[Haskell-cafe] Question about constraining functions to particular ADT constructors

[Erik Hesselink]

Hi Román,

Why not just pass Address to deliverPackage and Email to sendEmail,
instead of passing a user? That way each function gets exactly the
information it needs, and there's no need to muck with GADTs and
DataKinds.

Erik

On Thu, Jun 11, 2015 at 6:54 PM, Román González <romanandreg at gmail.com> wrote:
> Hello there,
>
> I've been thinking on different approaches to constraint particular
> functions to a particular constructor of an ADT in order to reduce
> representation of invalid states. Say for example I've this types:
>
> data Address = Address { ... }
> newtype Email = Email String
> data Package = Package { ... }
> data EmailMsg = EmailMsg { ... }
>
> data User
>   = RealUser Address
>    | VirtualUser Email
>
> And I would like to implement two functions:
>
> deliverPackage :: User -> Package -> IO Bool
> sendEmail :: User -> EmailMsg -> IO ()
>
> I would like to constraint both deliverPackage and sendEmail to receive only
> the semantically correct constructor of User.
>
> I know of an approach I could use, that is wrapping each constructor in it's
> own newtype, and create some smart constructor that way, that approach
> works, but I find it rather verbose.
>
> Is there any other well known approach to deal with this scenarios?
>
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