How can I pretty print a PersistentHashMap in Clojure to a string?

Question

How can I pretty print a PersistentHashMap in Clojure to a string? I am looking for something like:

(str (pprint {... hash map here...})

which I can pass around as a String

(str (pprint {:foo 1 :bar 2})) works for me => {:foo 1, :bar 2} — Maurits Rijk, Dec 29 '10 at 16:04

Shantanu Kumar · Answer 1 · 2012-12-08T14:58:37.897

47

(let [s (java.io.StringWriter.)]
  (binding [*out* s]
    (clojure.pprint/pprint {:a 10 :b 20}))
  (.toString s))

Edit: Equivalent succinct version:

(with-out-str (clojure.pprint/pprint {:a 10 :b 20}))

edited Dec 08 '12 at 14:58

answered Dec 29 '10 at 15:59

Shantanu Kumar

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You can also use with-out-str, which cuts out all the binding stuff. – Rayne Dec 29 '10 at 16:04
will this work if other threads output to *out*. Is it threadsafe? – yazz.com Dec 29 '10 at 16:15
1

@Zubair - The binding macro is always thread safe. The new var binding is created on a per thread basis and the root binding is unchanged. See http://clojure.org/vars – Alex Stoddard Dec 29 '10 at 21:28
1

@Rayne Thanks for mentioning. I could not find with-out-str on clojuredocs in the first pass, but can see it there now. So, the updated solution is this: (with-out-str (clojure.pprint/pprint {:a 10 :b 20})) – Shantanu Kumar Dec 30 '10 at 07:05
1

Please post this last comment as an answer - it's easy to miss ;) – bluegray Nov 15 '12 at 19:44
Convenient macro: (defmacro pprint-str [forms] `(with-out-str (clojure.pprint/pprint (do ~forms)))) – Paul Legato Jun 23 '15 at 21:49

score 20 · Answer 2 · answered Aug 12 '15 at 10:32

20

This should help:

(clojure.pprint/write {:a 1 :b 2} :stream nil)

according to clojure.pprint/write documentation

Returns the string result if :stream is nil or nil otherwise.

answered Aug 12 '15 at 10:32

smaant

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Works in ClojureScript. Excellent answer. – Chris Murphy Apr 22 '16 at 19:26

Abhinav Sarkar · Accepted Answer · 2012-03-16T15:25:32.433

16

user=> (import java.io.StringWriter)
java.io.StringWriter
user=> (use '[clojure.pprint :only (pprint)])
nil
user=> (defn hashmap-to-string [m] 
  (let [w (StringWriter.)] (pprint m w)(.toString w)))
#'user/hashmap-to-string
user=> (hashmap-to-string {:a 1 :b 2})
"{:a 1, :b 2}\n"

edited Mar 16 '12 at 15:25

answered Dec 29 '10 at 16:01

Abhinav Sarkar

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The compiler complains "CompilerException java.lang.RuntimeException: Unable to resolve symbol: w in this context". The original post works but Rayne's edits seem to have broken it! – AnnanFay Mar 15 '12 at 11:35

score 9 · Answer 4 · answered Dec 29 '10 at 15:59

9

(pr-str {:a 1 :b 2}) ;; => "{:a 1, :b 2}"

answered Dec 29 '10 at 15:59

mtyaka

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I tried this but I could only get it to work from the REPL. What should I import for this to work from within a program? – yazz.com Dec 29 '10 at 16:09
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pr-str is part of the clojure.core namespace. You shouldn't have to import anything special. – mtyaka Dec 29 '10 at 20:28
2

Yep, you're right, I was doing it wrong. It works now, except (pr-str {:a 1 :b {:a 2 :b 2}}) returns {:a 1, :b {:a 2, :b 2}}. Is there any way to get it to return this with carriage returns in it? – yazz.com Dec 30 '10 at 11:01
`pr-str` is not designed to 'pretty print' (e.g. with newlines). – David J. Oct 28 '15 at 17:23
`prn-str` includes newlines in the returned string. – Jeff Terrell Ph.D. Oct 25 '18 at 16:14

How can I pretty print a PersistentHashMap in Clojure to a string?

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