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[ASYNC-40] do not transform forms that have :no-transform metadata attached Created: 29/Nov/13  Updated: 05/May/14

Status: Open
Project: core.async
Component/s: None
Affects Version/s: None
Fix Version/s: None

Type: Enhancement Priority: Minor
Reporter: David Nolen Assignee: Unassigned
Resolution: Unresolved Votes: 3
Labels: None


Interaction between core.match clauses and the core.async transform is undesirable. core.async should respect some hook so that some forms are left alone. For example:

(match [x y]
  [1 2] ...
  [3 4] ...)

All code generated for [1 2] and [3 4] would have this metadata attached to it.

Comment by Rich Hickey [ 30/Nov/13 9:25 AM ]

Can you be more specific? This seems like a bad idea, and I wonder why it's desired.

Comment by David Nolen [ 30/Nov/13 2:12 PM ]

To show how bad the interaction is between core.async and a library like core.match that also generates a lot of code consider the following:

(defn foo [e]
  (match [e]
    [{:type :mouse :client-x x :client-y y}] [x y]
    [{:type :mouse :page-x x :page-y y}] [x y]
    [{:type :touch :page-x x :page-y y}] [x y]
    [{:type :key :char-code c}] c))

Without core.async, core.match produces a reasonable amount of code for this typical use of core.match to efficiently match maps - ~230 lines of pretty printed JavaScript.

But if the user wraps this typical expression in a go block:

(defn foo [in]
  (go (while true
        (let [[e c] (<! in)]
          (match [e]
            [{:type :mouse :client-x x :client-y y}] [x y]
            [{:type :mouse :page-x x :page-y y}] [x y]
            [{:type :touch :page-x x :page-y y}] [x y]
            [{:type :key :char-code c}] c)))))

It generates nearly 4200 lines of pretty-printed JavaScript. I fail to see the value of core.async transforming the optimized conditionals generated by core.match, it just generates 18X more code and the extra generated code is obviously useless - a user is matching a value, they cannot put arbitrary computations in the patterns.


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