Clojure: buffered reader for loop

Question

I have a large text file that I want to process in Clojure.
I need to process it 2 lines at a time.

I decided to use a for loop so that I could pull 2 ​​lines for each pass with the following binding (rdr is my reader):

[[line-a line-b] (partition 2 (line-seq rdr))] 

(I would be interested to know other ways to get 2 rows for each iteration of the loop, but that is not the question of my question).

When I try to get the loop to work (using a simpler binding for these tests), I see the following behavior, which I cannot explain:

Why

 (with-open [rdr (reader "path/to/file")] (for [line (line-seq rdr)] line)) 

triggers a thread exception with closure

while

 (with-open [rdr (reader "path/to/file")] (doseq [line (line-seq rdr)] (println line))) 

work?

Answer 1

for is lazy and just returns the head of the sequence, which will ultimately read the data from the file. The file is already closed when the content is printed . you can fix this pu shell for doall

 (with-open [rdr (reader "path/to/file")] (doall (for [line (line-seq rdr)] line))) 

Although this does not affect the sequence.

here is an example function from my misc.clj that lazily closes the file at the end:

 (defn byte-seq [rdr] "create a lazy seq of bytes in a file and close the file at the end" (let [result (. rdr read)] (if (= result -1) (do (. rdr close) nil) (lazy-seq (cons result (byte-seq rdr))))))