CODEWITHSUNDEEP
elm
Daniil Iaitskov
Daniil Iaitskov

Reputation: 6039

Update Elm record with multiple constructors

Is there a way to modify field for a type with multiple constructors?

type U = S | C { xxx : String }

This is not a record, so it has no fields to update!

5|   let c = C { xxx = "DD"} in case c of C {} -> { c | xxx = "ZZ" } 
                                                    ^
This `c` value is a:

    U

But I need a record!

Upvotes: 0

Views: 190

Answers (1)

glennsl
glennsl

Reputation: 29106

In type U = S | C { xxx : String }, xxx is not a field of U. It is a field of the record contained in C. Those are separate types that can be deconstructed into separate values. And the way to do that is to use case to match the C constructor and bind the contained value to a name (or further deconstruct it) so we can refer to it on the right side of the ->.

But then you also need to handle the possibility of c being S. What should be returned then? Maybe a default? Or maybe you actually want to return a U? I've assumed the former here, but the latter would just be construction values of U as you would anywhere else.

let
    c =
        C { xxx = "DD" }
in
case c of
    C record ->
        { record | xxx = "ZZ" }

    S ->
        -- What to do here?
        { xxx = "default?" }

Upvotes: 1

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