IllegalArgumentException Error in Clojure

Question

I get this error:

IllegalArgumentException Don't know how to create ISeq from: clojure.lang.Symbol clojure.lang.RT.seqFrom (RT.java:542)

when I call this function:

    (defn my-butlast [lista]
      (loop [c lista
             last ()]
        (if (= (count c) 1)
          last
          (recur (concat last (first c))
                 (pop c)))))

The function should return a list with the same elements as its input list but excluding the last element, or '()' if it's empty.

And the error only happens when the list has two or more elements like this:

(my-butlast '(a b))

Chris Murphy · Accepted Answer · 2017-08-21 13:11:19Z

2

concat: "concatenation of the elements in the supplied colls" (taken from https://clojuredocs.org/clojure.core/concat). Your error seems to be consistent with not passing concat the correct argument types. You could try using conj in place of concat, or else wrapping the second argument to concat in a vector.

If you choose conj be sure to understand the different behavior between conj-ing onto a list verses onto a vector: https://clojuredocs.org/clojure.core/conj

edited Aug 21, 2017 at 13:11

answered Aug 21, 2017 at 1:35

Chris Murphy

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Urke · Accepted Answer · 2017-08-21 14:53:26Z

1

I would suggest a diferent approach, a bit simpler in my opinion. Try something like (reverse (rest (reverse list))).

If that's all you need, no need to complicate. If you actually have a reason why you started like that, just read a bit about concat, that's where the error is coming from.

answered Aug 21, 2017 at 14:53

Urke

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Sam · Accepted Answer · 2017-08-21 16:33:36Z

1

As others have said, concat only takes collections.

Here is the offending expression: (concat last (first c))

Have a look at these links as they provide some useful techniques in order to help debug your Clojure code:

REPL DEBUGGING: NO STACKTRACE REQUIRED

Debugging with the Scientific Method - Stuart Halloway

answered Aug 21, 2017 at 16:33

Sam

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