Re: Runtime selection on types while capturing a dictionary?

Andreas Källberg <[email protected]>
Newsgroups gmane.comp.lang.haskell.cafe
Message-ID <[email protected]> 
> On 8 Oct 2024, at 00:14, Tom Ellis <[email protected]> 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)
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