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

[Markus Läll]

Right, I see how you mean.

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?
>
> Tom
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-- 
Markus Läll
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