Remove ugly IORef from fromRow and replace it by ugly compile time calculation :)

README.md

@@ -74,4 +74,9 @@ getScoreByAge conn = do
 - [ ] Implement JSON decoding
 - [ ] Implement (anonymous) composite types
 - [ ] Catch [UnicodeException](https://hackage.haskell.org/package/text-2.1/docs/Data-Text-Encoding-Error.html#t:UnicodeException) when decoding text
+    - This might not be necessary if Postgres guarantees us that having a textual OID on a field means that the field is encoded correctly.
 - [ ] Implement array decoding
+- [ ] Better docs and structure for `FromRow` module
+- [ ] Lexer for PostgreSQL that replaces $name by $1, $2, etc.
+- [ ] Tutorial
+- [ ] Rationale

lib/Database/PostgreSQL/Opium.hs

@@ -1,5 +1,4 @@
 {-# LANGUAGE DataKinds #-}
-{-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE LambdaCase #-}
@@ -7,47 +6,74 @@
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
-{-# LANGUAGE TypeOperators #-}
 
 module Database.PostgreSQL.Opium
-  ( ColumnTable
+    -- * Queries
+    --
+    -- | TODO: Add @newtype Query = Query Text@ with @IsString@ instance to make constructing query strings at run time harder.
+  ( fetch
+  , fetch_
+  , execute
+  , execute_
+    -- * Classes to Implement
+  , FromRow (..)
+  , FromField (..)
+  , ToParamList (..)
+  , ToField (..)
+    -- * Utility Stuff
   , Error (..)
   , ErrorPosition (..)
-  , FromField (..)
-  , FromRow (..)
   , RawField (..)
-  , fetch
-  , fetch_
-  , toListColumnTable
   )
   where
 
 import Control.Monad.IO.Class (liftIO)
 import Control.Monad.Trans.Except (ExceptT (..), except, runExceptT)
-import Data.ByteString (ByteString)
-import Data.IORef (IORef, modifyIORef', newIORef, readIORef)
 import Data.Proxy (Proxy (..))
 import Data.Text (Text)
-import Data.Vector (Vector)
 import Database.PostgreSQL.LibPQ
-  ( Column
+  ( Connection
-  , Connection
-  , Oid
   , Result
-  , Row
   )
-import GHC.Generics (C, D, Generic, K1 (..), M1 (..), Meta (..), Rec0, Rep, S, to, (:*:) (..))
-import GHC.TypeLits (KnownSymbol, symbolVal)
 
-import qualified Data.Text as Text
 import qualified Data.Text.Encoding as Encoding
-import qualified Data.Vector as Vector
 import qualified Database.PostgreSQL.LibPQ as LibPQ
 
 import Database.PostgreSQL.Opium.Error (Error (..), ErrorPosition (..))
-import Database.PostgreSQL.Opium.FromField (FromField (..), fromField, RawField (..))
+import Database.PostgreSQL.Opium.FromField (FromField (..), RawField (..))
+import Database.PostgreSQL.Opium.FromRow (FromRow (..))
+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]
+
+fetch_ :: forall a. FromRow a => Connection -> Text -> IO (Either Error [a])
+fetch_ conn query = fetch conn query ()
+
+execute
+  :: forall a. ToParamList a
+  => Connection
+  -> Text
+  -> a
+  -> IO (Either Error ())
+execute conn query params = runExceptT $ do
+  _ <- execParams conn query params
+  pure ()
+
+execute_ :: Connection -> Text -> IO (Either Error ())
+execute_ conn query = execute conn query ()
+
 execParams
   :: ToParamList a
   => Connection
@@ -66,126 +92,3 @@ execParams conn query params = do
         Just "" -> pure result
         Nothing -> pure result
         Just message -> except $ Left $ ErrorInvalidResult status $ Encoding.decodeUtf8 message
-
-fetch
-  :: forall a b. (ToParamList a, FromRow b)
-  => Connection
-  -> Text
-  -> a
-  -> IO (Either Error [b])
-fetch conn query params = runExceptT $ do
-  result <- execParams conn query params
-  columnTable <- ExceptT $ getColumnTable @b Proxy result
-  nRows <- liftIO $ LibPQ.ntuples result
-  mapM (ExceptT . fromRow result columnTable) [0..nRows - 1]
-
-fetch_ :: forall a. FromRow a => Connection -> Text -> IO (Either Error [a])
-fetch_ conn query = fetch conn query ()
-
-newtype ColumnTable = ColumnTable (Vector (Column, Oid))
-  deriving (Eq, Show)
-
-newColumnTable :: [(Column, Oid)] -> ColumnTable
-newColumnTable = ColumnTable . Vector.fromList
-
-indexColumnTable :: ColumnTable -> Int -> (Column, Oid)
-indexColumnTable (ColumnTable v) i = v `Vector.unsafeIndex` i
-
-toListColumnTable :: ColumnTable -> [(Column, Oid)]
-toListColumnTable (ColumnTable v) = Vector.toList v
-
-class FromRow a where
-  getColumnTable :: Proxy a -> Result -> IO (Either Error ColumnTable)
-  default getColumnTable :: (Generic a, GetColumnTable' (Rep a)) => Proxy a -> Result -> IO (Either Error ColumnTable)
-  getColumnTable Proxy = runExceptT . fmap newColumnTable . getColumnTable' @(Rep a) Proxy
-
-  fromRow :: Result -> ColumnTable -> Row -> IO (Either Error a)
-  default fromRow :: (Generic a, FromRow' (Rep a)) => Result -> ColumnTable -> Row -> IO (Either Error a)
-  fromRow result columnTable row = do
-    iRef <- newIORef 0
-    runExceptT $ to <$> fromRow' (FromRowCtx result columnTable iRef) row
-
-class GetColumnTable' f where
-  getColumnTable' :: Proxy (f p) -> Result -> ExceptT Error IO [(Column, Oid)]
-
-instance GetColumnTable' f => GetColumnTable' (M1 D c f) where
-  getColumnTable' Proxy = getColumnTable' @f Proxy
-
-instance GetColumnTable' f => GetColumnTable' (M1 C c f) where
-  getColumnTable' Proxy = getColumnTable' @f Proxy
-
-instance (GetColumnTable' f, GetColumnTable' g) => GetColumnTable' (f :*: g) where
-  getColumnTable' Proxy result =
-    (++) <$> getColumnTable' @f Proxy result <*> getColumnTable' @g Proxy result
-
-checkColumn :: forall a. FromField a => Proxy a -> String -> Result -> ExceptT Error IO [(Column, Oid)]
-checkColumn Proxy nameStr result = do
-  column <- ExceptT $ maybe (Left $ ErrorMissingColumn nameText) Right <$> LibPQ.fnumber result name
-  oid <- liftIO $ LibPQ.ftype result column
-  if validOid @a Proxy oid then
-    pure [(column, oid)]
-  else
-    except $ Left $ ErrorInvalidOid nameText oid
-  where
-    nameText = Text.pack nameStr
-    name = Encoding.encodeUtf8 nameText
-
-instance {-# OVERLAPPABLE #-} (FromField t, KnownSymbol nameSym) => GetColumnTable' (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 t)) where
-  getColumnTable' Proxy = checkColumn @t Proxy $ symbolVal @nameSym Proxy
-
-instance {-# OVERLAPPING #-} (KnownSymbol nameSym, FromField t) => GetColumnTable' (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 (Maybe t))) where
-  getColumnTable' Proxy = checkColumn @t Proxy $ symbolVal @nameSym Proxy
-
-data FromRowCtx = FromRowCtx Result ColumnTable (IORef Int)
-
-class FromRow' f where
-  fromRow' :: FromRowCtx -> Row -> ExceptT Error IO (f p)
-
-instance FromRow' f => FromRow' (M1 D c f) where
-  fromRow' ctx row = M1 <$> fromRow' ctx row
-
-instance FromRow' f => FromRow' (M1 C c f) where
-  fromRow' ctx row = M1 <$> fromRow' ctx row
-
-instance (FromRow' f, FromRow' g) => FromRow' (f :*: g) where
-  fromRow' ctx row = do
-    y <- fromRow' ctx row
-    z <- fromRow' ctx row
-    pure $ y :*: z
-
-decodeField
-  :: FromField t
-  => Text
-  -> (Row -> Maybe t -> Either Error t')
-  -> FromRowCtx
-  -> Row
-  -> ExceptT Error IO (M1 S m (Rec0 t') p)
-decodeField nameText g (FromRowCtx result columnTable iRef) row = do
-  i <- liftIO $ readIORef iRef
-  liftIO $ modifyIORef' iRef (+1)
-  let (column, oid) = columnTable `indexColumnTable` i
-  mbField <- liftIO $ LibPQ.getvalue result row column
-  mbValue <- except $ getValue oid mbField
-  value <- except $ g row mbValue
-  pure $ M1 $ K1 value
-  where
-    getValue :: FromField u => LibPQ.Oid -> Maybe ByteString -> Either Error (Maybe u)
-    getValue oid = maybe (Right Nothing) $ \field ->
-      mapLeft
-        (ErrorInvalidField (ErrorPosition row nameText) oid field)
-        (Just <$> fromField field)
-
-mapLeft :: (b -> c) -> Either b a -> Either c a
-mapLeft f (Left l) = Left $ f l
-mapLeft _ (Right r) = Right r
-
-instance {-# OVERLAPPABLE #-} (FromField t, KnownSymbol nameSym) => FromRow' (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 t)) where
-  fromRow' = decodeField nameText $ \row ->
-    maybe (Left $ ErrorUnexpectedNull $ ErrorPosition row nameText) Right
-    where
-      nameText = Text.pack $ symbolVal @nameSym Proxy
-
-instance {-# OVERLAPPING #-} (KnownSymbol nameSym, FromField t) => FromRow' (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 (Maybe t))) where
-  fromRow' = decodeField nameText $ const pure
-    where
-      nameText = Text.pack $ symbolVal @nameSym Proxy

lib/Database/PostgreSQL/Opium/FromRow.hs

@@ -0,0 +1,162 @@
+{-# LANGUAGE AllowAmbiguousTypes #-}
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE DefaultSignatures #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE UndecidableInstances #-}
+
+module Database.PostgreSQL.Opium.FromRow
+  -- * FromRow
+  ( FromRow (..)
+  -- * Internal
+  , toListColumnTable
+  ) where
+
+import Control.Monad.IO.Class (liftIO)
+import Control.Monad.Trans.Except (ExceptT (..), except, runExceptT)
+import Data.ByteString (ByteString)
+import Data.Bifunctor (first)
+import Data.Kind (Type)
+import Data.Proxy (Proxy (..))
+import Data.Text (Text)
+import Data.Vector (Vector)
+import Database.PostgreSQL.LibPQ
+  ( Column
+  , Oid
+  , Result
+  , Row
+  )
+import GHC.Generics (Generic, C, D, K1 (..), M1 (..), Meta (..), Rec0, Rep, S, to, (:*:) (..))
+import GHC.TypeLits (KnownNat, KnownSymbol, Nat, natVal, symbolVal, type (+))
+
+import qualified Data.Text as Text
+import qualified Data.Text.Encoding as Encoding
+import qualified Data.Vector as Vector
+import qualified Database.PostgreSQL.LibPQ as LibPQ
+
+import Database.PostgreSQL.Opium.Error (Error (..), ErrorPosition (..))
+import Database.PostgreSQL.Opium.FromField (FromField (..), fromField)
+
+class FromRow a where
+  getColumnTable :: Proxy a -> Result -> IO (Either Error ColumnTable)
+  default getColumnTable :: (Generic a, GetColumnTable' (Rep a)) => Proxy a -> Result -> IO (Either Error ColumnTable)
+  getColumnTable Proxy = runExceptT . fmap newColumnTable . getColumnTable' @(Rep a) Proxy
+
+  fromRow :: Result -> ColumnTable -> Row -> IO (Either Error a)
+  default fromRow :: (Generic a, FromRow' 0 (Rep a)) => Result -> ColumnTable -> Row -> IO (Either Error a)
+  fromRow result columnTable row =
+    runExceptT $ to <$> fromRow' @0 FRProxy (FromRowCtx result columnTable) row
+
+newtype ColumnTable = ColumnTable (Vector (Column, Oid))
+  deriving (Eq, Show)
+
+newColumnTable :: [(Column, Oid)] -> ColumnTable
+newColumnTable = ColumnTable . Vector.fromList
+
+indexColumnTable :: ColumnTable -> Int -> (Column, Oid)
+indexColumnTable (ColumnTable v) i = v `Vector.unsafeIndex` i
+
+toListColumnTable :: ColumnTable -> [(Column, Oid)]
+toListColumnTable (ColumnTable v) = Vector.toList v
+
+class GetColumnTable' f where
+  getColumnTable' :: Proxy (f p) -> Result -> ExceptT Error IO [(Column, Oid)]
+
+instance GetColumnTable' f => GetColumnTable' (M1 D c f) where
+  getColumnTable' Proxy = getColumnTable' @f Proxy
+
+instance GetColumnTable' f => GetColumnTable' (M1 C c f) where
+  getColumnTable' Proxy = getColumnTable' @f Proxy
+
+instance (GetColumnTable' f, GetColumnTable' g) => GetColumnTable' (f :*: g) where
+  getColumnTable' Proxy result =
+    (++) <$> getColumnTable' @f Proxy result <*> getColumnTable' @g Proxy result
+
+instance {-# OVERLAPPABLE #-} (FromField t, KnownSymbol nameSym) => GetColumnTable' (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 t)) where
+  getColumnTable' Proxy = checkColumn @t Proxy $ symbolVal @nameSym Proxy
+
+instance {-# OVERLAPPING #-} (KnownSymbol nameSym, FromField t) => GetColumnTable' (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 (Maybe t))) where
+  getColumnTable' Proxy = checkColumn @t Proxy $ symbolVal @nameSym Proxy
+
+-- | State kept for a call to 'fromRow'.
+data FromRowCtx = FromRowCtx
+  Result -- ^ Obtained from 'LibPQ.execParams'.
+  ColumnTable -- ^ 'Vector' of expected columns indices and OIDs.
+
+data FRProxy (n :: Nat) (f :: Type -> Type) = FRProxy
+
+class FromRow' (n :: Nat) (f :: Type -> Type) where
+  type Members f :: Nat
+
+  fromRow' :: FRProxy n f -> FromRowCtx -> Row -> ExceptT Error IO (f p)
+
+instance FromRow' n f => FromRow' n (M1 D c f) where
+  type Members (M1 D c f) = Members f
+
+  fromRow' FRProxy ctx row = M1 <$> fromRow' @n FRProxy ctx row
+
+instance FromRow' n f => FromRow' n (M1 C c f) where
+  type Members (M1 C c f) = Members f
+
+  fromRow' FRProxy ctx row = M1 <$> fromRow' @n FRProxy ctx row
+
+instance (FromRow' n f, FromRow' (n + Members f) g) => FromRow' n (f :*: g) where
+  type Members (f :*: g) = Members f + Members g
+
+  fromRow' FRProxy ctx row = (:*:) <$> fromRow' @n FRProxy ctx row <*> fromRow' @(n + Members 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
+  type Members (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 t)) = 1
+
+  fromRow' FRProxy = decodeField memberIndex nameText $ \row ->
+    maybe (Left $ ErrorUnexpectedNull $ ErrorPosition row nameText) Right
+    where
+      memberIndex = fromIntegral $ natVal @n Proxy
+      nameText = Text.pack $ symbolVal @nameSym Proxy
+
+instance {-# OVERLAPPING #-} (KnownNat n, KnownSymbol nameSym, FromField t) => FromRow' n (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 (Maybe t))) where
+  type Members (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 (Maybe t))) = 1
+
+  fromRow' FRProxy = decodeField memberIndex nameText $ const pure
+    where
+      memberIndex = fromIntegral $ natVal @n Proxy
+      nameText = Text.pack $ symbolVal @nameSym Proxy
+
+checkColumn :: forall a. FromField a => Proxy a -> String -> Result -> ExceptT Error IO [(Column, Oid)]
+checkColumn Proxy nameStr result = do
+  column <- ExceptT $ maybe (Left $ ErrorMissingColumn nameText) Right <$> LibPQ.fnumber result name
+  oid <- liftIO $ LibPQ.ftype result column
+  if validOid @a Proxy oid then
+    pure [(column, oid)]
+  else
+    except $ Left $ ErrorInvalidOid nameText oid
+  where
+    nameText = Text.pack nameStr
+    name = Encoding.encodeUtf8 nameText
+
+decodeField
+  :: FromField t
+  => Int
+  -> Text
+  -> (Row -> Maybe t -> Either Error t')
+  -> FromRowCtx
+  -> Row
+  -> ExceptT Error IO (M1 S m (Rec0 t') p)
+decodeField memberIndex nameText g (FromRowCtx result columnTable) row = do
+  let (column, oid) = columnTable `indexColumnTable` memberIndex
+  mbField <- liftIO $ LibPQ.getvalue result row column
+  mbValue <- except $ fromFieldIfPresent oid mbField
+  value <- except $ g row mbValue
+  pure $ M1 $ K1 value
+  where
+    fromFieldIfPresent :: FromField u => LibPQ.Oid -> Maybe ByteString -> Either Error (Maybe u)
+    fromFieldIfPresent oid = maybe (Right Nothing) $ \field ->
+      first
+        (ErrorInvalidField (ErrorPosition row nameText) oid field)
+        (Just <$> fromField field)
+

opium.cabal

@@ -62,13 +62,14 @@ library
     exposed-modules:
         Database.PostgreSQL.Opium,
         Database.PostgreSQL.Opium.FromField,
+        Database.PostgreSQL.Opium.FromRow,
         Database.PostgreSQL.Opium.ToField
 
     -- Modules included in this library but not exported.
     other-modules:
         Database.PostgreSQL.Opium.Error,
-        Database.PostgreSQL.Opium.ToParamList,
+        Database.PostgreSQL.Opium.Oid,
-        Database.PostgreSQL.Opium.Oid
+        Database.PostgreSQL.Opium.ToParamList
 
     -- LANGUAGE extensions used by modules in this package.
     -- other-extensions:

test/Database/PostgreSQL/OpiumSpec.hs

@@ -7,6 +7,7 @@
 module Database.PostgreSQL.OpiumSpec (spec) where
 
 import Data.ByteString (ByteString)
+import Data.Either (isLeft)
 import Data.Functor.Identity (Identity (..))
 import Data.Proxy (Proxy (..))
 import Data.Text (Text)
@@ -16,6 +17,7 @@ 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
@@ -53,10 +55,6 @@ newtype Only a = Only
 
 instance Opium.FromField a => Opium.FromRow (Only a) where
 
-isLeft :: Either a b -> Bool
-isLeft (Left _) = True
-isLeft _ = False
-
 shouldHaveColumns
   :: Opium.FromRow a
   => Proxy a
@@ -67,7 +65,7 @@ shouldHaveColumns
 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.toListColumnTable) columnTable
+  let actualColumns = fmap (map fst . Opium.FromRow.toListColumnTable) columnTable
   actualColumns `shouldBe` Right expectedColumns
 
 spec :: SpecWith Connection