re-frame/docs/coeffects.md

Introduction

This tutorial explains coeffects.

It explains what they are, how they help, how they can be "injected", and how to manage them in tests.

Table Of Contexts

• Introduction
• Table Of Contexts
• Coeffects
  • What Are They?
  • An Example
  • Let's Fix It
  • How Are Coeffect Babies Made?
  • So, Next Step
  • coeffect the function
  • Other Example Uses of coeffects
  • Meet reg-cofx
  • Examples
  • The 4 Point Summary
  • Secret Interceptors
  • Testing

Coeffects

What Are They?

coeffects are the data resources that an event handler needs to perform its computation.

Because the majority of event handlers only need db and event, there's a specific registration function, called reg-event-db, which delivers these two coeffects as arguments to an event handler, making this common case easy to program.

But sometimes an event handler needs other data inputs to perform its computation. Things like a random number, or a GUID, or the current datetime. It might even need access to a DataScript connection.

An Example

This handler obtains data directly from LocalStore

(reg-event-db
   :load-defaults
   (fn [coeffects _]
     (let [val (js->clj (.getItem js/localStorage "defaults-key"))]  ;; <-- Problem
       (assoc db :defaults val))))

Because it has accessed LocalStore, this event handler is not pure, and impure functions cause well-documented paper cuts.

How We Want It

Our goal in this tutorial is to rewrite this event handler so that data only comes from the arguments.

Our first change is to start using reg-event-fx (instead of reg-event-db).

Then we'll seek to have ALL the necessary extra data available in the first argument, typically called coeffects.

Previous tutorials have show us that we can obtain :db from coeffects. Well, not we want it to contain other useful data too.

(reg-event-fx                     ;; note: -fx 
   :load-defaults
   (fn [cofx event]                 ;; cofx means coeffects
     (let [val (:local-store cofx)  ;; <-- get data from cofx
           db  (:db cofx)]          ;; <-- more data from cofx
       {:db (assoc db :defaults val))})) ;; returns an effect

If we can find a way to achieve this, then we are back to writing pure event handlers.

But what must we do to data into cofx? How do we organise for it to contain a :local-store key, with the right value?

How Are Coeffect Babies Made?

Well, when two coeffects love each other very much ... no, stop ... this is a G-rated framework. Instead ...

Each time an event handler is executed, a brand new context is created, and within that context is a brand new :coeffect map, which is initially totally empty.

That pristine context value (containing a pristine :coeffect map) is then threaded through a chain of Interceptors before it is finally handled to our event handler which will be sitting on the end of chain, itself wrapped up in an interceptor. We know
this story well from a previous tutorial.

So, all members of the Interceptor chain have the opportunity to add to :coeffects via their :before function. This is where :coeffect gets made. This is where new keys are added to :coeffect, so that later our handler magically finds the right data in its parameter.

So, Next Step

If Interceptors put data in :coeffect, then we'd better put the right ones on our handler when we register it.

This handler is the same as before, except for one addition:

(reg-event-fx                     
   :load-defaults
   [ (inject-cofx :local-store "defaults-key") ]     ;; <-- this is new
   (fn [cofx event]         
     (let [val (:local-store cofx)
           db  (:db cofx)]  
       {:db (assoc db :defaults val))})) 

So we've added one Interceptor. It will inject the right value into context's :coeffeects and that :coeffects ends up being the first parameter to our handler.

inject-cofx

inject-cofx is part of re-frame API.

It is a function which returns an Interceptor whose :before function loads a value into a context's :coeffect map.

inject-cofx takes either one or two arguments. The first is always the id of the coeffect required (called a cofx-id). The 2nd is an optional addition value. .

So, in the case above, the cofx-id was :local-store and the additional value was "defaults-key" which was presumable the place to look in LocalStore

More inject-cofx

Here's some other examples of its use:

• (inject-cofx :random-int 10)
• (inject-cofx :guid)
• (inject-cofx :now)

So, if I wanted to, I could create an event handler which has access to 3 coeffects:

(reg-event-fx 
    :some-id 
    [(inject-cofx :random-int 10) (inject-cofx :now)  (inject-cofx :local-store "blah")]  ;; 3
    (fn [cofx _]
       ... in here I can access cofx's keys :now :local-store and :random-int)) 

Creating 3 coeffects for the one handler is probably just showing off, and not generally necessary.

And so to the final piece in the puzzle. How does inject-cofx know what to do when it is given :now or :local-store ? Each cofx-id requires a different action.

Meet reg-cofx

This function allows you associate acofx-id (like :now or :local-store) with a handler function that you supply.

The handler function you register for a given cofx-id will be passed two arguments:

• a :coeffects map, and
• the optional value supplied and it is expected to return a modified :coeffects map, presumably with an added key and value.

Examples

Above we wrote an event handler that wanted :now data to be available. Here is how a handler could be registered for :now:

(reg-cofx               ;; using this new registration function
   :now                 ;; what cofx-id are we registering
   (fn [cofx _]    ;; second parameter not used in this case
      (assoc cofx :now (js.Date.))))   ;; add :now key, with value

And then there's this example:

(reg-cofx               ;; new registration function
   :local-store 
   (fn [coeffects local-store-key]
      (assoc coeffects 
             :local-store
             (js->clj (.getItem js/localStorage local-store-key))))

With these two registrations in place, I can now use (inject-cofx :now) and (inject-cofx :local-store "blah") in an effect handler's interceptor chain.

The 4 Point Summary

Here's the overall picture, summarised, in note form ...

1. Event handlers should only source data from their arguments
2. So we have to "inject" required data into coeffect argument
3. So we use (inject-cofx :key) interceptor in registration of the event handler
4. There has to be a coefx handler registered for that :key (using reg-cofx)

Secret Interceptors

In a previous tutorial we learned that reg-events-db and reg-events-fx add Interceptors to front of any chain during registration.

We found they inserted an Interceptor called do-fx. I can now reveal that they also add (inject-cofx :db) at the front of each chain. (Last surprise, I promise)

Guess what that adds to the :coeffects of every event handler?

Testing

During testing, you may want to stub out certain