Child-Parent Communication in Elm: OutMsg vs Translator vs NoMap Patterns
When your app Elm start growing, you will want to break it apart in smaller pieces to be able to scale. I covered this in another blogpost: Structured TodoMVC example with Elm.
Part if this scaling eventually involves the need of sending a Msg from a module to its parent, like when navigational buttons on a specific view need to send a message to the top-level Router.
After some time I started noticing 3 different patterns for handling this, and I refactored Elm TodoMVC to all those different approach in this repository so you can compare them side by side.
I believe Folkertdev was the first one I saw to write about child-parent communication in Elm, his blogpost explains this approach pretty well.
But, to summarize, you basically return an extra value in your update function. So instead of returning this:
(Model, Cmd Msg)
You return this:
(Model, Cmd Msg, OutMsg)
Then, the parent update function is responsible for handling those. This way the child doesn’t need to know anything about its parent, but the parent needs to know about its child’s OutMsgs.
Another example that uses this approach is elm-datepicker.
The Translator Pattern is very similar to the OutMsg one, but instead of the parent knowing about child’s Msgs types, it is the parent that passes which Msgs will be generated, via a translator. Alex Lew explains his approach much better here.
Basically you have a translator which is a record like this:
type alias TranslationDictionary msg =
The elm-parent-child-update is a library the helps you with child-parent update which seems to follow this pattern.
This is something I noticed I was doing. The basic idea is to avoid doing
Html.map, so instead everybody needs to speak the same language, in other words, your update and view functions will need to return the top-level
With this you will probably have Msgs like
MsgForRouter, etc, so in your View you’d do something like:
button [ onClick (MsgForLogin SignUp) ] 
This is how I first refactored TodoMVC, in fact, first time I saw OutMsg I didn’t understood the reason for it, cause I was not mapping my Msgs.
Check out lightning-talk-app for a bigger example with this approach. Also, this app seems to follow Kris Jenkins’ way of structuring Elm apps, which favors this approach as he separates the Msgs types in a Types.elm file.
The elm-taco library kinda uses a mix of OutMsg and NoMap patterns by having a top-level “taco” which you can send messages to.
While researching and refactoring for those patterns, I noted somethings, which can be advantages or disadvantages depending of your needs:
- On NoMap, the parent’s update function keeps about the same as your app grows, while on OutMsg and Translate the parent’s update function may get very big, as you need to handle each child’s OutMsg (example)
- On OutMsg and Translate the nested modules doesn’t need to import anything from upper parents, making them more encapsulated, it would be easier to extract and publish some sub-module as a library for example
- For NoMap to work your Msgs should live in a separate file from Update, or else you’ll have a dependency loop. This is good cause force you to split things, but bad at the same time if you want to have a single file for each module (Home.elm, Login.elm, Router.elm)
- In NoMap, it is easier to send Msgs anywhere else, but it might be harder to follow all the state changes caused by it.
- As measured at the moment of this writing, for the TodoMVC refactors, the NoMap approach has 546 LOC, OutMsg 561 and Translator 612 if this matters to you
- On NoMap you eventually need to use the
_catch-all case to ignore Msgs from other places you don’t want to handle, so there is less help from the compiler, it cannot tell what you are missing (thanks for @mordrax for pointing that out on elm slack)
- In OutMsg and Translator you can just look at the types or translators to discover which child-parent communications are needed, so the compiler can guide you implementing those, while on NoMap this communication is more implicit
- The translator approach seems to be a good idea for giving your own Msgs to a external component, like elm-autocomplete
- I found the translator pattern difficult to follow with harder to understand error messages from Elm compiler while building it
- If you don’t modify the
(Model, Cmd Msg)standard you can use the fine elm-return library
- Some people consider not having
Html.mapas a good practice to avoid creating “components”
- You can get a lot of benefit from mixing those approaches, for example, you could just avoid
Html.mapfor views, while still use OutMsg for updates, or you could use NoMap only for the top-level Msgs, with OutMsgs down below, while rendering an external Translated component
I believe Child-Parent Communication is often more importante when scaling and doing SPAs, that’s why lots of stuff I found was reading this reddit thread about Scaling Elm Apps:
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