Don't include build tools in main lib output

README.md

@@ -33,8 +33,8 @@ data User = User
 
 instance Opium.FromRow User where
 
-getUsers :: Connection -> IO (Either Opium.Error [Users])
-getUsers conn = Opium.fetch_ conn "SELECT * FROM user"
+getUsers :: Connection -> IO (Either Opium.Error [User])
+getUsers = Opium.fetch_ "SELECT * FROM user"
 ```
 
 The `Opium.FromRow` instance is implemented generically for all product types ("records"). It looks up the field name in the query result and decodes the column value using `Opium.FromField`.
@@ -51,7 +51,7 @@ instance Opium.FromRow ScoreByAge where
 getScoreByAge :: Connection -> IO ScoreByAge
 getScoreByAge conn = do
   let query = "SELECT regr_intercept(score, age) AS t, regr_slope(score, age) AS m FROM user"
-  Right [x] <- Opium.fetch_ conn query
+  Right (Identity x) <- Opium.fetch_ query conn
   pure x
 ```
 
@@ -83,3 +83,4 @@ getScoreByAge conn = do
     - [ ] `FromRow`
     - [ ] Custom `FromField` impls
 - [ ] Improve type errors when trying to `instance` a type that isn't a record (e.g. sum type)
+- [ ] Improve documentation for `fromRow` module

flake.lock

@@ -1,23 +1,59 @@
 {
   "nodes": {
-    "nixpkgs": {
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
       "locked": {
-        "lastModified": 1681753173,
-        "narHash": "sha256-MrGmzZWLUqh2VstoikKLFFIELXm/lsf/G9U9zR96VD4=",
-        "owner": "NixOS",
-        "repo": "nixpkgs",
-        "rev": "0a4206a51b386e5cda731e8ac78d76ad924c7125",
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
         "type": "github"
       },
       "original": {
-        "id": "nixpkgs",
-        "type": "indirect"
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1752536923,
+        "narHash": "sha256-fdgPZR7VFSSRIQKOJLcs3qCJBWM64Uak0gAGtWTYAd8=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "c665e4d918eda5d78a175ed8d300809c44932160",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "release-25.05",
+        "repo": "nixpkgs",
+        "type": "github"
       }
     },
     "root": {
       "inputs": {
+        "flake-utils": "flake-utils",
         "nixpkgs": "nixpkgs"
       }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
     }
   },
   "root": "root",

flake.nix

@@ -1,35 +1,28 @@
 {
   description = "An opionated Postgres library";
 
-  outputs = { self, nixpkgs }:
-    let
-      pkgs = nixpkgs.legacyPackages.x86_64-linux;
-    in {
-      apps.x86_64-linux.cabal = {
-        type = "app";
-        program = "${nixpkgs.legacyPackages.x86_64-linux.cabal-install}/bin/cabal";
-      };
-      devShells.x86_64-linux.default = pkgs.mkShell {
-        packages = [
-          pkgs.cabal-install
-          pkgs.haskellPackages.implicit-hie
-          (pkgs.ghc.withPackages (hp: with hp; [
-            attoparsec
-            containers
-            bytestring
-            hspec
-            postgresql-libpq
-            text
-            time
-            transformers
-            vector
-          ]))
+  inputs.nixpkgs.url = "github:nixos/nixpkgs/release-25.05";
+  inputs.flake-utils.url = "github:numtide/flake-utils";
 
-          pkgs.haskell-language-server
-        ];
-        shellHook = ''
-          PS1="<opium> ''${PS1}"
-        '';
+  outputs = { self, nixpkgs, flake-utils }: flake-utils.lib.eachDefaultSystem (system:
+    let
+      pkgs = import nixpkgs { inherit system; };
+    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 [
+            pkgs.cabal-install
+            pkgs.haskellPackages.implicit-hie
+            pkgs.haskell-language-server
+          ];
+      }).env;
+    }
+  );
 }

lib/Database/PostgreSQL/Opium.hs

@@ -1,13 +1,20 @@
-{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE TypeApplications #-}
 
 module Database.PostgreSQL.Opium
+    -- * Connection Management
+  ( Connection
+  , ConnectionError (..)
+  , connect
+  , close
     -- * Queries
     --
+    -- Functions for performing queries. @fetch@ retrieves rows, @execute@ doesn't.
+    -- The 'Connection' parameter comes last to facilitate currying for implicitly passing in the connection, e.g. from some framework's connection pool.
+    --
     -- | TODO: Add @newtype Query = Query Text@ with @IsString@ instance to make constructing query strings at run time harder.
-  ( fetch
+  , fetch
   , fetch_
   , execute
   , execute_
@@ -23,51 +30,68 @@ module Database.PostgreSQL.Opium
   )
   where
 
-import Control.Monad (void)
+import Control.Monad (unless, void)
 import Control.Monad.IO.Class (liftIO)
-import Control.Monad.Trans.Except (ExceptT (..), except, runExceptT)
+import Control.Monad.Trans.Except (ExceptT (..), runExceptT, throwE)
+import Data.Functor.Identity (Identity (..))
 import Data.Proxy (Proxy (..))
 import Data.Text (Text)
-import Database.PostgreSQL.LibPQ
-  ( Connection
-  , Result
-  )
+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.Error (Error (..), ErrorPosition (..))
 import Database.PostgreSQL.Opium.FromField (FromField (..), RawField (..))
-import Database.PostgreSQL.Opium.FromRow (FromRow (..))
+import Database.PostgreSQL.Opium.FromRow (FromRow (..), ColumnTable)
 import Database.PostgreSQL.Opium.ToField (ToField (..))
 import Database.PostgreSQL.Opium.ToParamList (ToParamList (..))
 
--- The order of the type parameters is important, because it is more common to use type applications for providing the row type.
-fetch
-  :: forall a b. (ToParamList b, FromRow a)
-  => Connection
-  -> Text
-  -> b
-  -> IO (Either Error [a])
-fetch conn query params = runExceptT $ do
-  result <- execParams conn query params
-  columnTable <- ExceptT $ getColumnTable @a Proxy result
-  nRows <- liftIO $ LibPQ.ntuples result
-  mapM (ExceptT . fromRow result columnTable) [0..nRows - 1]
+class RowContainer c where
+  extract :: FromRow a => Result -> LibPQ.Row -> ColumnTable -> ExceptT Error IO (c a)
 
-fetch_ :: forall a. FromRow a => Connection -> Text -> IO (Either Error [a])
-fetch_ conn query = fetch conn query ()
+instance RowContainer [] where
+  extract result nRows columnTable = do
+    mapM (ExceptT . fromRow result columnTable) [0..nRows - 1]
+
+instance RowContainer Maybe where
+  extract result nRows columnTable
+    | nRows == 0 = pure Nothing
+    | nRows == 1 = Just <$> ExceptT (fromRow result columnTable 0)
+    | otherwise = throwE $ ErrorMoreThanOneRow $ fromEnum nRows
+
+instance RowContainer Identity where
+  extract result nRows columnTable = do
+    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.
+fetch
+  :: forall a b c. (ToParamList c, FromRow a, RowContainer b)
+  => Text
+  -> c
+  -> Connection
+  -> IO (Either Error (b a))
+fetch query params conn = runExceptT $ do
+  result <- execParams conn query params
+  nRows <- liftIO $ LibPQ.ntuples result
+  columnTable <- ExceptT $ getColumnTable @a Proxy result
+  extract result nRows columnTable
+
+fetch_ :: forall a c. (FromRow a, RowContainer c) => Text -> Connection -> IO (Either Error (c a))
+fetch_ query = fetch query ()
 
 execute
   :: forall a. ToParamList a
-  => Connection
-  -> Text
+  => Text
   -> a
+  -> Connection
   -> IO (Either Error ())
-execute conn query params = runExceptT $ void $ execParams conn query params
+execute query params conn = runExceptT $ void $ execParams conn query params
 
-execute_ :: Connection -> Text -> IO (Either Error ())
-execute_ conn query = execute conn query ()
+execute_ :: Text -> Connection -> IO (Either Error ())
+execute_ query = execute query ()
 
 execParams
   :: ToParamList a
@@ -76,14 +100,23 @@ execParams
   -> a
   -> ExceptT Error IO Result
 execParams conn query params = do
-  let queryBytes = Encoding.encodeUtf8 query
-  liftIO (LibPQ.execParams conn queryBytes (toParamList params) LibPQ.Binary) >>= \case
-    Nothing ->
-      except $ Left ErrorNoResult
-    Just result -> do
-      status <- liftIO $ LibPQ.resultStatus result
-      mbMessage <- liftIO $ LibPQ.resultErrorMessage result
-      case mbMessage of
-        Just "" -> pure result
-        Nothing -> pure result
-        Just message -> except $ Left $ ErrorInvalidResult status $ Encoding.decodeUtf8 message
+  -- Actually run the query while locking the connection.
+  mbResult <- ExceptT runQuery
+  -- Check whether the result is valid or nah.
+  result <- mbResult `orThrow` ErrorNoResult
+  status <- liftIO $ LibPQ.resultStatus result
+  mbMessage <- liftIO $ LibPQ.resultErrorMessage result
+  case mbMessage of
+    Just "" -> pure result
+    Nothing -> pure result
+    Just message -> throwE $ ErrorInvalidResult status $ Encoding.decodeUtf8 message
+
+  where
+    orThrow (Just x) _ = pure x
+    orThrow Nothing  e = throwE e
+
+    queryBytes = Encoding.encodeUtf8 query
+    runQuery =
+      flip withRawConnection conn $ \mbRawConn -> runExceptT $ do
+        rawConn <- mbRawConn `orThrow` ErrorConnectionClosed
+        liftIO $ LibPQ.execParams rawConn queryBytes (toParamList params) LibPQ.Binary

lib/Database/PostgreSQL/Opium/Connection.hs

@@ -0,0 +1,68 @@
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE OverloadedRecordDot #-}
+{-# LANGUAGE OverloadedStrings #-}
+
+module Database.PostgreSQL.Opium.Connection
+  ( Connection
+  , ConnectionError (..)
+  , unsafeWithRawConnection
+  , withRawConnection
+  , connect
+  , close
+  ) 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 (..))
+import GHC.Stack (HasCallStack)
+
+import qualified Data.Text.Encoding as Encoding
+import qualified Database.PostgreSQL.LibPQ as LibPQ
+
+newtype Connection = Connection
+  { rawConnection :: MVar (Maybe LibPQ.Connection)
+  }
+
+newtype ConnectionError = ConnectionError Text
+  deriving (Show)
+
+instance Exception ConnectionError
+
+withRawConnection
+  :: HasCallStack
+  => (Maybe LibPQ.Connection -> IO a)
+  -> Connection
+  -> IO a
+withRawConnection f conn = withMVar conn.rawConnection f
+
+unsafeWithRawConnection
+  :: HasCallStack
+  => (LibPQ.Connection -> IO a)
+  -> Connection
+  -> IO a
+unsafeWithRawConnection f = withRawConnection $ \case
+  Nothing -> error "raw connection is missing! perhaps the connection was already closed."
+  Just rawConn -> f rawConn
+
+connect :: Text -> IO (Either ConnectionError Connection)
+connect connectionString = do
+  -- Appending the client_encoding setting overrides any previous setting in the connection string.
+  -- We set the client encoding here to make sure we can use it below for decoding connection
+  -- error messages.
+  rawConn <- LibPQ.connectdb $ Encoding.encodeUtf8 $ connectionString <> " client_encoding=UTF8"
+  status <- LibPQ.status rawConn
+  if status == ConnectionOk then
+    Right . Connection <$> newMVar (Just rawConn)
+  else do
+    rawError <- fromMaybe "" <$> LibPQ.errorMessage rawConn
+    pure $ Left $ ConnectionError $ Encoding.decodeUtf8Lenient rawError
+
+close :: Connection -> IO ()
+close conn = modifyMVar_ conn.rawConnection $ \case
+  Just rawConn -> do
+    LibPQ.finish rawConn
+    pure Nothing
+  Nothing ->
+    pure Nothing

lib/Database/PostgreSQL/Opium/Error.hs

@@ -17,6 +17,9 @@ data Error
   | ErrorInvalidOid Text Oid
   | ErrorUnexpectedNull ErrorPosition
   | ErrorInvalidField ErrorPosition Oid ByteString String
+  | ErrorNotExactlyOneRow Int
+  | ErrorMoreThanOneRow Int
+  | ErrorConnectionClosed
   deriving (Eq, Show)
 
 instance Exception Error where

lib/Database/PostgreSQL/Opium/FromField.hs

@@ -61,7 +61,7 @@ instance FromField ByteString where
 -- | See https://www.postgresql.org/docs/current/datatype-character.html.
 -- Accepts @text@, @character@ and @character varying@.
 instance FromField Text where
-  validOid Proxy = eq Oid.text \/ eq Oid.character \/ eq Oid.characterVarying
+  validOid Proxy = eq Oid.name \/ eq Oid.text \/ eq Oid.character \/ eq Oid.characterVarying
   parseField = Encoding.decodeUtf8 <$> AP.takeByteString
 
 -- Accepts @text@, @character@ and @character varying@.

lib/Database/PostgreSQL/Opium/FromRow.hs

@@ -14,6 +14,7 @@ module Database.PostgreSQL.Opium.FromRow
   -- * FromRow
   ( FromRow (..)
   -- * Internal
+  , ColumnTable
   , toListColumnTable
   ) where
 
@@ -52,12 +53,34 @@ class FromRow a where
   fromRow result columnTable row =
     runExceptT $ to <$> fromRow' @0 FRProxy (FromRowCtx result columnTable) row
 
+instance
+  ( Generic a
+  , GetColumnTable' (Rep a)
+  , FromRow' 0 (Rep a)
+  , Generic b
+  , GetColumnTable' (Rep b)
+  , FromRow' (NumberOfMembers (Rep a)) (Rep b)
+  ) => FromRow (a, b) where
+  getColumnTable Proxy result = runExceptT $ do
+    ctA <- newColumnTable <$> getColumnTable' @(Rep a) Proxy result
+    ctB <- newColumnTable <$> getColumnTable' @(Rep b) Proxy result
+    pure $ ctA `concatColumnTables` ctB
+
+  fromRow result ct row = runExceptT $ do
+    x <- to <$> fromRow' @0 FRProxy (FromRowCtx result ct) row
+    y <- to <$> fromRow' @(NumberOfMembers (Rep a)) FRProxy (FromRowCtx result ct) row
+    pure (x, y)
+
 newtype ColumnTable = ColumnTable (Vector (Column, Oid))
   deriving (Eq, Show)
 
 newColumnTable :: [(Column, Oid)] -> ColumnTable
 newColumnTable = ColumnTable . Vector.fromList
 
+concatColumnTables :: ColumnTable -> ColumnTable -> ColumnTable
+concatColumnTables (ColumnTable a) (ColumnTable b) =
+  ColumnTable $ a <> b
+
 indexColumnTable :: ColumnTable -> Int -> (Column, Oid)
 indexColumnTable (ColumnTable v) i = v `Vector.unsafeIndex` i
 
@@ -89,7 +112,7 @@ type family NumberOfMembers f where
   NumberOfMembers (M1 D _ f) = NumberOfMembers f
   -- The constructor has as many members as the type that it contains.
   NumberOfMembers (M1 C _ f) = NumberOfMembers f
-  -- A product type has as many types as its subtypes have together.
+  -- A product type has as many members as its subtypes have together.
   NumberOfMembers (f :*: g) = NumberOfMembers f + NumberOfMembers g
   -- A selector has/is exactly one member.
   NumberOfMembers (M1 S _ f) = 1
@@ -99,19 +122,23 @@ data FromRowCtx = FromRowCtx
   Result -- ^ Obtained from 'LibPQ.execParams'.
   ColumnTable -- ^ 'Vector' of expected columns indices and OIDs.
 
+-- Specialized proxy type to be used instead of `Proxy (n, f)`
 data FRProxy (n :: Nat) (f :: Type -> Type) = FRProxy
 
 class FromRow' (n :: Nat) (f :: Type -> Type) where
   fromRow' :: FRProxy n f -> FromRowCtx -> Row -> ExceptT Error IO (f p)
 
 instance FromRow' n f => FromRow' n (M1 D c f) where
-  fromRow' FRProxy ctx row = M1 <$> fromRow' @n FRProxy ctx row
+  fromRow' FRProxy ctx row =
+    M1 <$> fromRow' @n FRProxy ctx row
 
 instance FromRow' n f => FromRow' n (M1 C c f) where
-  fromRow' FRProxy ctx row = M1 <$> fromRow' @n FRProxy ctx row
+  fromRow' FRProxy ctx row =
+    M1 <$> fromRow' @n FRProxy ctx row
 
 instance (FromRow' n f, FromRow' (n + NumberOfMembers f) g) => FromRow' n (f :*: g) where
-  fromRow' FRProxy ctx row = (:*:) <$> fromRow' @n FRProxy ctx row <*> fromRow' @(n + NumberOfMembers f) FRProxy ctx row
+  fromRow' FRProxy ctx row =
+    (:*:) <$> fromRow' @n FRProxy ctx row <*> fromRow' @(n + NumberOfMembers f) FRProxy ctx row
 
 instance {-# OVERLAPPABLE #-} (KnownNat n, KnownSymbol nameSym, FromField t) => FromRow' n (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 t)) where
   fromRow' FRProxy = decodeField memberIndex nameText $ \row ->
@@ -158,4 +185,3 @@ decodeField memberIndex nameText g (FromRowCtx result columnTable) row = do
       first
         (ErrorInvalidField (ErrorPosition row nameText) oid field)
         (Just <$> fromField field)
-

lib/Database/PostgreSQL/Opium/Oid.hs

@@ -9,6 +9,9 @@ bytea = Oid 17
 
 -- string types
 
+name :: Oid
+name = Oid 19
+
 text :: Oid
 text = Oid 25
 

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:            0.1.0.0
+version:            1.1.1.0
 
 -- A short (one-line) description of the package.
 -- synopsis:
@@ -63,6 +63,7 @@ library
         Database.PostgreSQL.Opium,
         Database.PostgreSQL.Opium.FromField,
         Database.PostgreSQL.Opium.FromRow,
+        Database.PostgreSQL.Opium.Connection,
         Database.PostgreSQL.Opium.ToField
 
     -- Modules included in this library but not exported.
@@ -117,13 +118,16 @@ test-suite opium-test
     other-modules:
         SpecHook,
         Database.PostgreSQL.OpiumSpec,
-        Database.PostgreSQL.Opium.FromFieldSpec
+        Database.PostgreSQL.Opium.FromFieldSpec,
+        Database.PostgreSQL.Opium.FromRowSpec,
+        Database.PostgreSQL.Opium.ToFieldSpec
 
     -- Test dependencies.
     build-depends:
         base,
         opium,
         bytestring,
+        containers,
         hspec,
         postgresql-libpq,
         time,

test/Database/PostgreSQL/Opium/FromFieldSpec.hs

@@ -14,7 +14,6 @@ import Data.Time
   , timeOfDayToTime
   )
 import Data.Text (Text)
-import Database.PostgreSQL.LibPQ (Connection)
 import Database.PostgreSQL.Opium (FromRow)
 import GHC.Generics (Generic)
 import Test.Hspec (SpecWith, describe, it, shouldBe, shouldSatisfy)
@@ -113,15 +112,15 @@ newtype ARawField = ARawField
 
 instance FromRow ARawField where
 
-shouldFetch :: (Eq a, FromRow a, Show a) => Connection -> Text -> [a] -> IO ()
+shouldFetch :: (Eq a, FromRow a, Show a) => Opium.Connection -> Text -> [a] -> IO ()
 shouldFetch conn query expectedRows = do
-  actualRows <- Opium.fetch_ conn query
+  actualRows <- Opium.fetch_ query conn
   actualRows `shouldBe` Right expectedRows
 
 (/\) :: (a -> Bool) -> (a -> Bool) -> a -> Bool
 p /\ q = \x -> p x && q x
 
-spec :: SpecWith Connection
+spec :: SpecWith Opium.Connection
 spec = do
   describe "FromField Int" $ do
     it "Decodes smallint" $ \conn -> do
@@ -213,15 +212,15 @@ spec = do
       shouldFetch conn "SELECT 4.2::real AS float" [AFloat 4.2]
 
     it "Decodes NaN::real" $ \conn -> do
-      Right [AFloat value] <- Opium.fetch_ conn "SELECT 'NaN'::real AS float"
+      Right [AFloat value] <- Opium.fetch_ "SELECT 'NaN'::real AS float" conn
       value `shouldSatisfy` isNaN
 
     it "Decodes Infinity::real" $ \conn -> do
-      Right [AFloat value] <- Opium.fetch_ conn "SELECT 'Infinity'::real AS float"
+      Right [AFloat value] <- Opium.fetch_ "SELECT 'Infinity'::real AS float" conn
       value `shouldSatisfy` (isInfinite /\ (> 0))
 
     it "Decodes -Infinity::real" $ \conn -> do
-      Right [AFloat value] <- Opium.fetch_ conn "SELECT '-Infinity'::real AS float"
+      Right [AFloat value] <- Opium.fetch_ "SELECT '-Infinity'::real AS float" conn
       value `shouldSatisfy` (isInfinite /\ (< 0))
 
   describe "FromField Double" $ do
@@ -229,22 +228,22 @@ spec = do
       shouldFetch conn "SELECT 4.2::double precision AS double" [ADouble 4.2]
 
     it "Decodes NaN::double precision" $ \conn -> do
-      Right [ADouble value] <- Opium.fetch_ conn "SELECT 'NaN'::double precision AS double"
+      Right [ADouble value] <- Opium.fetch_ "SELECT 'NaN'::double precision AS double" conn
       value `shouldSatisfy` isNaN
 
     it "Decodes Infinity::double precision" $ \conn -> do
-      Right [ADouble value] <- Opium.fetch_ conn "SELECT 'Infinity'::double precision AS double"
+      Right [ADouble value] <- Opium.fetch_ "SELECT 'Infinity'::double precision AS double" conn
       value `shouldSatisfy` (isInfinite /\ (> 0))
 
     it "Decodes -Infinity::double precision" $ \conn -> do
-      Right [ADouble value] <- Opium.fetch_ conn "SELECT '-Infinity'::double precision AS double"
+      Right [ADouble value] <- Opium.fetch_ "SELECT '-Infinity'::double precision AS double" conn
       value `shouldSatisfy` (isInfinite /\ (< 0))
 
     it "Decodes {inf,-inf}::double precision" $ \conn -> do
-      Right [ADouble value0] <- Opium.fetch_ conn "SELECT 'inf'::double precision AS double"
+      Right [ADouble value0] <- Opium.fetch_ "SELECT 'inf'::double precision AS double" conn
       value0 `shouldSatisfy` (isInfinite /\ (> 0))
 
-      Right [ADouble value1] <- Opium.fetch_ conn "SELECT '-inf'::double precision AS double"
+      Right [ADouble value1] <- Opium.fetch_ "SELECT '-inf'::double precision AS double" conn
       value1 `shouldSatisfy` (isInfinite /\ (< 0))
 
   describe "FromField Bool" $ do

test/Database/PostgreSQL/Opium/FromRowSpec.hs

@@ -0,0 +1,92 @@
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE TypeApplications #-}
+
+module Database.PostgreSQL.Opium.FromRowSpec (spec) where
+
+import Data.ByteString (ByteString)
+import Data.Proxy (Proxy (..))
+import Test.Hspec (SpecWith, aroundWith, describe, it, shouldBe)
+
+import qualified Database.PostgreSQL.LibPQ as LibPQ
+
+import Database.PostgreSQL.Opium.Connection (unsafeWithRawConnection)
+import Database.PostgreSQL.OpiumSpec (ManyFields (..), MaybeTest (..), Person (..), Only (..))
+
+import qualified Database.PostgreSQL.Opium as Opium
+import qualified Database.PostgreSQL.Opium.FromRow as Opium.FromRow
+
+shouldHaveColumns
+  :: Opium.FromRow a
+  => Proxy a
+  -> LibPQ.Connection
+  -> ByteString
+  -> [LibPQ.Column]
+  -> IO ()
+shouldHaveColumns proxy conn query expectedColumns = do
+  Just result <- LibPQ.execParams conn query [] LibPQ.Binary
+  columnTable <- Opium.getColumnTable proxy result
+  let actualColumns = fmap (map fst . Opium.FromRow.toListColumnTable) columnTable
+  actualColumns `shouldBe` Right expectedColumns
+
+-- These test the mapping from Result to ColumnTable/FromRow instances.
+  -- They use the raw LibPQ connection for retrieving the Results.
+spec :: SpecWith Opium.Connection
+spec = aroundWith unsafeWithRawConnection $ do
+  describe "getColumnTable" $ do
+    it "Gets the column table for a result" $ \conn -> do
+      shouldHaveColumns @Person Proxy conn
+        "SELECT name, age FROM person"
+        [0, 1]
+
+    it "Gets the numbers right for funky configurations" $ \conn -> do
+      shouldHaveColumns @Person Proxy conn
+        "SELECT age, name FROM person"
+        [1, 0]
+
+      shouldHaveColumns @Person Proxy conn
+        "SELECT 0 AS a, 1 AS b, 2 AS c, age, 4 AS d, name FROM person"
+        [5, 3]
+
+    it "Fails for missing columns" $ \conn -> do
+      Just result <- LibPQ.execParams conn "SELECT 0 AS a FROM person" [] LibPQ.Binary
+      columnTable <- Opium.getColumnTable @Person Proxy result
+      columnTable `shouldBe` Left (Opium.ErrorMissingColumn "name")
+
+  describe "fromRow" $ do
+    it "Decodes rows in a Result" $ \conn -> do
+      Just result <- LibPQ.execParams conn "SELECT * FROM person" [] LibPQ.Binary
+      Right columnTable <- Opium.getColumnTable @Person Proxy result
+
+      row0 <- Opium.fromRow @Person result columnTable 0
+      row0 `shouldBe` Right (Person "paul" 25)
+
+      row1 <- Opium.fromRow @Person result columnTable 1
+      row1 `shouldBe` Right (Person "albus" 103)
+
+    it "Decodes NULL into Nothing for Maybes" $ \conn -> do
+      Just result <- LibPQ.execParams conn "SELECT NULL AS a" [] LibPQ.Binary
+      Right columnTable <- Opium.getColumnTable @MaybeTest Proxy result
+
+      row <- Opium.fromRow result columnTable 0
+      row `shouldBe` Right (MaybeTest Nothing)
+
+    it "Decodes values into Just for Maybes" $ \conn -> do
+      Just result <- LibPQ.execParams conn "SELECT 'abc' AS a" [] LibPQ.Binary
+      Right columnTable <- Opium.getColumnTable @MaybeTest Proxy result
+
+      row <- Opium.fromRow result columnTable 0
+      row `shouldBe` Right (MaybeTest $ Just "abc")
+
+    it "Works for many fields" $ \conn -> do
+      Just result <- LibPQ.execParams conn "SELECT 'abc' AS a, 42 AS b, 1.0::double precision AS c, 'test' AS d, true AS e" [] LibPQ.Binary
+      Right columnTable <- Opium.getColumnTable @ManyFields Proxy result
+
+      row <- Opium.fromRow result columnTable 0
+      row `shouldBe` Right (ManyFields "abc" 42 1.0 "test" True)
+
+    it "Decodes multiple records into a tuple" $ \conn -> do
+      Just result <- LibPQ.execParams conn "SELECT 'albus' AS name, 123 AS age, 42 AS only" [] LibPQ.Binary
+      Right columnTable <- Opium.getColumnTable @(Person, Only Int) Proxy result
+
+      row <- Opium.fromRow @(Person, Only Int) result columnTable 0
+      row `shouldBe` Right (Person "albus" 123, Only 42)

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)

test/Database/PostgreSQL/OpiumSpec.hs

@@ -4,20 +4,16 @@
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE TypeApplications #-}
 
-module Database.PostgreSQL.OpiumSpec (spec) where
+module Database.PostgreSQL.OpiumSpec (ManyFields (..), MaybeTest (..), Person (..), Only (..), spec) where
 
-import Data.ByteString (ByteString)
 import Data.Either (isLeft)
 import Data.Functor.Identity (Identity (..))
-import Data.Proxy (Proxy (..))
 import Data.Text (Text)
-import Database.PostgreSQL.LibPQ (Connection)
 import GHC.Generics (Generic)
 import Test.Hspec (SpecWith, describe, it, shouldBe, shouldSatisfy)
 
 import qualified Database.PostgreSQL.LibPQ as LibPQ
 import qualified Database.PostgreSQL.Opium as Opium
-import qualified Database.PostgreSQL.Opium.FromRow as Opium.FromRow
 
 data Person = Person
   { name :: Text
@@ -55,99 +51,63 @@ newtype Only a = Only
 
 instance Opium.FromField a => Opium.FromRow (Only a) where
 
-shouldHaveColumns
-  :: Opium.FromRow a
-  => Proxy a
-  -> Connection
-  -> ByteString
-  -> [LibPQ.Column]
-  -> IO ()
-shouldHaveColumns proxy conn query expectedColumns = do
-  Just result <- LibPQ.execParams conn query [] LibPQ.Binary
-  columnTable <- Opium.getColumnTable proxy result
-  let actualColumns = fmap (map fst . Opium.FromRow.toListColumnTable) columnTable
-  actualColumns `shouldBe` Right expectedColumns
-
-spec :: SpecWith Connection
+spec :: SpecWith Opium.Connection
 spec = do
-  describe "getColumnTable" $ do
-    it "Gets the column table for a result" $ \conn -> do
-      shouldHaveColumns @Person Proxy conn
-        "SELECT name, age FROM person"
-        [0, 1]
-
-    it "Gets the numbers right for funky configurations" $ \conn -> do
-      shouldHaveColumns @Person Proxy conn
-        "SELECT age, name FROM person"
-        [1, 0]
-
-      shouldHaveColumns @Person Proxy conn
-        "SELECT 0 AS a, 1 AS b, 2 AS c, age, 4 AS d, name FROM person"
-        [5, 3]
-
-    it "Fails for missing columns" $ \conn -> do
-      Just result <- LibPQ.execParams conn "SELECT 0 AS a FROM person" [] LibPQ.Binary
-      columnTable <- Opium.getColumnTable @Person Proxy result
-      columnTable `shouldBe` Left (Opium.ErrorMissingColumn "name")
-
-  describe "fromRow" $ do
-    it "Decodes rows in a Result" $ \conn -> do
-      Just result <- LibPQ.execParams conn "SELECT * FROM person" [] LibPQ.Binary
-      Right columnTable <- Opium.getColumnTable @Person Proxy result
-
-      row0 <- Opium.fromRow @Person result columnTable 0
-      row0 `shouldBe` Right (Person "paul" 25)
-
-      row1 <- Opium.fromRow @Person result columnTable 1
-      row1 `shouldBe` Right (Person "albus" 103)
-
-    it "Decodes NULL into Nothing for Maybes" $ \conn -> do
-      Just result <- LibPQ.execParams conn "SELECT NULL AS a" [] LibPQ.Binary
-      Right columnTable <- Opium.getColumnTable @MaybeTest Proxy result
-
-      row <- Opium.fromRow result columnTable 0
-      row `shouldBe` Right (MaybeTest Nothing)
-
-    it "Decodes values into Just for Maybes" $ \conn -> do
-      Just result <- LibPQ.execParams conn "SELECT 'abc' AS a" [] LibPQ.Binary
-      Right columnTable <- Opium.getColumnTable @MaybeTest Proxy result
-
-      row <- Opium.fromRow result columnTable 0
-      row `shouldBe` Right (MaybeTest $ Just "abc")
-
-    it "Works for many fields" $ \conn -> do
-      Just result <- LibPQ.execParams conn "SELECT 'abc' AS a, 42 AS b, 1.0::double precision AS c, 'test' AS d, true AS e" [] LibPQ.Binary
-      Right columnTable <- Opium.getColumnTable @ManyFields Proxy result
-
-      row <- Opium.fromRow result columnTable 0
-      row `shouldBe` Right (ManyFields "abc" 42 1.0 "test" True)
-
   describe "fetch" $ do
     it "Passes numbered parameters and retrieves a list of rows" $ \conn -> do
-      rows <- Opium.fetch conn "SELECT ($1 + $2) AS only" (17 :: Int, 25 :: Int)
+      rows <- Opium.fetch "SELECT ($1 + $2) AS only" (17 :: Int, 25 :: Int) conn
       rows `shouldBe` Right [Only (42 :: Int)]
 
     it "Uses Identity to pass single parameters" $ \conn -> do
-      rows <- Opium.fetch conn "SELECT count(*) AS only FROM person WHERE name = $1" $ Identity ("paul" :: Text)
+      rows <- Opium.fetch "SELECT count(*) AS only FROM person WHERE name = $1" (Identity ("paul" :: Text)) conn
       rows `shouldBe` Right [Only (1 :: Int)]
 
   describe "fetch_" $ do
     it "Retrieves a list of rows" $ \conn -> do
-      rows <- Opium.fetch_ conn "SELECT * FROM person"
+      rows <- Opium.fetch_ "SELECT * FROM person" conn
       rows `shouldBe` Right [Person "paul" 25, Person "albus" 103]
 
     it "Fails for invalid queries" $ \conn -> do
-      rows <- Opium.fetch_ @Person conn "MRTLBRNFT"
+      rows <- Opium.fetch_ @Person @[] "MRTLBRNFT" conn
       rows `shouldSatisfy` isLeft
 
     it "Fails for unexpected NULLs" $ \conn -> do
-      rows <- Opium.fetch_ @Person conn "SELECT NULL AS name, 0 AS age"
+      rows <- Opium.fetch_ @Person @[] "SELECT NULL AS name, 0 AS age" conn
       rows `shouldBe` Left (Opium.ErrorUnexpectedNull (Opium.ErrorPosition 0 "name"))
 
     it "Fails for the wrong column type" $ \conn -> do
-      rows <- Opium.fetch_ @Person conn "SELECT 'quby' AS name, 'indeterminate' AS age"
+      rows <- Opium.fetch_ @Person @[] "SELECT 'quby' AS name, 'indeterminate' AS age" conn
       rows `shouldBe` Left (Opium.ErrorInvalidOid "age" $ LibPQ.Oid 25)
 
     it "Works for the readme regression example" $ \conn -> do
-      rows <- Opium.fetch_ @ScoreByAge conn "SELECT regr_intercept(score, age) AS t, regr_slope(score, age) AS m FROM person"
+      rows <- Opium.fetch_ @ScoreByAge @[] "SELECT regr_intercept(score, age) AS t, regr_slope(score, age) AS m FROM person" conn
       rows `shouldSatisfy` \case { (Right [ScoreByAge _ _]) -> True; _ -> False }
+
+    it "Accepts exactly one row when Identity is the row container type" $ \conn -> do
+      row <- Opium.fetch_ "SELECT 42 AS only" conn
+      row `shouldBe` Right (Identity (Only (42 :: Int)))
+
+    it "Does not accept zero rows when Identity is the row container type" $ \conn -> do
+      row <- Opium.fetch_ @(Only Int) @Identity "SELECT 42 AS only WHERE false" conn
+      row `shouldSatisfy` isLeft
+
+    it "Does not accept two rows when Identity is the row container type" $ \conn -> do
+      row <- Opium.fetch_ @(Only Int) @Identity "SELECT 17 AS only UNION ALL SELECT 25 AS only" conn
+      row `shouldSatisfy` isLeft
+
+    it "Accepts zero rows when Maybe is the row container type" $ \conn -> do
+      row <- Opium.fetch_ @(Only Int) @Maybe "SELECT 17 AS only WHERE false" conn
+      row `shouldBe` Right Nothing
+
+    it "Accepts one row when Maybe is the row container type" $ \conn -> do
+      row <- Opium.fetch_ @(Only Int) @Maybe "SELECT 42 AS only" conn
+      row `shouldBe` Right (Just (Only 42))
+
+    it "Does not accept two rows when Maybe is the row container type" $ \conn -> do
+      row <- Opium.fetch_ @(Only Int) @Maybe "SELECT 17 AS only UNION ALL SELECT 25 AS only" conn
+      row `shouldSatisfy` isLeft
+
+  describe "close" $ do
+    it "Does not crash when called twice on the same connection" $ \conn -> do
+      Opium.close conn
+      Opium.close conn

test/SpecHook.hs

@@ -3,16 +3,15 @@
 module SpecHook (hook) where
 
 import Control.Exception (bracket)
-import Database.PostgreSQL.LibPQ (Connection)
 import System.Environment (lookupEnv)
 import Test.Hspec (Spec, SpecWith, around)
 import Text.Printf (printf)
 
 import qualified Data.Text as Text
-import qualified Data.Text.Encoding as Encoding
-import qualified Database.PostgreSQL.LibPQ as LibPQ
 
-setupConnection :: IO Connection
+import qualified Database.PostgreSQL.Opium as Opium
+
+setupConnection :: IO Opium.Connection
 setupConnection = do
   Just dbUser <- lookupEnv "DB_USER"
   Just dbPass <- lookupEnv "DB_PASS"
@@ -20,24 +19,17 @@ setupConnection = do
   Just dbPort <- lookupEnv "DB_PORT"
 
   let dsn = printf "host=localhost user=%s password=%s dbname=%s port=%s" dbUser dbPass dbName dbPort
-  conn <- LibPQ.connectdb $ Encoding.encodeUtf8 $ Text.pack dsn
-  _ <- LibPQ.setClientEncoding conn "UTF8"
+  Right conn <- Opium.connect $ Text.pack dsn
 
-  _ <- LibPQ.exec conn "CREATE TABLE person (name TEXT NOT NULL, age INT NOT NULL, score DOUBLE PRECISION NOT NULL, motto TEXT)"
-  _ <- LibPQ.exec conn "INSERT INTO person VALUES ('paul', 25, 30), ('albus', 103, 50.42)"
+  Right _ <- Opium.execute_ "DROP TABLE IF EXISTS person" conn
+  Right _ <- Opium.execute_ "CREATE TABLE person (name TEXT NOT NULL, age INT NOT NULL, score DOUBLE PRECISION NOT NULL, motto TEXT)" conn
+  Right _ <- Opium.execute_ "INSERT INTO person VALUES ('paul', 25, 30), ('albus', 103, 50.42)" conn
 
   pure conn
 
-teardownConnection :: Connection -> IO ()
+teardownConnection :: Opium.Connection -> IO ()
 teardownConnection conn = do
-  _ <- LibPQ.exec conn "DROP TABLE person"
-  LibPQ.finish conn
+  Opium.close conn
 
-class SpecInput a where
-  hook :: SpecWith a -> Spec
-
-instance SpecInput Connection where
-  hook = around $ bracket setupConnection teardownConnection
-
-instance SpecInput () where
-  hook = id
+hook :: SpecWith Opium.Connection -> Spec
+hook = around $ bracket setupConnection teardownConnection