Hi all.

Anyone up for a little assistence in diagnosing a NPE thrown when trying to build an associative tree from a list of paths? Specifically in fairly simple code

(defn as-tree []
  (reduce
    (fn [the-map row]
      (let [[prod-type class sub-class] row]
      (assoc
        the-map
        prod-type
        (assoc

          (the-map prod-type)
          
class
          (conj ((the-map prod-type) class) sub-class)))))
    
{}
    [["a" "b" "c"][["a" "b" "d"]]))

@paulspencerwilliams: (the-map prod-type) could return nil, you can’t invoke nil like you are in ((the-map prod-type) class)

@dnolen: cheers. I know it CAN, but didn't think it would for this trivial list of lists. I assume the outer assoc will have added "a" to the map by the time I'm trying to add b as a child.

@paulspencerwilliams: that would only happen if the map is mutable, but it isn't

@dnolen: I thought that, but it works for the first, but not second level if that makes sense

even so that wouldn’t be the evaluation order, inner expressions will always evaluate before outer ones in this case

so this approach using nested assoc is not viable?

@paulspencerwilliams: I’m only suggesting why the current implementation does not work

dnolen: understood and thank you. Obviously, nil checks would be a kludge. Would building the maps upwards (leaves first) be more idiomatic / reliable?

@paulspencerwilliams: in this case Clojure already provides all the functions to accomplish what you want, but it’s not clear to me that’s what you’re looking for?

dnolen: I'm trying to convert [["a" "b" "c"]["a" "b" "d"]] into {"a" {"b" ["c" "d"]}}

yes I know

Just trying to gauge how much of the solution you want.

Okay, in the overall, I'm looking for an idiomatic way of acheving it, but in the short term I wondered why my solution wouldn't work.

right but I already explained why your solution can’t work

maps are not mutable, your notion of evaluation order is not the right one

yep, I understand the mutable consequences. That's why I thought approaching it in reverse might work. In effect, the inner loop returning lists, the middle loop associng it to a map and the top level combining these maps. Apologies for my vocabulary, I'm still learning the terms and functions that clojurians use / provide.

if you build your values inside out, that would work

dnolen: yep.

you do need to add logic for the initial vector

yes, the first time a subtype of "b" is found?

I would try this and then look at how update-in combined with fnil handles all of this for you.

@paulspencerwilliams: yep

dnolen: I will. Thanks for the help. I really enjoy writing in Clojure even though / especially because it warps the mind of someone with 15 years of imperative coding experience.

I have a button that :onClick calls a function, which sends http/get to the server. This returns a PDF.

I'd like to open this PDF in a new window. I can do this with a straight-up link, but then I can't pass a parameter to the server. How can I replicate the [:a {:href "/make-pdf" :target "_blank"} "make it!"]] with my button/function?

(js/window.open response) opens a new window, but this just mangles an odd string.

Never mind - I was able to do it by just wrapping the link in a button. No function call necessary. I'll leave Cljs I/O for another day...