Use associated type family for payment key index

Co-authored-by: Mateusz Galazyn <[email protected]>

cardano-api/internal/Cardano/Api/Keys/Mnemonics.hs

@@ -69,14 +69,15 @@ instance Error MnemonicToSigningKeyError where
   prettyError (InvalidAccountNumberError accNo) = "Invalid account number: " <> pretty accNo
   prettyError (InvalidPaymentKeyNoError keyNo) = "Invalid payment key number: " <> pretty keyNo
 
-class ExtendedSigningKeyRole keyrole indexType | keyrole -> indexType where
+class ExtendedSigningKeyRole keyrole where
+  type EskIndex keyrole
   -- | Derive an extended private key of the keyrole from an account extended private key
   deriveSigningKeyFromAccount
     :: AsType keyrole
     -- ^ Type of the extended signing key to generate.
     -> Shelley 'AccountK XPrv
     -- ^ The account extended private key from which to derivate the private key for the keyrole.
-    -> indexType
+    -> EskIndex keyrole
     -- ^ The payment key number in the derivation path (as 'Word32') if applicable for
     -- the given key role, otherwise '()'. First key is 0.
     -> Either Word32 (SigningKey keyrole)