Returning sub-map with select, not value

[LusciousPear]

Hello! Been a while.

I'm writing some ETL code which basically filters a map by another map. Specter seems right for this. For example:

{:a 
  {:b {:c 4 :d 5} 
   :e {:f 6 :g 7} 
   :h {:i 8 :j 9} }}

filter by [ [:a :b :c] [:a :e :f] ]

 ---> {:a 
        {:b {:c 4}}
         :e {:f 6}}

I think the main thing I'm not understanding is getting select to return a sub-map, instead of the value. For instance (select [ALL :a :b :c]) returns "4" instead of {:a {:b {:c 4}}.

What concept am I missing?

[nathanmarz]

Here's one way to write your function:

(defn filter-map [paths input]
 (reduce
   (fn [m p] (setval p (select-one p input) m))
   {}
   paths
   ))

(filter-map [[:a :b :c] [:a :e :f]]
 {:a 
  {:b {:c 4 :d 5} 
   :e {:f 6 :g 7} 
   :h {:i 8 :j 9} }})
=>  {:a {:b {:c 4}, :e {:f 6}}}

If you precompile those paths this will execute very fast.

[LusciousPear]

Interesting -- I didn't think of building a new map instead of filtering the original. Thanks!