera]

On Wednesday, 1 July 2015 12:55:28 UTC+1, J. Pablo Fernández wrote:

Hello Clojurists,

Today I was surprised by the result of (flatten 1) which is '(). I was expecting '(1) or an error. Talking in some other people in #clojure @ clojurians.net, not everybody agrees that '(1) is a good result but that '() is somewhat surprising. Would it be better if it raised an error when the attribute is not sequential?

From an array programming / core.matrix perspective '(1) would be the most logical result.

Consider the logical sequence:

[[[1]]] = 3 dimensional array with elements '(1) 

[[1]] = 2 dimensional array with elements '(1) 

[1] =1 dimensional array with elements '(1) 

1 = 0 dimensional array (or "scalar") with elements '(1) 

I am not saying that this is necessarily the best behaviour to follow for flatten, but it is an analogy worth considering.

[J. Pablo Fernández]

I agree with you Mikera, it also maintains the homogeneity of always returning a sequence but some people disagreed with it, so an error might be better.

--
J. Pablo Fernández <pup...@pupeno.com>
http://pupeno.com

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[icamts]

Hi Pablo,

I think you're right. Have a look at flatten source

(defn flatten

  "Takes any nested combination of sequential things (lists, vectors,

  etc.) and returns their contents as a single, flat sequence.

  (flatten nil) returns an empty sequence."

  {:added "1.2"

   :static true}

  [x]

  (filter (complement sequential?)

          (rest (tree-seq sequential? seq x))))

it is the rest function that causes this behavior and it seems to be just an optimization to avoid filtering the first element of tree-seq that is known to be the whole sequence. A simpler definition of flatten seems to have the behavior you expected.

(defn flatten1 [x] (filter (complement sequential?) (tree-seq sequential? seq x)))

[J. Pablo Fernández]

Yes, reading the source code and trying to understand why rest was being called there is how I came up with this case.

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J. Pablo Fernández <pup...@pupeno.com> (http://pupeno.com)