On Wednesday, February 15, 2017 at 8:49:13 PM UTC, Rupert Smith wrote:
>
> I am trying to build a model with states, such that fields are only
> available that are actually needed in the relevant state. The model is like
> this:
>
> type alias WithPosition a =
> { a | rect : Rectangle }
>
>
> type alias WithValue a =
> { a | value : String }
>
>
> type State
> = Hidden
> | Aware (WithPosition {})
> | Active (WithPosition (WithValue {}))
> | Inactive (WithPosition (WithValue {}))
>
Having played around with this for a while, I don't think this pattern is
worth continuing with.
It is much easier to use non-extensible records and take advantage of
tagged union constructors letting you have >1 argument. Like this:
type State
= Hidden
| Aware WithPosition
| Active WithPosition WithValue
| Inactive WithPosition
(Of course I could just use Rectangle and String directly - but the actual
model I am working with has more fields)
This avoids the need to unpack and repack record types unnecessarily to try
and work around the typing issues.
It has been a learning experience to play around with them, but I think
without an existential qualifier, extensible records are not terribly
useful at all - except for the simple case where you want to write a
function that projects the fields of a record onto a sub set. As
existential qualifier would break type inference and that is too useful -
so extensible records are best avoided.
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