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+# ADR-016: Backwards and Forwards Compatibility
+
+**Status:** Proposed
+
+**Date:** 2025-10-07
+
+## Context
+
+We are working on [a new API](./ADR-004-Support-only-for-mainnet-and-upcoming-eras.md) for `cardano-api` that is centred around the `Era era` type. We would like to maintain backwards compatibility with the "old" api under two conditions:
+
+1) Existing exposed functionality that uses the "old" api which may be in use.
+2) Any functionality needed to enable QA's ability to test hardforking from Byron to the current era.
+
+## Proposed Solution
+
+The solution proposed below is specific to certificates but can be expanded to any era depdendent type.
+
+Our "old" Certificate definition is as follows:
+
+```haskell
+data Certificate era where
+  -- Pre-Conway
+  ShelleyRelatedCertificate
+    :: Typeable era
+    => ShelleyToBabbageEra era
+    -> Ledger.ShelleyTxCert (ShelleyLedgerEra era)
+    -> Certificate era
+  -- Conway onwards
+  ConwayCertificate
+    :: Typeable era
+    => ConwayEraOnwards era
+    -> Ledger.ConwayTxCert (ShelleyLedgerEra era)
+    -> Certificate era
+``` 
+
+`Cardano.Api.Certificate` exposes several helper functions that use this "old" type. E.g `makeStakeAddressDelegationCertificate`, `makeStakeAddressRegistrationCertificate` and `makeStakeAddressUnregistrationCertificate` to name a few. All of these "old" functions will be deprecated in favour of a new API that allows us to expose functionality from any era required without adding significant boilerplate as seen in the above definition of `data Certificate era`. We achieve this by the following:
+
+
+Replace era-variant GADTs (like Certificate era) with a uniform wrapper (Certificate :: L.TxCert era -> Certificate era) and using open type families to resolve era-specific types (e.g., Delegatee era). Era constraints (IsEra, IsShelleyBasedEra) gate access to functionality.
+
+I will make this clearer with an example below.
+
+**NB**: We will only implement functionality constrained by `IsShelleyBasedEra era` and `IsEra era` that either is currently exposed by the "old" api or specifically required by QA. If we do not need backwards compatibility then we will use`Era era` or `IsEra era` without implementing an additional type synonym family.
+
+
+Let's use `makeStakeAddressDelegationCertificate` as an example.
+
+```haskell
+data StakeDelegationRequirements era where
+  StakeDelegationRequirementsConwayOnwards
+    :: ConwayEraOnwards era
+    -> StakeCredential
+    -> Ledger.Delegatee
+    -> StakeDelegationRequirements era
+  StakeDelegationRequirementsPreConway
+    :: ShelleyToBabbageEra era
+    -> StakeCredential
+    -> PoolId
+    -> StakeDelegationRequirements era
+
+makeStakeAddressDelegationCertificate :: StakeDelegationRequirements era -> Certificate era
+makeStakeAddressDelegationCertificate = \case
+  StakeDelegationRequirementsConwayOnwards cOnwards scred delegatee ->
+    conwayEraOnwardsConstraints cOnwards $
+      ConwayCertificate cOnwards $
+        Ledger.mkDelegTxCert (toShelleyStakeCredential scred) delegatee
+  StakeDelegationRequirementsPreConway atMostBabbage scred pid ->
+    shelleyToBabbageEraConstraints atMostBabbage $
+      ShelleyRelatedCertificate atMostBabbage $
+        Ledger.mkDelegStakeTxCert (toShelleyStakeCredential scred) (unStakePoolKeyHash pid)
+
+-- The `StakeDelegationRequirements era` GADT will necessitate more data constructors if there are any changes to the requirements of stake delegation in future eras. 
+-- This is already seen in the transition from Babbage (can only delegate to stake pools) -> Conway (can delegate to stake pools and dreps). 
+-- This obviously does not scale in the face of possible incoming changes and requires the propagation of an additional data constructor.
+
+-- I propose the following:
+
+-- Already exists in Cardano.Api.Experimental.Tx.Internal.Certificate
+data Certificate era where
+  Certificate :: L.EraTxCert era => L.TxCert era -> Certificate era
+
+-- Type family definition capturing the change of possible delegatees. We can now delegate to stake pools or dreps or delegate
+-- to dreps and stake pools simultaneously. This is captured in the `Ledger.Delegatee` type. 
+type family Delegatee era where
+  Delegatee ConwayEra = Ledger.Delegatee
+  Delegatee BabbageEra = Api.Hash Api.StakePoolKey
+  Delegatee AlonzoEra = Api.Hash Api.StakePoolKey 
+  ... 
+  ...
+  Delegate ShelleyEra = Api.Hash Api.StakePoolKey
+
+makeStakeAddressDelegationCertificate
+  :: forall era
+   . IsEra era
+  => StakeCredential
+  -> Delegatee era
+  -> Certificate (LedgerEra era)
+makeStakeAddressDelegationCertificate sCred delegatee =
+  case useEra @era of
+    e@ConwayEra ->
+      obtainCommonConstraints e $
+        Certificate $
+          Ledger.mkDelegTxCert (Api.toShelleyStakeCredential sCred) delegatee
+
+
+--- BELOW IS IN A SEPARATE "COMPATIBLE" MODULE DESIGNED FOR BACKWARDS COMPATIBILITY.
+
+makeStakeAddressDelegationCertificate
+  :: forall era
+   . IsShelleyBasedEra era
+  => StakeCredential
+  -> Delegatee era
+  -> Certificate (LedgerEra era)
+makeStakeAddressDelegationCertificate sCred delegatee =
+  case shelleyBasedEra @era of
+    ...
+
+-- All functions using the old Certificate era GADT will be marked {-# DEPRECATED #-}
+
+``` 
+
+We remove entirely the need to define a type like `StakeDelegationRequirements era`. The type synonym family is now responsible for resolving the era dependent type.
+
+## Decision
+
+The ADR gets adopted in `cardano-api`.
+
+## Consequences
+
+- An additional type synonym family per era dependent type.
+- Removal of all the "old" api's data definitions that capture the notion of backwards compatibility.
+- Removal of all helper functions that use these data definitions described in the aforementioned point.
+- Implementation of new equivalent helper functions that utilize a type synonym family for the era dependent type.
+
+