Constructing a map on anonymous function in Clojure

Question

I am just learning the language and I've got a simple question. Why does this work (constructs {:key "value"}):

(#(assoc {} :key %) "value")

But this doesn't:

(#({:key %}) "value")
ArityException Wrong number of args (0) passed to: PersistentArrayMap  clojure.lang.AFn.throwArity (AFn.java:429)

On Python the latter syntax is perfectly valid:

> (lambda v: {'key': v})('value')
{'key': 'value'}

edit: thanks for great answers, it is apparent I need to stop thinking # as equivalent to lambda in Python.

It's a littly hacky but if you want to return an element from an anon fn: #(-> {:key %}) — ClojureMostly
– ClojureMostly, Commented Jun 10, 2016 at 5:28

Nathan Davis · Accepted Answer · 2016-06-09 21:56:34Z

14

#(f %) is expanded by the reader into (fn [%] (f %). Likewise, #({:key %}) is expanded into (fn [%] ({:key %}). The python equivalent of this would be lambda v: {'key': v}(), which has the exact same problem as the Clojure version.

What you are looking for is something equivalent to (fn [v] {:key v}). If you really want to use #(...) notation, you could use #(do {:key %}).

Incidentally, I personally never use #(...). I think it's more difficult to grok (as examples such as this evidence), and is only very slightly more compact than an equivalent fn form. Then there's also the limitation that #(...) forms can not be nested.

edited Jun 9, 2016 at 21:56

answered Jun 9, 2016 at 21:49

Nathan Davis

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ntalbs · Accepted Answer · 2016-06-09 20:21:38Z

5

That is the limitation of #() reader. fn will work fine.

user=> ((fn [x] {:key x}) "value")
{:key "value"}

Please take a look at the document Anonymous function literal (#())

answered Jun 9, 2016 at 20:21

ntalbs

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2 Comments

Nathan Davis Over a year ago

What limitation are you referring to? The only limitation I see from the link you posted is that "#() forms cannot be nested." But that doesn't apply in this situation....

ntalbs Over a year ago

According to the document, #(…) will be translated to (fn [args] (…)). So #({:key %}) will be translated to (fn [x] ({:key x})). This will cause error because {:key x} is in function position and doesn't make sense.

jmargolisvt · Accepted Answer · 2016-06-09 19:44:16Z

0

{:key %} is a PersistentArrayMap. You have it in the "verb position" in your function call. You need a Clojure method of some type in there to avoid that error, as you can see in your first (working) example.

answered Jun 9, 2016 at 19:44

jmargolisvt

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