[Haskell-cafe] Combining RequiredTypeArguments and type class methods?

[Anthony Clayden]

On Sun, Jul 13, 2025 at 11:47:36AM +0100, Tom Ellis wrote:

> >* There is, however, a known trick. This works:
*>


Ah yes. Same solution here

https://discourse.haskell.org/t/how-to-use-requiredtypearguments-with-typeclasse-methods/12187/3


Currently, `RequiredTypeArguments` is documented as " only partially
implemented in GHC". Full specification to be found in proposal #281.


That proposal doesn't give examples of how to write instances for
`sizeOf`, as and when we get Dependent Haskell in full. Will it still
need the type equality trick?


>

>* (But I prefer your [Viktor's] solution: the wrapper.)*

*>*


Yes. That in effect still uses a term-level proxy for the type, as argument
to _sizeOf. For example I could go


    pattern Bool :: Bool

    pattern Bool = undefined

    ... _sizeOf Bool ...
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