How to use RequiredTypeArguments with typeclasse methods?

3

The recommended way, currently, is just to use a separate function:

{-# LANGUAGE RequiredTypeArguments, AllowAmbiguousTypes #-}

import Data.Kind (Type)

class Storable (a :: Type) where
  _sizeOf :: Int
  
instance Storable Int where
  _sizeOf = 8

sizeOf :: forall a -> Storable a => Int
sizeOf a = _sizeOf @a

Source: https://github.com/ghc-proposals/ghc-proposals/pull/267#issuecomment-528449235

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