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

9 Oct 2024


      On Wed, Oct 09, 2024 at 09:33:30AM +0200, Andreas Källberg wrote:
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On 8 Oct 2024, at 00:14, Tom Ellis  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)
Yes, you're right, thanks (and I'm not sure why I thought that
required a GADT).

Tom