[Haskell-cafe] Is there a handy refl or refl generator for converting GADT types?

Roman effectfully at gmail.com
Sat Jun 27 15:09:29 UTC 2020

You can define a view

    viewGlobalRef :: GlobalRef s -> Either InstanceName (GlobalRef s')
    viewGlobalRef FromIntegerGlobal     = Right FromIntegerGlobal
    viewGlobalRef FromDecimalGlobal     = Right FromDecimalGlobal
    viewGlobalRef (InstanceGlobal name) = Left name

but that loses the `s ~ Resolved` information in the `InstanceGlobal`
case, so I'd define and use

        :: GlobalRef s
        -> (s ~ Resolved => InstanceName -> r)
        -> (forall s'. GlobalRef s' -> r)
        -> r
    matchGlobalRef globalRef handleInstanceGlobal handleOther =
        case globalRef of
            FromIntegerGlobal   -> handleOther FromIntegerGlobal
            FromDecimalGlobal   -> handleOther FromDecimalGlobal
            InstanceGlobal name -> handleInstanceGlobal name

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