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

[Markus Läll]

Got it! I think I've found another way to think about this:
- Void is a type-level empty: you can't even construct it
- () is a value-level empty: you can construct it but the information
content of the value is empty

On Mon, Nov 1, 2021 at 7:04 PM Tom Ellis <
tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:

> 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?
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-- 
Markus Läll
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