[Haskell-cafe] Runtime selection on types while capturing a dictionary?

[Andreas Källberg]

> On 8 Oct 2024, at 00:14, Tom Ellis <tom-lists-haskell-cafe-2023 at jaguarpaw.co.uk> wrote:
>    data PossiblyRootedTree lengths labels where
>        RootedTree :: AugmentedTree [NodeId] lengths labels
>        UnrootedTree :: AugmentedTree () lengths labels

Isn’t that invalid syntax? Wouldn’t you need either

data PossiblyRootedTree lengths labels where
       RootedTree :: AugmentedTree [NodeId] lengths labels -> PossiblyRootedTree lengths labels 
       UnrootedTree :: AugmentedTree () lengths labels  -> PossiblyRootedTree lengths labels

or simply (avoiding GADT syntax altogether):

data PossiblyRootedTree lengths labels =
       RootedTree (AugmentedTree [NodeId] lengths labels)
       | UnrootedTree (AugmentedTree () lengths labels)