draft an initial interface for a DB layer

cardano-wallet.cabal

@@ -59,6 +59,7 @@ library
   hs-source-dirs:
       src
   exposed-modules:
+      Cardano.DBLayer
       Cardano.NetworkLayer
       Cardano.NetworkLayer.HttpBridge
       Cardano.NetworkLayer.HttpBridge.Api
@@ -154,6 +155,7 @@ test-suite unit
   main-is:
       Main.hs
   other-modules:
+      Cardano.DBLayerSpec
       Cardano.Wallet.AddressDerivationSpec
       Cardano.Wallet.AddressDiscoverySpec
       Cardano.Wallet.Binary.PackfileSpec

src/Cardano/DBLayer.hs

@@ -0,0 +1,51 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE ExistentialQuantification #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE RankNTypes #-}
+
+module Cardano.DBLayer
+    ( DBLayer(..)
+    ) where
+
+import Cardano.Wallet
+    ( Wallet )
+import Control.Monad.Except
+    ( ExceptT )
+import Data.Word
+    ( Word64 )
+import GHC.TypeLits
+    ( Symbol )
+
+
+-- | A Database interface for storing various things in a DB. In practice,
+-- we'll need some extra contraints on the wallet state that allows us to
+-- serialize and unserialize it (e.g. @forall s. (Serialize s) => ...@)
+data DBLayer m = forall s. DBLayer
+    -- Wallet checkpoints, checkpoints are handled as a bounded FIFO, where we
+    -- eventually store @k@ values (e.g. k=2160) at the same time.
+    { enqueueCheckpoint -- Add a checkpoint on top of the queue
+        :: PrimaryKey "wallet"
+        -> Wallet s
+        -> ExceptT ErrEnqueueCheckpoint m ()
+    , dequeueCheckpoints -- Discard a number of checkpoints from the end
+        :: PrimaryKey "wallet"
+        -> Word64
+        -> ExceptT ErrDequeueCheckpoints m ()
+    , checkpoints --
+        :: PrimaryKey "wallet"
+        -> ExceptT ErrCheckpoints m [Wallet s]
+    }
+
+-- | A primary key which can take many forms depending on the value. This may
+-- become a type family as we move forward, but for now, it illustrate that
+-- some queries are ran against some sort of store;
+--
+-- As a matter of fact, we may manipulate multiple wallets at the same time, so,
+-- functions like 'enqueueCheckpoint' needs to be associated to a corresponding
+-- wallet. Some other may not because they are information valid for all wallets
+-- (like for instance, the last known network tip).
+data PrimaryKey (resource :: Symbol)
+
+data ErrEnqueueCheckpoint
+data ErrDequeueCheckpoints
+data ErrCheckpoints

test/unit/Cardano/DBLayerSpec.hs

@@ -0,0 +1,14 @@
+module Cardano.DBLayerSpec
+    ( spec
+    ) where
+
+import Prelude
+
+import Cardano.DBLayer
+    ()
+import Test.Hspec
+    ( Spec )
+
+
+spec :: Spec
+spec = return ()