clojure 2021-03-14 | Slack Archive

lmergen18:03:48

i wasn't able to find anything on this -- how would i go about defining my own custom global vars, such as *assert* ? let's say i want to introduce a *my-assert* that's either set to true or false at compile time (configurable through lein), what would be the way?

lmergen18:03:55

i know i can probably use java system properties, but i was aiming for an approach that's portable between clj and cljs

thegeez18:03:44

@lmergen

thegeez19:03:04

(def ^:dynamic *my-thing*)

lmergen19:03:36

but can i then easily toggle *my-thing* through compile-time flags ?

lmergen19:03:18

i'll want something like

:global-vars {*my-thing* false}

in profiles, in this specific case to enable/disable some instrumentation

thegeez19:03:15

Something like

:injections [(set! my-thing)]

lmergen19:03:31

:thinking_face: ok that works indeed

vemv19:03:27

Leiningen also offers :global-vars precisely for that IMO neither :injections or :global-vars are clean solutions because they don't transparently translate to an official Clojure feature so instead I'd compile (or run) my code while explicitly binding the given var

lmergen21:03:11

how do you explicitly bind your var at compile time though? do you use a compiler option of some sort? let’s say I want to turn on/off some instrumentation that affects defn definitions, how would you approach this without any compiler flags?

vemv21:03:15

let's say that you're app's main ns is called I'd create a ns called my.main. It would look like the following:

(ns my.main
  ;; important! no requires here
  )

(defn -main [& _]
  (binding [*assert* false]
    (require ') ;; compile `` with a specific binding
    ))

This way my.main would be the entrypoint in production. The technique works with AOT too

borkdude19:03:39

@lmergen Not portable, but CLJS offers goog-define for this. I bet you can make it portable using some .cljc though

lmergen19:03:19

yeah I think I’ll do something like that

lilactown22:03:39

I'm trying to figure out when the appropriate time is to use a transducer's single arity

ghadi22:03:46

the completion arity?

lilactown22:03:09

I'm making a thing that is sort of like a stream. it has an "input" function which sets up listeners to other streams and returns the next value, and I want to accept a transducer to transform that. e.g.:

(stream (remove even?) #(inc @counter))

when the stream is initialized, it needs to run the input function and set its initial value. however, if e.g. counter starts at -1, then (remove even?) should reject it

(even? 0)
;; => true

but right now, when the stream doesn't have a value yet, I thought I should use the single-arity of the transducer. but that simply passes the first value to the reducer :thinking_face: meaning that the first value of the stream is 0

lilactown22:03:32

ah, it's meant for completion?

ghadi22:03:31

[] [x] [x input] which arity do you mean?

lilactown22:03:36

[x]

lilactown22:03:48

1-arity

ghadi22:03:12

that's the completion arity

(transduce
  (map inc)
  (fn
    ([sb] (.toString sb))
    ([sb input] (doto sb (.append (str input)))))
  (StringBuilder.)
  [1 2 3 4 5])

ghadi22:03:50

example is illustrative, it has reflective calls

ghadi22:03:27

user=> (transduce (map inc) (completing conj! persistent!) (transient []) [1 2 3 4 5])
[2 3 4 5 6]

ghadi22:03:57

completing is a higher-order function that wraps a completing arity persistent! on an existing reduction function conj!

ghadi22:03:44

completion arity is useful for finalizing something after the "stepping" process is done

lilactown22:03:01

I see

lilactown22:03:22

I guess then perhaps I should follow in the tradition of reduce transduce and allow the consumer to provide an initial value to the stream?

lilactown22:03:53

(stream (remove even?) #(inc @counter) 1)

ghadi22:03:57

dunno

ghadi22:03:21

not all transducible contexts take initial values, nor produce a finalized output. e.g. sequence or chan

seancorfield22:03:10

With next.jdbc and reducible result sets, there's no obvious initial value because you don't know how the rows are going to be processed. Is the user aggregating numbers? Building a new collection? So users always have to provide their own initial value.

2021-03-14

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