How to read all lines from stdin in Clojure

Question

I'm writing a Brainf*** interpreter in Clojure. I want to pass a program in using stdin. However, I still need to read from stdin later for user input.

Currently, I'm doing this:

$ cat sample_programs/hello_world.bf | lein trampoline run

My Clojure code is only reading the first line though, using read-line:

(defn -main
  "Read a BF program from stdin and evaluate it."
  []
  ;; FIXME: only reads the first line from stdin
  (eval-program (read-line)))

How can I read all the lines in the file I've piped in? *in* seems to be an instance of java.io.Reader, but that only provides .read (one char), .readLine (one line) and read(char[] cbuf, int off, int len) (seems very low level).

possible duplicate of [How to read lines from stdin (\*in\*) in clojure](http://stackoverflow.com/questions/2034059/how-to-read-lines-from-stdin-in-in-clojure) — Shepmaster, Dec 29 '13 at 16:13

score 12 · Answer 1 · edited May 23 '17 at 10:32

12

It's simple enough to read all input data as a single string:

(defn -main []
  (let [in (slurp *in*)]
    (println in)))

This works fine if your file can fit in available memory; for reading large files lazily, see this answer.

edited May 23 '17 at 10:32

Community

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answered Dec 30 '13 at 04:13

JohnJ

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d.j.sheldrick · Accepted Answer · 2013-12-30T08:28:06.943

10

you could get a lazy seq of lines from *in* like this:

(take-while identity (repeatedly #(.readLine *in*)))

or this:

(line-seq (java.io.BufferedReader. *in*))

which are functionally identical.

edited Dec 30 '13 at 08:28

answered Dec 29 '13 at 14:00

d.j.sheldrick

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`line-seq` does exactly that, doesn't it? – xsc Dec 29 '13 at 14:03
Yep. I'll add that to the answer. – d.j.sheldrick Dec 29 '13 at 14:05
The latter doesn't seem to work. `(line-seq *in*)` gives me `ClassCastException clojure.lang.LineNumberingPushbackReader cannot be cast to java.io.BufferedReader`. Have I missed something? – Wilfred Hughes Dec 29 '13 at 23:05
@WilfredHughes yes, you missed the fact that `*in*` needs to be wrapped in a `java.io.BufferedReader`, as shown in the example. – d.j.sheldrick Dec 30 '13 at 08:30

How to read all lines from stdin in Clojure

2 Answers2