Clojure减少需要val参数来累积向量

Question

I am doing a reduce where I want to accumulate and return a vector. It works fine when done like this:

(reduce
  (fn [[res-vec last-rgb] rgb]
    (let [calc-res (mu/colour-distance-L2 last-rgb rgb)
          accum-results (conj res-vec calc-res)]
      [accum-results rgb]))
  []
  colour-selections)

However when the [] parameter is left out I get Caused by: java.lang.IllegalArgumentException: Don't know how to create ISeq from: java.lang.Long . This occurs because colour-distance-L2 is being given at least one java.lang.Long rather than two vectors as it expects (both vectors of the type [rgb] - for red green and blue).

This is exactly the kind of reduce I am doing.

I do not need to use the 3-arity version of reduce, because colour-selections will always have a size of at least 2. I just put the [] as part of the trial and error of getting past the run-time error. And as it turns out doing this somehow has a good effect.

Is there some way I can use the 2-arity version of reduce here?

Answer 1

You can only use 2-arity reduce provided your reducing function has a signature like (A,A) → A - taking two values of the same type and producing another value of that type.
Yours has a signature of (A,B) → A, where A is a pair [res-vec last-rgb]
and B is a triple [rgb] . If you want to reduce it over a sequence of B, providing the first A to start the sequence is necessary , because the function doesn't know how to make two Bs into an A.

That being said, I'm not quite seeing the need for a reduce here. Assuming res-vec is the interesting value from your reduction it could also be produced by mapping colour-distance over colour-selections and itself less 1 element. Something like

(mapv mu/colour-distance-L2 colour-selections (drop 1 colour-selections))