Writing clojure maps/arrays to a file and reading them back

Question

I need to save clojure maps to a file and read them back later to process them.

This is what I could come up with. Is there a better way to accomplish the same thing?

Answer 1

It is easiest to read a single form to and from the file, so I usually put my data into a vector. I also prefer to use pr or pr-str rather than print because it is guaranteed to produce readable data,

(def my-data [{:a "foo" :b [1 2 3]} "asdf" 42 #{1 2 3}]) 
(spit "/tmp/data.edn" (with-out-str (pr my-data)))
nil
(read-string (slurp "/tmp/data.edn"))
[{:a "foo", :b [1 2 3]} "asdf" 42 #{1 2 3}] 

vs:

(spit "/tmp/data.edn" (with-out-str (print my-data)))
(read-string (slurp "/tmp/data.edn"))
[{:a foo, :b [1 2 3]} asdf 42 #{1 2 3}] 

notice how the string `"asdf" was read back as a symbol.

.toString also works fine:

(spit "/tmp/data.edn" (.toString my-data)) 
(read-string (slurp "/tmp/data.edn"))
[{:a "foo", :b [1 2 3]} "asdf" 42 #{1 2 3}] 

Answer 2

Yes - spit and prn-str for writing, slurp and read-string for reading.

user=> (def a [1 2 3 4])
#'user/a
user=> (prn-str a)
"[1 2 3 4]\n"
user=> (spit "stored-array.dat" (prn-str a))
nil

(in a new REPL session)

user=> (require 'clojure.edn)
nil
user=> (slurp "stored-array.dat")
"[1 2 3 4]\n"
user=> (clojure.edn/read-string (slurp "stored-array.dat"))
[1 2 3 4]

I used a vector for that example, but any data (e.g. maps) should work just as well.

Note that there is a read-string in the main Clojure language, but this is explicitly documented as not being safe for untrusted data, so it's better to use the version from clojure.edn.

Answer 3

Take a look at this question that deals with print-dup and serializing forms. It has a better coverage of the trade-offs and security issues involved with each method.