Clojure get nested map value

Question

So I'm used to having a nested array or map of settings in my applications. I tried setting one up in Clojure like this:

(def gridSettings
  {:width 50
   :height 50
   :ground {:variations 25}
   :water {:variations 25}
   })

And I wondered if you know of a good way of retrieving a nested value? I tried writing

(:variations (:ground gridSettings))

Which works, but it's backwords and rather cumbersome, especially if I add a few levels.

Both mtaka's and dbyrne's answers are common ways to solve this issue. Use whichever strikes your fancy! I would note that get-in makes your intent clearer. — WolfeFan
– WolfeFan, Commented Mar 26, 2013 at 14:37
I think it's time to accept one of these answers. My heart says dbyrne but my head says mtyaka :) — joelittlejohn
– joelittlejohn, Commented May 2, 2013 at 16:07
@joelittlejohn Hard to choose an answer because they're all right and useful and I think people are best served by reading them all! — Joakim Tall
– Joakim Tall, Commented May 6, 2013 at 13:30

mtyaka · Accepted Answer · 2013-03-26 14:06:06Z

40

That's what get-in does:

(get-in gridSettings [:ground :variations])

From the docstring:

clojure.core/get-in
([m ks] [m ks not-found])
  Returns the value in a nested associative structure,
  where ks is a sequence of keys. Returns nil if the key
  is not present, or the not-found value if supplied.

answered Mar 26, 2013 at 14:06

mtyaka

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Comments

dbyrne · Accepted Answer · 2013-03-26 14:42:31Z

31

You can use the thread-first macro:

(-> gridSettings :ground :variations)

I prefer -> over get-in except for two special cases:

When the keys are an arbitrary sequence determined at runtime.
When supplying a not-found value is useful.

edited Mar 26, 2013 at 14:42

answered Mar 26, 2013 at 14:21

dbyrne

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5 Comments

arajek Over a year ago

what will be the result if (:ground gridSettings) is nil?

dbyrne Over a year ago

The result of the whole expression will just be nil. There won't be an exception thrown.

danneu Over a year ago

(:ground nil) ;=> nil. Using -> unless you deliberately need something else also seems to be idiomatic Clojure.

danneu Over a year ago

But -> can be used with or to also nicely return a not-found value. (get-in grid-settings [:ground :variations] "not found!") vs. (-> grid-settings :ground :variations (or "not found!"))

user100464 Over a year ago

You can also use -> when you need predicates that aren't keywords, i.e. when finding the next step on the navigation path requires calling a function.

Ankur · Accepted Answer · 2013-03-26 15:37:30Z

14

Apart from what other answers has mentioned (get-in and -> macro), sometimes you want to fetch multiple values from a map (nested or not), in those cases de-structuring can be really helpful

(let [{{gv :variations} :ground
       {wv :variations} :water} gridSettings]
  [gv wv])

answered Mar 26, 2013 at 15:37

Ankur

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Comments

01es · Accepted Answer · 2017-07-24 12:58:17Z

3

Maps are partial functions (as in not total). Thus, one can simply apply them as functions. Based on the map from the question:

(gridSettings :ground)
;=> {:variations 25}

The result is a map. So, it can be applied again, which results in a very similar (but not backwards) "syntax" as proposed in the question:

((gridSettings :ground) :variations)
;=>25

edited Jul 24, 2017 at 12:58

answered Jul 24, 2017 at 12:51

01es

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