A short way to update a nested value inside a record in Elm (0.18)

Question

I am looking for a short way to update a nested value inside an entry in Elm (0.18).

In the following example:

person = { name = "Steven", address = { country = "Spain", city = "Barcelona" } } 

I can update person.name to "Steve" using the following expression:

 { person | name = "Steve" } 

However, I am looking for a way to update a nested value. For example, I would like to update person.address.city to Madrid. I tried the following:

 { person | address.city = "Madrid" } { person | address = { address | city = "Madrid" } } { person | address = { person.address | city = "Madrid" } } 

The compiler rejects all of these options. The shortest valid parameter that I see is the following:

 let personAddress = person.address in { person | address = { personAddress | city = "Madrid" } } 

This seems to be too much code to update a nested value. Do you know if there is a better / shorter way to achieve this?

Answer 1

Your last example with let / in syntax is as brief as possible in Elm 0.18 without resorting to additional packages.

As they say, in functional languages ​​you often find the concept of lenses useful for updating nested records. In arturopala / elm-monocle there is an Elm package that provides the ability to create and execute lenses to more accurately obtain and set the values ​​of nested records.

Using this package, you can create lenses that allow you to do such things:

 personWithUpdatedCity = personCity.set "Madrid" person getCityOfPerson = personCity.get person 

The disadvantage is that you must write all the lens wiring code yourself. In Haskell, this wiring can be done by the compiler. At Elm, we don’t have that luxury.

The Elm code required for the above lenses will be as follows:

 addressCityLens : Lens Address String addressCityLens = Lens .city (\cn a -> { a | city = cn }) personAddressLens : Lens Person Address personAddressLens = Lens .address (\ap -> { p | address = a }) personCity : Lens Person String personCity = compose personAddressLens addressCityLens 

As you can see, this is tedious and a lot more code than you can expect to set a nested value. Because of this boredom, you can stick with the let / in example, unless your code uses nested sets everywhere.

There is an older discussion on the topic , which makes it easier to set the value in Elm, but it has been inactive for some time.

Answer 2

If I need to do a lot of such an update, I create a helper

 updateAddress : ( Address -> Address ) -> Model -> Model updateAddress fn m = {m | address = fn m.address } 

and use it for example

 updateAddress (\a -> { a | city = Madrid }) model 

Answer 3

You can create a function that receives Person (which is a type alias for { name: String, address: { city: String, country: String } ) and String and returns an updated record, for example:

 updateCityAdress : Person -> String -> Person updateCityAddress person newCity = { name: person.name, address = { country: person.address.country, city = newCity } > updateCityAddress person "Madrid" > { name = "Steven", address = { country = "Spain", city = "Madrid" } }