On Tuesday, February 28, 2017 at 1:24:32 AM UTC, Aaron Strick wrote:
>
> Another solution, one which developers it seems tend towards, is using
> extensible types to represent the states:
>
> ```
> type alias PersonBase a =
> { a
> | id : Int
> , name : Int
> }
>
> type alias Person =
> PersonBase { address : Address }
>
> type alias HomelessPerson =
> PersonBase {}
> ```
>
> This enables us to give raise decoding errors, and to make impossible
> states impossible. It becomes unwieldy very fast to start creating the
> constructors for when the records grow in size.
>
I think trying to represent sub-types using extensible records in Elm is a
mistake and is going to lead to all sorts of other difficulties in your
code. I explored using extensible records for this purpose in this thread,
and basically came to the conclusion that as elm does not support
sub-typing (and cannot without losing full type inference), they should be
avoided:
https://groups.google.com/forum/#!topic/elm-discuss/NVoWA0ZYZNM
How about decoding to this data structure:
type Person =
Housed { id: Int, name: Int, address: Address }
| Homeless { id: Int, name: Int }
or
type alias Named = { id: Int, name: Int }
type Person =
Housed Named Address
| Homeless Named
?
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