Don't include build tools in main lib output

flake.nix

@@ -7,7 +7,14 @@
   outputs = { self, nixpkgs, flake-utils }: flake-utils.lib.eachDefaultSystem (system:
     let
       pkgs = import nixpkgs { inherit system; };
-      opium = pkgs.haskellPackages.developPackage {
+    in {
+      packages.opium = pkgs.haskell.lib.overrideCabal (pkgs.haskellPackages.developPackage { root = ./.; }) {
+        # Currently the tests require a running Postgres instance.
+        # This is not automated yet, so don't export the tests.
+        doCheck = false;
+      };
+
+      devShells.default = (pkgs.haskellPackages.developPackage {
         root = ./.;
         modifier = drv:
           pkgs.haskell.lib.addBuildTools drv [
@@ -15,15 +22,7 @@
             pkgs.haskellPackages.implicit-hie
             pkgs.haskell-language-server
           ];
-      };
+      }).env;
-    in {
-      packages.opium = pkgs.haskell.lib.overrideCabal opium {
-        # Currently the tests require a running Postgres instance.
-        # This is not automated yet, so don't export the tests.
-        doCheck = false;
-      };
-
-      devShells.default = opium.env;
     }
   );
 }

lib/Database/PostgreSQL/Opium.hs

@@ -5,7 +5,7 @@
 module Database.PostgreSQL.Opium
     -- * Connection Management
   ( Connection
-  , ConnectionError
+  , ConnectionError (..)
   , connect
   , close
     -- * Queries
@@ -41,7 +41,7 @@ import Database.PostgreSQL.LibPQ (Result)
 import qualified Data.Text.Encoding as Encoding
 import qualified Database.PostgreSQL.LibPQ as LibPQ
 
-import Database.PostgreSQL.Opium.Connection (Connection, ConnectionError, connect, close, withRawConnection)
+import Database.PostgreSQL.Opium.Connection (Connection, ConnectionError (..), connect, close, withRawConnection)
 import Database.PostgreSQL.Opium.Error (Error (..), ErrorPosition (..))
 import Database.PostgreSQL.Opium.FromField (FromField (..), RawField (..))
 import Database.PostgreSQL.Opium.FromRow (FromRow (..), ColumnTable)
@@ -59,11 +59,11 @@ instance RowContainer Maybe where
   extract result nRows columnTable
     | nRows == 0 = pure Nothing
     | nRows == 1 = Just <$> ExceptT (fromRow result columnTable 0)
-    | otherwise = throwE ErrorMoreThanOneRow
+    | otherwise = throwE $ ErrorMoreThanOneRow $ fromEnum nRows
 
 instance RowContainer Identity where
   extract result nRows columnTable = do
-    unless (nRows == 1) $ throwE ErrorNotExactlyOneRow
+    unless (nRows == 1) $ throwE $ ErrorNotExactlyOneRow $ fromEnum nRows
     Identity <$> ExceptT (fromRow result columnTable 0)
 
 -- The order of the type parameters is important, because it is more common to use type applications for providing the row type and row container type.

lib/Database/PostgreSQL/Opium/Connection.hs

@@ -4,7 +4,7 @@
 
 module Database.PostgreSQL.Opium.Connection
   ( Connection
-  , ConnectionError
+  , ConnectionError (..)
   , unsafeWithRawConnection
   , withRawConnection
   , connect
@@ -12,6 +12,7 @@ module Database.PostgreSQL.Opium.Connection
   ) where
 
 import Control.Concurrent.MVar (MVar, newMVar, modifyMVar_, withMVar)
+import Control.Exception (Exception)
 import Data.Maybe (fromMaybe)
 import Data.Text (Text)
 import Database.PostgreSQL.LibPQ (ConnStatus (..))
@@ -27,6 +28,8 @@ newtype Connection = Connection
 newtype ConnectionError = ConnectionError Text
   deriving (Show)
 
+instance Exception ConnectionError
+
 withRawConnection
   :: HasCallStack
   => (Maybe LibPQ.Connection -> IO a)

lib/Database/PostgreSQL/Opium/Error.hs

@@ -17,8 +17,8 @@ data Error
   | ErrorInvalidOid Text Oid
   | ErrorUnexpectedNull ErrorPosition
   | ErrorInvalidField ErrorPosition Oid ByteString String
-  | ErrorNotExactlyOneRow
+  | ErrorNotExactlyOneRow Int
-  | ErrorMoreThanOneRow
+  | ErrorMoreThanOneRow Int
   | ErrorConnectionClosed
   deriving (Eq, Show)
 

lib/Database/PostgreSQL/Opium/ToField.hs

@@ -7,8 +7,10 @@ module Database.PostgreSQL.Opium.ToField
 
 import Data.Bits (Bits (..))
 import Data.ByteString (ByteString)
+import Data.Int (Int32)
 import Data.List (singleton)
 import Data.Text (Text)
+import Data.Time (Day, diffDays, fromGregorian)
 import Data.Word (Word32)
 import Database.PostgreSQL.LibPQ (Format (..), Oid)
 import Unsafe.Coerce (unsafeCoerce)
@@ -48,3 +50,11 @@ instance ToField Float where
 
 instance ToField Double where
   toField x = Just (Oid.doublePrecision, encodeBigEndian @Word 8 $ unsafeCoerce x, Binary)
+
+toPostgresJulian :: Day -> Integer
+toPostgresJulian date = diffDays date postgresEpoch
+  where
+    postgresEpoch = fromGregorian 2000 1 1
+
+instance ToField Day where
+  toField x = Just (Oid.date, encodeBigEndian @Int32 4 $ fromIntegral $ toPostgresJulian x, Binary)

opium.cabal

@@ -20,7 +20,7 @@ name:               opium
 -- PVP summary:     +-+------- breaking API changes
 --                  | | +----- non-breaking API additions
 --                  | | | +--- code changes with no API change
-version:            1.1.0.0
+version:            1.1.1.0
 
 -- A short (one-line) description of the package.
 -- synopsis:
@@ -119,7 +119,8 @@ test-suite opium-test
         SpecHook,
         Database.PostgreSQL.OpiumSpec,
         Database.PostgreSQL.Opium.FromFieldSpec,
-        Database.PostgreSQL.Opium.FromRowSpec
+        Database.PostgreSQL.Opium.FromRowSpec,
+        Database.PostgreSQL.Opium.ToFieldSpec
 
     -- Test dependencies.
     build-depends:

test/Database/PostgreSQL/Opium/ToFieldSpec.hs

@@ -0,0 +1,30 @@
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE OverloadedStrings #-}
+
+module Database.PostgreSQL.Opium.ToFieldSpec (spec) where
+
+import Data.Functor.Identity (Identity (..))
+import Data.Text (Text)
+import Data.Time (fromGregorian)
+import GHC.Generics (Generic)
+import Test.Hspec (SpecWith, describe, it, shouldBe)
+
+import qualified Database.PostgreSQL.Opium as Opium
+
+newtype Result = Result
+  { a :: Bool
+  } deriving (Show, Eq, Generic)
+
+instance Opium.FromRow Result
+
+shouldBeTrue :: Opium.ToParamList p => Opium.Connection -> Text -> p -> IO ()
+shouldBeTrue conn query params = do
+  result <- Opium.fetch query params conn
+  result `shouldBe` Right (Identity (Result True))
+
+spec :: SpecWith Opium.Connection
+spec = do
+  describe "ToField Day" $ do
+    it "Encodes Day" $ \conn -> do
+      shouldBeTrue conn "SELECT $1 = date '1997-09-29' AS a" (Identity $ fromGregorian 1997 9 29)
+      shouldBeTrue conn "SELECT $1 = date '2025-07-28' AS a" (Identity $ fromGregorian 2025 7 28)