Using Sequences in clojure macros

Question

I am reading data from a yaml file that results in data like this:

{:test1 (1 2 3)}

I can lookup the key :test1 and get a clojure.lang.LazySeq with the elements 1 2 3. But when I want to use this data in a macro it gets expanded to a function call and not to a quoted list.

For example:

(defmacro mmerge
  [map1 map2]
  `(assoc ~(merge map1 map2) :merged true))

(mmerge {:test1 (1 2 3)} {:test2 (4 5 6)})

This gets expanded to:

(clojure.core/assoc {:test2 (4 5 6), :test1 (1 2 3)} :merged true)

Is there a possibility to somehow get this to work?

Thanks in advance

why is this a macro?

noisesmith
– noisesmith

2014-11-12 21:20:20 +00:00
Commented Nov 12, 2014 at 21:20 — noisesmith
– noisesmith, Commented Nov 12, 2014 at 21:20

Community · Accepted Answer · 2017-05-23 10:26:26Z

5

You can achieve the same with a function if you write the map arguments' values as quoted lists:

(defn mmerge*
  [map1 map2]
  (assoc (merge map1 map2) :merged true))

(mmerge* {:test1 '(1 2 3)} {:test2 '(4 5 6)})
;= {:merged true, :test2 (4 5 6), :test1 (1 2 3)}

If you still want a macro, you need to quote the result from the merge operation in the form returned by the macro (or as @fl00r mentioned it is absolutely correct if you just quote the lists :P):

(defmacro mmerge
  [map1 map2]
  `(assoc '~(merge map1 map2) :merged true))

Which results in the following macroexpansion:

(clojure.core/assoc '{:test2 (4 5 6), :test1 (1 2 3)} :merged true)

edited May 23, 2017 at 10:26

CommunityBot

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answered Nov 12, 2014 at 21:33

juan.facorro

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