do not enforce non-empty outputs for unsigned tx

  Actually, there are many use-cases for empty outputs. This occurs in
  particular often when performing a selection for delegation: in this
  scenario, there are typically no outputs at all and, depending on the
  size of the selected input(s), there might be no change at all
  (because of the minUTxO value).

lib/core/src/Cardano/Wallet.hs

@@ -1634,7 +1634,7 @@ signTx
     -> Maybe TxMetadata
     -> UnsignedTx (TxIn, TxOut)
     -> ExceptT ErrSignPayment IO (Tx, TxMeta, UTCTime, SealedTx)
-signTx ctx wid pwd md (UnsignedTx inpsNE outsNE) = db & \DBLayer{..} -> do
+signTx ctx wid pwd md (UnsignedTx inpsNE outs) = db & \DBLayer{..} -> do
     withRootKey @_ @s ctx wid pwd ErrSignPaymentWithRootKey $ \xprv scheme -> do
         let pwdP = preparePassphrase scheme pwd
         nodeTip <- withExceptT ErrSignPaymentNetwork $ currentNodeTip nl
@@ -1658,7 +1658,6 @@ signTx ctx wid pwd md (UnsignedTx inpsNE outsNE) = db & \DBLayer{..} -> do
     tl = ctx ^. transactionLayer @t @k
     nl = ctx ^. networkLayer @t
     inps = NE.toList inpsNE
-    outs = NE.toList outsNE
 
 -- | Makes a fully-resolved coin selection for the given set of payments.
 selectCoinsExternal
@@ -1687,8 +1686,7 @@ selectCoinsExternal ctx wid argGenChange selectCoins = do
     UnsignedTx
         <$> (fullyQualifiedInputs s' cs'
                 (ErrSelectCoinsExternalUnableToAssignInputs cs'))
-        <*> ensureNonEmpty (outputs cs')
-                (ErrSelectCoinsExternalUnableToAssignOutputs cs')
+        <*> pure (outputs cs')
   where
     db = ctx ^. dbLayer @s @k
 
@@ -1697,7 +1695,6 @@ data ErrSelectCoinsExternal e
     | ErrSelectCoinsExternalForPayment (ErrSelectForPayment e)
     | ErrSelectCoinsExternalForDelegation ErrSelectForDelegation
     | ErrSelectCoinsExternalUnableToAssignInputs CoinSelection
-    | ErrSelectCoinsExternalUnableToAssignOutputs CoinSelection
     deriving (Eq, Show)
 
 signDelegation
@@ -2265,7 +2262,6 @@ data ErrSelectForDelegation
     = ErrSelectForDelegationNoSuchWallet ErrNoSuchWallet
     | ErrSelectForDelegationFee ErrAdjustForFee
     | ErrSelectForDelegationUnableToAssignInputs ErrNoSuchWallet
-    | ErrSelectForDelegationUnableToAssignOutputs ErrNoSuchWallet
     deriving (Show, Eq)
 
 -- | Errors that can occur when signing a delegation certificate.
@@ -2283,7 +2279,6 @@ data ErrJoinStakePool
     | ErrJoinStakePoolSubmitTx ErrSubmitTx
     | ErrJoinStakePoolCannotJoin ErrCannotJoin
     | ErrJoinStakePoolUnableToAssignInputs CoinSelection
-    | ErrJoinStakePoolUnableToAssignOutputs CoinSelection
     deriving (Generic, Eq, Show)
 
 data ErrQuitStakePool
@@ -2293,7 +2288,6 @@ data ErrQuitStakePool
     | ErrQuitStakePoolSubmitTx ErrSubmitTx
     | ErrQuitStakePoolCannotQuit ErrCannotQuit
     | ErrQuitStakePoolUnableToAssignInputs CoinSelection
-    | ErrQuitStakePoolUnableToAssignOutputs CoinSelection
     deriving (Generic, Eq, Show)
 
 -- | Errors that can occur when fetching the reward balance of a wallet

lib/core/src/Cardano/Wallet/Api/Server.hs

@@ -1659,7 +1659,7 @@ migrateWallet ctx (ApiT wid) migrateData = do
             ti
             (txId tx)
             (fmap Just <$> NE.toList (W.unsignedInputs cs))
-            (NE.toList (W.unsignedOutputs cs))
+            (W.unsignedOutputs cs)
             (tx ^. #withdrawals)
             (meta, time)
             Nothing
@@ -1699,7 +1699,7 @@ assignMigrationAddresses addrs selections =
     makeTx :: CoinSelection -> [Address] -> UnsignedTx (TxIn, TxOut)
     makeTx sel addrsSelected = UnsignedTx
         (NE.fromList (sel ^. #inputs))
-        (NE.fromList (zipWith TxOut addrsSelected (sel ^. #change)))
+        (zipWith TxOut addrsSelected (sel ^. #change))
 
 {-------------------------------------------------------------------------------
                                     Network
@@ -1916,6 +1916,7 @@ mkApiCoinSelection mcerts (UnsignedTx inputs outputs) =
                 ]
       where
         apiStakePath = ApiT <$> xs
+
     mkAddressAmount :: TxOut -> AddressAmount (ApiT Address, Proxy n)
     mkAddressAmount (TxOut addr (Coin c)) =
         AddressAmount (ApiT addr, Proxy @n) (Quantity $ fromIntegral c)
@@ -2273,11 +2274,8 @@ instance Buildable e => LiftHandler (ErrSelectCoinsExternal e) where
         ErrSelectCoinsExternalUnableToAssignInputs e ->
             apiError err500 UnableToAssignInputOutput $ mconcat
                 [ "I'm unable to assign inputs from coin selection: "
-                , pretty e]
-        ErrSelectCoinsExternalUnableToAssignOutputs e ->
-            apiError err500 UnableToAssignInputOutput $ mconcat
-                [ "I'm unable to assign outputs from coin selection: "
-                , pretty e]
+                , pretty e
+                ]
 
 instance Buildable e => LiftHandler (ErrCoinSelection e) where
     handler = \case
@@ -2543,7 +2541,6 @@ instance LiftHandler ErrSelectForDelegation where
                 , "delegation certificate. I need: ", showT cost, " Lovelace."
                 ]
         ErrSelectForDelegationUnableToAssignInputs e -> handler e
-        ErrSelectForDelegationUnableToAssignOutputs e -> handler e
 
 instance LiftHandler ErrSignDelegation where
     handler = \case
@@ -2582,10 +2579,6 @@ instance LiftHandler ErrJoinStakePool where
             apiError err500 UnableToAssignInputOutput $ mconcat
                 [ "I'm unable to assign inputs from coin selection: "
                 , pretty e]
-        ErrJoinStakePoolUnableToAssignOutputs  e ->
-            apiError err500 UnableToAssignInputOutput $ mconcat
-                [ "I'm unable to assign outputs from coin selection: "
-                , pretty e]
 
 instance LiftHandler ErrFetchRewards where
     handler = \case
@@ -2626,10 +2619,6 @@ instance LiftHandler ErrQuitStakePool where
             apiError err500 UnableToAssignInputOutput $ mconcat
                 [ "I'm unable to assign inputs from coin selection: "
                 , pretty e]
-        ErrQuitStakePoolUnableToAssignOutputs e ->
-            apiError err500 UnableToAssignInputOutput $ mconcat
-                [ "I'm unable to assign outputs from coin selection: "
-                , pretty e]
 
 instance LiftHandler ErrCreateRandomAddress where
     handler = \case

lib/core/src/Cardano/Wallet/Api/Types.hs

@@ -410,7 +410,7 @@ data ApiCertificate
 
 data ApiCoinSelection (n :: NetworkDiscriminant) = ApiCoinSelection
     { inputs :: !(NonEmpty (ApiCoinSelectionInput n))
-    , outputs :: !(NonEmpty (AddressAmount (ApiT Address, Proxy n)))
+    , outputs :: ![AddressAmount (ApiT Address, Proxy n)]
     , certificates :: Maybe (NonEmpty ApiCertificate)
     } deriving (Eq, Generic, Show)
 

lib/core/src/Cardano/Wallet/Primitive/Types.hs

@@ -970,7 +970,7 @@ data UnsignedTx input = UnsignedTx
     { unsignedInputs
         :: NonEmpty input
     , unsignedOutputs
-        :: NonEmpty TxOut
+        :: [TxOut]
     }
     deriving (Eq, Show)
 

lib/core/test/unit/Cardano/Wallet/Primitive/CoinSelectionSpec.hs

@@ -145,7 +145,7 @@ prop_coinValuesPreserved (CoinSelectionsSetup cs addrs) = do
     let selsCoinValue =
             sum $ getCoinValueFromInp . inputs . getCS <$> cs
     let getCoinValueFromTxOut (UnsignedTx _ txouts) =
-            sum $ map (\(TxOut _ (Coin c)) -> c) $ NE.toList txouts
+            sum $ map (\(TxOut _ (Coin c)) -> c) txouts
     let txsCoinValue =
             sum . map getCoinValueFromTxOut
     txsCoinValue (assignMigrationAddresses addrs sels) === selsCoinValue
@@ -164,7 +164,7 @@ prop_coinValuesPreservedPerTx f (CoinSelectionsSetup cs addrs) = do
             f . map (\(_, TxOut _ (Coin c)) -> c)
     let selsCoinValue = getCoinValueFromInp . inputs . getCS <$> cs
     let getCoinValueFromTxOut (UnsignedTx _ txouts) =
-            f $ map (\(TxOut _ (Coin c)) -> c) $ NE.toList txouts
+            f $ map (\(TxOut _ (Coin c)) -> c) txouts
     let txsCoinValue = map getCoinValueFromTxOut
     txsCoinValue (assignMigrationAddresses addrs sels) === selsCoinValue
 
@@ -202,8 +202,7 @@ prop_fairAddressesRecycled
 prop_fairAddressesRecycled (CoinSelectionsSetup cs addrs) = do
     let sels = getCS <$> cs
     let getAllAddrPerTx (UnsignedTx _ txouts) =
-            map (\(TxOut addr _) -> addr) $
-            NE.toList txouts
+            map (\(TxOut addr _) -> addr) txouts
     let getAllAddrCounts =
             Map.elems .
             foldr (\x -> Map.insertWith (+) x (1::Int)) Map.empty .

specifications/api/swagger.yaml

@@ -520,7 +520,7 @@ x-transactionInputs: &transactionInputs
 x-transactionOutputs: &transactionOutputs
   description: A list of target outputs
   type: array
-  minItems: 1
+  minItems: 0
   items:
     type: object
     required:
@@ -2947,7 +2947,7 @@ paths:
         Random-Improve coin selection algorithm</a>.
 
         <b>Note: </b> Not supported for Byron random wallets.
-        
+
       parameters:
         - *parametersWalletId
       requestBody: