Implications of storing functions in model or msg type

[MartinS]

There have been two other discussions here asking a similar question

Consequences of functions in the model with 0.19?

Hi, I was looking at rtfeldman/elm-sorter-experiment and its Sorter.Dict implementation and noticed that it stores the Sorter function in the type. It seems to be also the case for all Dict implementations that support any key type (all the ones I looked at use a k -> comparable function argument for Dict.empty), for example elm-dictset or any-dict. A known rule of The Elm Architecture is to never put functions in your Model or Msg types, so I wonder what are the exact impacts in 0.19 to do s…

Why functions cannot be part of Msg?

Why functions cannot be part of message? Why not serialize them as javascript object {body: ..., args: [ ... ]}? As an example (plus 77) will be serialized (in javascript) as {body: a => b => a + b, args: [77]}

but I feel that there hasn’t been any comprehensive discussion about the implications of storing functions in the application model or msg type.

From what I’ve read, the following are implications with doing this:

• Potentially harder to understand code (though perhaps not always?)
• It prevents exporting debugger history (regardless if it’s the model or msg containing functions)
• Using == on a data structure containing a function causes a runtime error
• Prevents compiler optimizations (what kind of optimizations does the compiler do that are affected?)
• Breaks hot reloading (at least, it does in create-elm-app but I haven’t tested other tools. Also I’m not sure if this is only a problem for storing functions in the model or the msg type also.)
• It does not break Html.lazy due to lazy using reference equality

Are all these statements true? Are there things I’ve missed?

[brian]

I don’t know about all of them, but at least these are true:

• prevents exporting debugger history (this is why hot reloading breaks too)
• prevents using == on Model values

It may or may not break lazy. If the function changes, for example by being partially applied with different arguments, then the reference is different and the view will be re-rendered.