Dhall Association List

Question

I have a Union type that I wish one field to be an association list.

let Blah = < First : { name : Text, params: XXX } | Second : { name : Text } >

Here, I wish params to be an association list so when I type check Record values it will pass, e.g.

Blah.First { name = "Alex", params: [{ mapKey = "a", mapValue = 1 }] }

So, what type should XXX be in Blah?

Answer 1

The answer depends on the type of value stored in the association list. In the most general case, you can parametrize the Blah type on the type of the mapValue, like this:

let Blah =
        λ(a : Type)
      → < First :
            { name : Text, params : List { mapKey : Text, mapValue : a } }
        | Second :
            { name : Text }
        >

in  (Blah Natural).First
    { name = "Alex", params = [ { mapKey = "a", mapValue = 1 } ] }

If you know the desired mapValue type ahead of time, you can hard-code it instead of making Blah a function of a type. Or, if you plan to use Blah for the same mapValue type multiple times, you can do something like this:

let Blah =
        λ(a : Type)
      → < First :
            { name : Text, params : List { mapKey : Text, mapValue : a } }
        | Second :
            { name : Text }
        >

let Foo = Blah Natural

in  [ Foo.First { name = "Alex", params = [ { mapKey = "a", mapValue = 1 } ] }
    , Foo.Second { name = "John" }
    ]