I've been away from clojure for a while. Can someone remind my the clojure idiom for the following lispy idiom? I.e., the outer mapcat maps a binary function over two sequences qs and transitions, not concentrically but in parallel, and the inner map iterates a unary function over a list of pairs, while binding label and y for each pair.

(mapcat (fn [x pairs]
              (map (fn [[label y]]
                     [x label (qs y)])
                   pairs))
            qs transitions)

Question about group-by If I want to partition a given sequence into equivalence classes according to a probe function f, I can use (group-by f my-seq). E.g., (group-by first my-seq) will transform [[1 2 3] [10 20 30] [100 200 300] [1 5 6 7]] -> {1 -> [[1 2 3] [1 5 6 7]], 10 -> [[10 20 30]], 100 -> [[100 200 300]]} , In Scala I have a useful varient of this which does not collect the elements such as [1 2 3] , [10 20 30] , but rather applies a given function and collects those values. e.g., (group-map first my-seq rest) will give me a map from the first element of each sequence to list sequence of rests giving me the equivalent of {1 -> [[2 3] [5 6 7]], 10 -> [[20 30]], 100 -> [[200 300]]} Do we have such a cousin of group-by in scala? Of course I can create one in 2 lines, but the 2-line version creates lots of intermediate sequences.

(defn group-map [f seq g]  ;; untested
  (into {} (for [[k vs] (group-by f seq)]
             [k (map g vs)])))

Having these two ways of destructuring:

(fn [{:keys [name surname age]}]
  (str name ", " surname ", " age))

(fn [{:keys [:name :surname :age]}]
  (str name ", " surname ", " age))

I'm surprised to learn that (mapcat f set-of-sequences) returns a list rather than a set even in the case that f returns a set. I filled up memory with a huge list of repeated elements. E.g., (mapcat set #{[1 2 3] [2 3 4] [3 4 5]}) this returns (4 3 5 4 3 2 1 3 2) not #{1 2 3 4 5}

this is weirdly inkeeping with the documentation as (concat (set [1 2 3]) (set [2 3 4])) returns (1 2 3 2 3 4) not #{1 2 3 4}. that's shocking though

shouldn't the concatenation of two sets be the union? not the concatenation of the sets each converted to lists?

Depends on the abstraction. Union of the sets is, well, a union. Concatenation in Clojure operates at the seq level - from its POV, sets are just collections, it doesn't care about the intrinsics.

concat is also lazy. The notion of a set does not really allow for laziness

good point. but still this was a surprising bug in my program. I thought it was an infinite loop. no it was just building a huge seqence of the same thing over and over. I was thinking the semantics were set-like, not sequence like. lesson learned!

yeah, the mental model I believe we are meant to have is "sequence operations take in 'seq-able' things and output a sequence."

for your case, using the transducer form and into might do exactly what you want

(into #{} (mapcat f) set-of-sequences)

Or use clojure.set/union

Are reader tags tagging meta data or the form that the meta data is attached to? 1. |#a ^:a| a b 2. |#a ^:a a| b

I have successfully generated a Java class from clojure code using :gen-class I'm able to export it as a JAR, and use that class from another Java Project now I need to write clojure tests for that generated java class (not the clojure namespace that contains the code the generate the class) in my test namespace I naively (:imported my class (instead of the clj namespace), but its complaining that the class does not exists. It makes sense after all someone needs to compile that clj file into a class and add into the class path whats the best way of achieving this?