Why is line-seq returning clojure.lang.Cons instead of clojure.lang.LazySeq?

Question

According to the ClojureDocs entry for line-seq (http://clojuredocs.org/clojure_core/clojure.core/line-seq) and the accepted answer for the Stack question (In Clojure 1.3, How to read and write a file), line-seq should return a lazy seq when passed a java.io.BufferedReader.

However when I test this in the REPL, the type is listed as clojure.lang.Cons. See the code below:

=> (ns stack-question
     (:require [clojure.java.io :as io]))
nil
=> (type (line-seq (io/reader "test-file.txt")))
clojure.lang.Cons
=> (type (lazy-seq (line-seq (io/reader "test-file.txt"))))
clojure.lang.LazySeq

Wrapping up the line-seq call in a lazy-seq call gives a lazy seq, but according to the docs, this shouldn't be necessary: line-seq should return a lazy seq anyway.

Note: Inside the REPL (I'm using nrepl) it seems that lazy seqs get fully realized, so I thought perhaps it was just a quirk of the REPL; however the same problem exists when I test it with Speclj. Plus, I don't think realizing a lazy seq has to do with what is going on anyway.

EDIT: So I went to check the source code after mobyte's answer said there is a lazy seq in the tail of the cons...

1   (defn line-seq
2     "Returns the lines of text from rdr as a lazy sequence of strings.
3     rdr must implement java.io.BufferedReader."
4     {:added "1.0"}
5     [^java.io.BufferedReader rdr]
6     (when-let [line (.readLine rdr)]
7       (cons line (lazy-seq (line-seq rdr)))))

That call to cons would explain why the type of the return value of line-seq is clojure.lang.Cons.

mobyte mobyte · Accepted Answer · 2013-03-03T05:36:45

You don't need "wrap" output Cons because it already has lazy seq as "tail":

(type (line-seq (io/reader "test-file.txt")))
=> clojure.lang.Cons

(type (rest (line-seq (io/reader "test-file.txt"))))
=> clojure.lang.LazySeq

(type (cons 'a (rest (line-seq (io/reader "test-file.txt")))))
=> clojure.lang.Cons

Edit.

Note: Inside the REPL (I'm using nrepl) it seems that lazy seqs get fully realized

Not correct. You can test it:

(with-open [r (io/reader "test-file.txt")] (line-seq r))
=> IOException Stream closed  java.io.BufferedReader.ensureOpen (BufferedReader.java:97)

It's because line-seq returns lazy-seq which is not fully realized and reader is already closed when repl tries to realize result later to print it. But if you realize it explicitly it would give normal result without any exceptions:

(with-open [r (io/reader "/home/mobyte/2")] (doall (line-seq r)))
=> ...  output ...

Why is line-seq returning clojure.lang.Cons instead of clojure.lang.LazySeq?

1 Answers