[Haskell-cafe] No "fields of ... not initialized" warning with RecordWildCards and Void?

[Tom Ellis]

Ah, I think you really do want () there then, not Void.  Void you
*can* use.  In fact you can use it to make anything!

    absurd :: Void -> a

() you can't really use because you can't use it to make anything that
wasn't there in the first place.

Tom

On Mon, Nov 01, 2021 at 06:58:16PM +0200, Markus Läll wrote:
> My use of Void is similar to your example: it's a polymorphic field that I
> don't want to be able to use sometimes, and with Void I thought I wouldn't
> (at the value level, that is), but as you say I shouldn't be able to
> construct it either (which I do want to do). In short, what I wanted to
> remind myself was that this field is empty, and it seemed a better
> candidate than the unit as the unit I could handle at runtime. But perhaps
> the correct solution is to refactor the type into separate data types
> instead.
> 
> On Mon, Nov 1, 2021 at 4:23 PM Tom Ellis <
> tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
> 
> > On Mon, Nov 01, 2021 at 04:07:02PM +0200, Markus Läll wrote:
> > > Would it make sense to suppress the "fields of ... not initialized" when
> > > the type of the field is Void, or any other type with no data
> > constructors?
> > > As the only way to construct void would be `undefined :: Void`, and the
> > > field already is undefined.
> >
> > It makes the opposite of sense to me.  The warning is there to tell
> > you when you've failed to initialize a field.  Whether you *can't*
> > initialise a field because its type has no values makes no difference.
> > You're still not initialising it!
> >
> > I sometimes use a polymorphic field to indicate whether a constructor
> > can be present or not.  For example
> >
> > data Expr a = Zero | One | Sum Expr Expr | Product a Expr Expr
> >
> > Now `type ProductExpr = Expr ()` is an `Expr` which might contain
> > `Product`s. `type NoProductExpr = Expr Void` is an `Expr` which
> > cannot because I "can't" write a `Product` constructor for it (at
> > least not without getting a warning).
> >
> > I'm curious: how come you're in the situation where you need to fill
> > in product types with `Void` entries?