clojure.core/with-open

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Type: macro Added: Clojure 1.0 Examples: 4 Runnable: 1

([bindings & body])

bindings => [name init ...]

Evaluates body in a try expression with names bound to the values of the inits, and a finally clause that calls (.close name) on each name in reverse order.

Examples

by gstamp on 2010-08-16 . May have evaluation errors. ▶

;; Opens the file 'myfile.txt' and prints out the contents. The ;; 'with-open' ensures that the reader is closed at the end of the ;; form. ;; ;; Please note that reading a file a character at a time is not ;; very efficient. (with-open [r (clojure.java.io/input-stream "myfile.txt")] (loop [c (.read r)] (if (not= c -1) (do (print (char c)) (recur (.read r))))))

by octopusgrabbus on 2012-11-07 ▶

(defn write-csv-file "Writes a csv file using a key and an s-o-s (sequence of sequences)" [out-sos out-file] (spit out-file "" :append false) (with-open [out-data (io/writer out-file)] (csv/write-csv out-data out-sos)))

by jcburley on 2018-04-03 . May have evaluation errors. ▶

;; Try to read 3 lines of text from a test file and return them (to the REPL) ;; as a list: (with-open [r (clojure.java.io/reader "test-0.txt")] (binding [*in* r] (repeatedly 3 read-line))) ;; The above returns a lazy seq without reading any lines while *in* is bound ;; to the file, resulting in the original *in* (usually stdin) being read. ;; To fix, wrap the body within the *in* binding in (doall ...): (with-open [r (clojure.java.io/reader "test-0.txt")] (binding [*in* r] (doall (repeatedly 3 read-line)))) ;; That ensures the sequence will be fully realized with the binding of *in* ;; still in effect, thus reading all 3 lines from the test file.

by NielsvanKlaveren on 2018-12-07 . May have evaluation errors. ▶

;Process multiple files with a transducer as if it's one file ;Not lazy, but processes a line at a time (defn process-file [t file] (with-open [rdr (io/reader file)] (into [] t (line-seq rdr)))) (defn process-files [t files] (into [] (comp (mapcat (fn [file] (process-file t file)))) files)) (apply + (process-files (comp (map count)) ["test-0.txt" "test-1.txt" "test-2.txt"]))

See also

• clojure.core/slurp (2)
• clojure.java.io/input-stream

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