feat!: rework error handling

2024-06-25 13:53:09 -05:00 · 2024-06-25 13:53:09 -05:00 · 11c1645173
commit 11c1645173
parent c9e5ee9205
20 changed files with 529 additions and 256 deletions
--- a/spago.lock
+++ b/spago.lock
@ -15,11 +15,13 @@ workspace:
        - foldable-traversable: ">=6.0.0 <7.0.0"
        - foreign: ">=7.0.0 <8.0.0"
        - fork: ">=6.0.0 <7.0.0"
+        - functors
        - integers: ">=6.0.0 <7.0.0"
        - js-bigints: ">=2.2.1 <3.0.0"
        - lists: ">=7.0.0 <8.0.0"
        - maybe: ">=6.0.0 <7.0.0"
        - mmorph: ">=7.0.0 <8.0.0"
+        - monad-control
        - newtype: ">=5.0.0 <6.0.0"
        - node-buffer: ">=9.0.0 <10.0.0"
        - node-event-emitter: ">=3.0.0 <4.0.0"
@ -33,6 +35,7 @@ workspace:
        - prelude: ">=6.0.1 <7.0.0"
        - profunctor: ">=6.0.1 <7.0.0"
        - record: ">=4.0.0 <5.0.0"
+        - refs
        - simple-json: ">=9.0.0 <10.0.0"
        - strings: ">=6.0.1 <7.0.0"
        - tailrec: ">=6.1.0 <7.0.0"
@ -728,8 +731,8 @@ packages:
      - unsafe-coerce
  node-stream-pipes:
    type: registry
-    version: 2.1.0
-    integrity: sha256-pYBOQY4bGEZzI5UHsUxJAhsKqtmE6CC1sHmFqgj64V8=
+    version: 2.1.1
+    integrity: sha256-j7ZE+Vtc9gwXlH8s8pyVsbbCMd91AIRk05IOsZAO7x0=
    dependencies:
      - aff
      - arrays
--- a/spago.yaml
+++ b/spago.yaml
@ -13,6 +13,9 @@ package:
    strict: true
    pedanticPackages: true
  dependencies:
+    - functors
+    - monad-control
+    - refs
    - aff: ">=7.1.0 <8.0.0"
    - aff-promise: ">=4.0.0 <5.0.0"
    - arrays: ">=7.3.0 <8.0.0"
--- a/src/Control.Monad.Postgres.Base.purs
+++ b/src/Control.Monad.Postgres.Base.purs
@ -2,18 +2,13 @@ module Control.Monad.Postgres.Base where

 import Prelude

-import Control.Alt (class Alt)
-import Control.Alternative (class Alternative, class Plus)
-import Control.Monad.Error.Class (class MonadError, class MonadThrow, catchError, throwError)
-import Control.Monad.Fork.Class (class MonadBracket, class MonadFork, class MonadKill, bracket, kill, never, uninterruptible)
-import Control.Monad.Morph (class MFunctor, class MMonad)
+import Control.Monad.Error.Class (catchError, throwError)
+import Control.Monad.Fork.Class (class MonadBracket, bracket)
+import Control.Monad.Morph (hoist)
 import Control.Monad.Postgres.Cursor (class MonadCursor, CursorT)
-import Control.Monad.Postgres.Session (class MonadSession, SessionT, exec, exec_, query, streamIn, streamOut)
-import Control.Monad.Reader (class MonadAsk, class MonadReader, ReaderT, ask, local, runReaderT)
-import Control.Monad.Rec.Class (class MonadRec)
-import Control.Monad.Trans.Class (class MonadTrans, lift)
-import Control.Parallel (class Parallel, parallel, sequential)
-import Data.Newtype (class Newtype, unwrap, wrap)
+import Control.Monad.Postgres.Session (class MonadSession, SessionT, endSession, exec, exec_, startSession)
+import Control.Monad.Reader (ask, runReaderT)
+import Data.Newtype (unwrap)
 import Data.Postgres (RepT)
 import Data.Postgres.Query (class AsQuery, asQuery)
 import Data.Postgres.Raw (Raw)
@ -22,9 +17,12 @@ import Data.Tuple.Nested ((/\))
 import Effect.Aff.Class (class MonadAff, liftAff)
 import Effect.Aff.Postgres.Pool (Pool)
 import Effect.Aff.Postgres.Pool as Pool
-import Effect.Aff.Unlift (class MonadUnliftAff)
-import Effect.Class (class MonadEffect, liftEffect)
-import Effect.Unlift (class MonadUnliftEffect)
+import Effect.Class (liftEffect)
+import Effect.Exception as Effect
+import Effect.Postgres.Error (RE)
+import Effect.Postgres.Error as E
+import Effect.Postgres.Error.Except as X
+import Effect.Postgres.Error.RE as RE
 import Prim.Row (class Union)

 -- | Monad handling pool resource acquisition & release
@ -51,53 +49,8 @@ import Prim.Row (class Union)
 -- |       res <- Client.query "select * from foo" client
 -- |       pure $ res == 1
 -- | ```
-newtype PostgresT :: forall k. (k -> Type) -> k -> Type
-newtype PostgresT m a = PostgresT (ReaderT Pool m a)
-
-derive instance Newtype (PostgresT m a) _
-derive newtype instance (Functor m) => Functor (PostgresT m)
-derive newtype instance (Apply m) => Apply (PostgresT m)
-derive newtype instance (Alternative m) => Alternative (PostgresT m)
-derive newtype instance (Applicative m) => Applicative (PostgresT m)
-derive newtype instance (Plus m) => Plus (PostgresT m)
-derive newtype instance (Alt m) => Alt (PostgresT m)
-derive newtype instance (Bind m) => Bind (PostgresT m)
-derive newtype instance (Monad m) => Monad (PostgresT m)
-derive newtype instance (MonadEffect m) => MonadEffect (PostgresT m)
-derive newtype instance (MonadAff m) => MonadAff (PostgresT m)
-derive newtype instance (MonadUnliftEffect m) => MonadUnliftEffect (PostgresT m)
-derive newtype instance (MonadUnliftAff m) => MonadUnliftAff (PostgresT m)
-derive newtype instance MonadRec m => MonadRec (PostgresT m)
-derive newtype instance MonadTrans (PostgresT)
-derive newtype instance (MonadThrow e m) => MonadThrow e (PostgresT m)
-derive newtype instance (MonadError e m) => MonadError e (PostgresT m)
-derive newtype instance (MonadFork f m) => MonadFork f (PostgresT m)
-derive newtype instance MFunctor PostgresT
-derive newtype instance MMonad PostgresT
-instance (Apply m, Apply p, Parallel p m) => Parallel (PostgresT p) (PostgresT m) where
-  parallel = wrap <<< parallel <<< unwrap
-  sequential = wrap <<< sequential <<< unwrap
-
-instance (Monad m, MonadKill e f m) => MonadKill e f (PostgresT m) where
-  kill a b = lift $ kill a b
-
-instance (Monad m, MonadBracket e f (ReaderT Pool m), MonadBracket e f m) => MonadBracket e f (PostgresT m) where
-  bracket acq rel m = wrap $ bracket (unwrap acq) (\a b -> unwrap $ rel a b) (unwrap <<< m)
-  uninterruptible a = wrap $ uninterruptible $ unwrap a
-  never = lift $ never
-
-instance Monad m => MonadAsk Pool (PostgresT m) where
-  ask = wrap ask
-
-instance Monad m => MonadReader Pool (PostgresT m) where
-  local f m = wrap $ local f $ unwrap m
-
-instance (MonadBracket e f m, MonadAff m) => MonadSession (PostgresT m) where
-  query = session <<< query
-  exec = session <<< exec
-  exec_ = session <<< exec_
-  streamIn = session <<< streamIn
-  streamOut = session <<< streamOut
+type PostgresT :: (Type -> Type) -> Type -> Type
+type PostgresT = RE Pool

 -- | Typeclass generalizing `PostgresT`. Allows for dependency-injecting different
 -- | implementations of the idea of a postgres connection.
@ -117,13 +70,23 @@ class (Monad m, MonadSession session, MonadCursor cursor ct) <= MonadPostgres m
  -- | yielded by the cursor
  cursorWith :: forall q. AsQuery q => (Array Raw -> RepT ct) -> String -> q -> cursor ~> m

-instance (MonadBracket e f m, MonadAff m, MonadSession (SessionT m), MonadCursor (CursorT t (SessionT m)) t) => MonadPostgres (PostgresT m) (SessionT m) (CursorT ct (SessionT m)) ct where
+instance
+  ( MonadBracket Effect.Error f m
+  , MonadAff m
+  , MonadSession (SessionT m)
+  , MonadCursor (CursorT t (SessionT m)) t
+  ) => MonadPostgres
+         (PostgresT m)
+         (SessionT m)
+         (CursorT ct (SessionT m))
+         ct
+  where
  session m = do
    pool <- ask
-    let
-      acq = liftAff $ Pool.connect pool
-      rel _ c = liftEffect $ Pool.release pool c
-    lift $ bracket acq rel (runReaderT m)
+    client <- RE.liftExcept $ hoist liftAff $ startSession pool
+    RE.finally
+      (RE.liftExcept $ hoist liftEffect $ endSession pool client)
+      (RE.liftExcept $ RE.toExcept m client)
  transaction m =
    let
      begin = void $ exec "begin;"
@ -133,27 +96,29 @@ instance (MonadBracket e f m, MonadAff m, MonadSession (SessionT m), MonadCursor
      session $ begin *> catchError commit rollback
  cursorWith f cur q m =
    transaction do
-      q' <- liftEffect $ asQuery q
+      q' <- RE.liftExcept $ X.printing $ asQuery q
      exec_ $ "declare " <> cur <> " cursor for (" <> (unwrap q').text <> ");"
      runReaderT (unwrap m) (cur /\ f)

 -- | Create a server-side cursor for a query in a transaction,
 -- | and execute a `CursorT` with a view to the new cursor.
 cursor :: forall @cursort t session cursor q a. MonadPostgres t session cursor cursort => AsQuery q => FromRow cursort => String -> q -> cursor a -> t a
-cursor = cursorWith fromRow
+cursor = cursorWith (fromRow 0)

 -- | Execute a `PostgresT` using an existing connection pool.
 -- |
 -- | This will not invoke `Pool.end` after executing.
-withPool :: forall m a. PostgresT m a -> Pool -> m a
-withPool = runReaderT <<< unwrap
+withPool :: forall m a. PostgresT m a -> Pool -> E.Except m a
+withPool m p = runReaderT (unwrap m) p

 -- | Create a new connection pool from the provided config and execute
 -- | the postgres monad, invoking `Effect.Aff.Postgres.Pool.end` afterwards.
-runPostgres :: forall m a missing trash r e f. MonadBracket e f m => MonadAff m => Union r missing (Pool.Config trash) => Record r -> PostgresT m a -> m a
+runPostgres :: forall m a missing trash r f. MonadBracket Effect.Error f m => MonadAff m => Union r missing (Pool.Config trash) => Record r -> PostgresT m a -> E.Except m a
 runPostgres cfg m =
  let
+    acq :: RE Unit m Pool
    acq = liftEffect $ Pool.make @r @missing @trash cfg
-    rel _ p = liftAff $ Pool.end p
+    rel :: _ -> Pool -> RE Unit m Unit
+    rel _ p = RE.liftExcept $ hoist liftAff $ Pool.end p
  in
-    bracket acq rel $ withPool m
+    RE.toExcept (bracket acq rel $ RE.liftExcept <<< withPool m) unit
--- a/src/Control.Monad.Postgres.Cursor.purs
+++ b/src/Control.Monad.Postgres.Cursor.purs
@ -22,6 +22,7 @@ import Data.Tuple.Nested (type (/\), (/\))
 import Effect.Aff.Class (class MonadAff)
 import Effect.Aff.Unlift (class MonadUnliftAff)
 import Effect.Class (class MonadEffect, liftEffect)
+import Effect.Postgres.Error as E
 import Effect.Unlift (class MonadUnliftEffect)

 data Move
@ -118,7 +119,7 @@ instance (MonadSession m) => MonadCursor (CursorT t m) t where
    RowsAffected n' <- query $ ("move relative $1 from " <> cur) /\ n
    pure n'

-instance (MonadSession m) => MonadSession (CursorT t m) where
+instance (MonadThrow E.E m, MonadSession m) => MonadSession (CursorT t m) where
  query = lift <<< query
  exec = lift <<< exec
  exec_ = lift <<< exec_
--- a/src/Control.Monad.Postgres.Session.purs
+++ b/src/Control.Monad.Postgres.Session.purs
@ -1,21 +1,50 @@
 module Control.Monad.Postgres.Session where

-import Prelude
+import Prelude hiding (join)

-import Control.Monad.Reader (ReaderT, ask)
+import Control.Monad.Error.Class (class MonadError, class MonadThrow, catchError, throwError)
+import Control.Monad.Fork.Class (class MonadBracket)
+import Control.Monad.Morph (hoist)
+import Control.Monad.Reader (ask)
+import Control.Monad.Trans.Class (lift)
+import Data.Newtype (wrap)
 import Data.Postgres.Query (class AsQuery)
 import Data.Postgres.Result (class FromRows)
+import Effect (Effect)
+import Effect.Aff (Aff)
 import Effect.Aff.Class (class MonadAff, liftAff)
 import Effect.Aff.Postgres.Client (Client)
 import Effect.Aff.Postgres.Client as Client
-import Effect.Class (liftEffect)
-import Node.Stream (Writable, Readable)
+import Effect.Aff.Postgres.Pool (Pool)
+import Effect.Aff.Postgres.Pool as Pool
+import Effect.Class (class MonadEffect, liftEffect)
+import Effect.Exception as Effect
+import Effect.Postgres.Error (RE)
+import Effect.Postgres.Error as E
+import Effect.Postgres.Error.Except as X
+import Effect.Postgres.Error.RE as RE
+import Effect.Ref as Ref
+import Node.EventEmitter as Event
+import Node.Stream (Readable, Stream, Writable)
+import Node.Stream as Stream

-type SessionT :: forall k. (k -> Type) -> k -> Type
-type SessionT = ReaderT Client
+type SessionT :: (Type -> Type) -> Type -> Type
+type SessionT = RE Client
+
+class MonadStartSession a where
+  startSession :: a -> E.Except Aff Client
+  endSession :: a -> Client -> E.Except Effect Unit
+
+instance MonadStartSession Pool where
+  startSession = Pool.connect
+  endSession p c = Pool.release p c
+
+instance MonadStartSession Client where
+  startSession = pure
+  endSession _ _ = pure unit

 -- | A monad representing a connected session to a database
-class MonadAff m <= MonadSession m where
+class (MonadThrow E.E m, MonadAff m) <= MonadSession m where
  -- | Executes a query and unmarshals the result into `r`
  query :: forall q r. AsQuery q => FromRows r => q -> m r
  -- | Executes a query and returns the number of rows affected
@ -41,17 +70,44 @@ class MonadAff m <= MonadSession m where
  -- | ```
  streamOut :: String -> m (Readable ())

-instance MonadAff m => MonadSession (SessionT m) where
+instance (MonadStartSession s, MonadBracket Effect.Error f m, MonadAff m) => MonadSession (RE s m) where
  query q = do
-    client <- ask
-    liftAff $ Client.query q client
+    pool <- ask
+    client <- hoist liftAff $ RE.liftExcept $ startSession pool
+    RE.finally
+      (hoist liftEffect $ RE.liftExcept $ endSession pool client)
+      (wrap $ lift $ hoist liftAff $ Client.query q client)
  exec q = do
-    client <- ask
-    liftAff $ Client.exec q client
+    pool <- ask
+    client <- hoist liftAff $ RE.liftExcept $ startSession pool
+    RE.finally
+      (hoist liftEffect $ RE.liftExcept $ endSession pool client)
+      (wrap $ lift $ hoist liftAff $ Client.exec q client)
  exec_ = void <<< exec
  streamIn q = do
-    client <- ask
-    liftEffect $ Client.execWithStdin q client
+    pool <- ask
+    client <- hoist liftAff $ RE.liftExcept $ startSession pool
+    handleStream (X.run $ endSession pool client) (RE.liftExcept $ hoist liftEffect $ Client.execWithStdin q client)
  streamOut q = do
-    client <- ask
-    liftEffect $ Client.queryWithStdout q client
+    pool <- ask
+    client <- hoist liftAff $ RE.liftExcept $ startSession pool
+    handleStream (X.run $ endSession pool client) (RE.liftExcept $ hoist liftEffect $ Client.queryWithStdout q client)
+
+handleStream :: forall e m r. MonadEffect m => MonadError e m => Effect Unit -> m (Stream r) -> m (Stream r)
+handleStream onError getStream =
+    flip catchError
+      (\e -> liftEffect onError *> throwError e)
+      do
+        stream <- getStream
+        liftEffect $ onErrorOrClose stream onError
+        pure stream
+
+onErrorOrClose :: forall r. Stream r -> Effect Unit -> Effect Unit
+onErrorOrClose stream eff = do
+  did <- Ref.new false
+  let
+    onE = do
+      did' <- Ref.read did
+      when (not did') (Ref.write true did *> eff)
+  Event.once_ Stream.errorH (const onE) stream
+  Event.once_ Stream.closeH onE stream
--- a/src/Data.Postgres.Query.purs
+++ b/src/Data.Postgres.Query.purs
@ -2,10 +2,11 @@ module Data.Postgres.Query where

 import Prelude

+import Control.Monad.Cont (lift)
 import Data.Maybe (Maybe(..))
 import Data.Newtype (class Newtype)
 import Data.Nullable (Nullable, toNullable)
-import Data.Postgres (class Rep, serialize, smash)
+import Data.Postgres (class Rep, RepT, serialize, smash)
 import Data.Postgres.Raw (Raw)
 import Data.Tuple.Nested (type (/\), (/\))
 import Effect (Effect)
@ -30,26 +31,29 @@ derive newtype instance Show Query
 emptyQuery :: Query
 emptyQuery = Query { text: "", values: [], name: Nothing }

+stringQuery :: String -> Query
+stringQuery s = Query { text: s, values: [], name: Nothing }
+
 -- | Any value that can be converted to an array of query parameters
 class AsQueryParams a where
-  asQueryParams :: a -> Effect (Array Raw)
+  asQueryParams :: a -> RepT (Array Raw)

 instance AsQueryParams (Array Raw) where
  asQueryParams = pure
 else instance (Rep a, AsQueryParams b) => AsQueryParams (a /\ b) where
  asQueryParams (a /\ tail) = do
-    a' <- map pure $ smash $ serialize a
+    a' <- map pure $ serialize a
    tail' <- asQueryParams tail
    pure $ a' <> tail'
 else instance (Rep a) => AsQueryParams a where
-  asQueryParams = map pure <<< smash <<< serialize
+  asQueryParams = map pure <<< serialize

 -- | Values that can be rendered as a SQL query
 class AsQuery a where
-  asQuery :: a -> Effect Query
+  asQuery :: a -> RepT Query

 instance AsQuery a => AsQuery (Effect a) where
-  asQuery a = asQuery =<< a
+  asQuery a = asQuery =<< lift a

 instance AsQuery Query where
  asQuery = pure
--- a/src/Data.Postgres.Result.purs
+++ b/src/Data.Postgres.Result.purs
@ -2,7 +2,7 @@ module Data.Postgres.Result where

 import Prelude

-import Control.Monad.Error.Class (liftMaybe, throwError)
+import Control.Monad.Error.Class (catchError, liftMaybe, throwError)
 import Data.Array as Array
 import Data.Generic.Rep (class Generic)
 import Data.Int as Int
@ -43,9 +43,9 @@ class FromRows a where
 instance FromRows RowsAffected where
  fromRows a _ = pure a
 else instance (FromRow a) => FromRows (Array a) where
-  fromRows _ = traverse fromRow
+  fromRows _ = traverse (fromRow 0)
 else instance (FromRow a) => FromRows (Maybe a) where
-  fromRows _ = map Array.head <<< traverse fromRow
+  fromRows _ = map Array.head <<< traverse (fromRow 0)
 else instance (FromRow a) => FromRows a where
  fromRows a =
    let
@ -89,29 +89,34 @@ class FromRow (a :: Type) where
  -- | Minimum length of row for type `a`
  minColumnCount :: forall g. g a -> Int
  -- | Performs the conversion
-  fromRow :: Array Raw -> RepT a
+  fromRow :: Int -> Array Raw -> RepT a

 instance (Deserialize a, FromRow b) => FromRow (a /\ b) where
  minColumnCount _ = minColumnCount (Proxy @b) + 1
-  fromRow r =
+  fromRow n r =
    let
      minLen = minColumnCount (Proxy @(Tuple a b))
      lengthMismatch = pure $ TypeMismatch ("Expected row to have at least " <> show minLen <> " columns") ("Found row of length " <> show (Array.length r))
    in
      do
+        let
+          de a =
+            catchError
+              (deserialize @a a)
+              (\e -> throwError $ ErrorAtIndex n <$> e)
        when (Array.length r < minLen) (throwError lengthMismatch)
-        a <- deserialize =<< liftMaybe lengthMismatch (Array.head r)
-        b <- fromRow =<< liftMaybe lengthMismatch (Array.tail r)
+        a <- de =<< liftMaybe lengthMismatch (Array.head r)
+        b <- fromRow (n + 1) =<< liftMaybe lengthMismatch (Array.tail r)
        pure $ a /\ b
 else instance FromRow (Array Raw) where
  minColumnCount _ = 0
-  fromRow = pure
+  fromRow _ = pure
 else instance FromRow Unit where
  minColumnCount _ = 0
-  fromRow _ = pure unit
+  fromRow _ _ = pure unit
 else instance Deserialize a => FromRow a where
  minColumnCount _ = 1
-  fromRow r =
+  fromRow _ r =
    let
      err = pure $ TypeMismatch "Expected row of length >= 1" "Empty row"
    in
--- a/src/Effect.Aff.Postgres.Client.purs
+++ b/src/Effect.Aff.Postgres.Client.purs
@ -1,4 +1,4 @@
-module Effect.Aff.Postgres.Client (connected, connect, end, exec, execWithStdin, queryWithStdout, query, queryRaw, __connect, __end, __query, __execStreamStdin, __execStreamStdout, module X) where
+module Effect.Aff.Postgres.Client (connected, connect, end, exec, execWithStdin, queryWithStdout, query, queryRaw, __connect, __end, __query, __execStreamStdin, __execStreamStdout, module Reexport) where

 import Prelude

@ -7,14 +7,15 @@ import Control.Promise as Promise
 import Data.Functor (voidRight)
 import Data.Maybe (fromMaybe)
 import Data.Newtype (wrap)
-import Data.Postgres (smash)
-import Data.Postgres.Query (class AsQuery, QueryRaw, asQuery, __queryToRaw)
+import Data.Postgres.Query (class AsQuery, QueryRaw, __queryToRaw, asQuery, stringQuery)
 import Data.Postgres.Result (class FromRows, Result, fromRows, rows, rowsAffected)
 import Effect (Effect)
 import Effect.Aff (Aff)
 import Effect.Class (liftEffect)
-import Effect.Postgres.Client (Client, ClientConfigRaw, Config, Notification, NotificationRaw, __make, __uncfg, endE, errorE, make, noticeE, notificationE) as X
+import Effect.Postgres.Client (Client, ClientConfigRaw, Config, Notification, NotificationRaw, __make, __uncfg, endE, errorE, make, noticeE, notificationE) as Reexport
 import Effect.Postgres.Client (Client, Config, make)
+import Effect.Postgres.Error as E
+import Effect.Postgres.Error.Except as X
 import Node.Stream (Readable, Writable)
 import Prim.Row (class Union)

@ -23,8 +24,8 @@ import Prim.Row (class Union)
 -- | The config parameter `r` is `Config` with all keys optional.
 -- |
 -- | This is a shorthand for `(voidRight <*> connect) =<< liftEffect (make cfg)`
-connected :: forall r missing trash. Union r missing (Config trash) => Record r -> Aff Client
-connected cfg = (voidRight <*> connect) =<< liftEffect (make @r @missing @trash cfg)
+connected :: forall r missing trash. Union r missing (Config trash) => Record r -> E.Except Aff Client
+connected cfg = X.with E.Connecting $ (voidRight <*> connect) =<< liftEffect (make @r @missing @trash cfg)

 -- | Connects the client to the database
 -- |
@ -35,41 +36,43 @@ connect = Promise.toAffE <<< __connect
 -- | Disconnects the client from the database
 -- |
 -- | <https://node-postgres.com/apis/client#clientend>
-end :: Client -> Aff Unit
-end = Promise.toAffE <<< __end
+end :: Client -> E.Except Aff Unit
+end = X.with E.Disconnecting <<< Promise.toAffE <<< __end

 -- | Performs a query, returning the raw `Result` object
 -- |
 -- | <https://node-postgres.com/apis/client#clientquery>
-queryRaw :: forall q. AsQuery q => q -> Client -> Aff Result
+queryRaw :: forall q. AsQuery q => q -> Client -> E.Except Aff Result
 queryRaw q c = do
-  q' <- __queryToRaw <$> liftEffect (asQuery q)
-  Promise.toAffE $ __query q' c
+  q' <- X.printing $ asQuery q
+  let q'' = __queryToRaw q'
+  X.executing q' $ Promise.toAffE $ __query q'' c

 -- | Performs a query that we expect to not yield any rows,
 -- | returning the number of rows affected by the statement.
 -- |
 -- | <https://node-postgres.com/apis/client#clientquery>
-exec :: forall q. AsQuery q => q -> Client -> Aff Int
+exec :: forall q. AsQuery q => q -> Client -> E.Except Aff Int
 exec q = map (fromMaybe 0 <<< rowsAffected) <<< queryRaw q

 -- | Performs a query that we expect to yield rows,
 -- | returning them unmarshalled into destination type `r`.
 -- |
 -- | <https://node-postgres.com/apis/client#clientquery>
-query :: forall q r. AsQuery q => FromRows r => q -> Client -> Aff r
+query :: forall q r. AsQuery q => FromRows r => q -> Client -> E.Except Aff r
 query q c = do
+  q' <- X.printing $ asQuery q
  raw <- queryRaw q c
  let
    affected = rowsAffected raw
    rows' = rows raw
-  liftEffect $ smash $ fromRows (wrap $ fromMaybe 0 affected) rows'
+  X.parsing q' $ fromRows (wrap $ fromMaybe 0 affected) rows'

-execWithStdin :: String -> Client -> Effect (Writable ())
-execWithStdin q c = __execStreamStdin q c
+execWithStdin :: String -> Client -> E.Except Effect (Writable ())
+execWithStdin q c = X.executing (stringQuery q) $ __execStreamStdin q c

-queryWithStdout :: String -> Client -> Effect (Readable ())
-queryWithStdout q c = __execStreamStdout q c
+queryWithStdout :: String -> Client -> E.Except Effect (Readable ())
+queryWithStdout q c = X.executing (stringQuery q) $ __execStreamStdout q c

 -- | FFI binding to `Client#connect`
 foreign import __connect :: Client -> Effect (Promise Unit)
--- a/src/Effect.Aff.Postgres.Pool.purs
+++ b/src/Effect.Aff.Postgres.Pool.purs
@ -7,8 +7,25 @@ import Control.Promise as Promise
 import Effect (Effect)
 import Effect.Aff (Aff)
 import Effect.Postgres.Client (Client)
+import Effect.Postgres.Error as E
+import Effect.Postgres.Error.Except as E.Except
 import Effect.Postgres.Pool (Pool)
-import Effect.Postgres.Pool as X
+import Effect.Postgres.Pool
+  (Config
+  , Pool
+  , PoolConfigRaw
+  , acquireE
+  , clientCount
+  , clientIdleCount
+  , clientWaitingCount
+  , connectE
+  , destroy
+  , errorE
+  , make
+  , release
+  , releaseE
+  , removeE
+  ) as X

 -- | Acquires a client from the pool.
 -- | 
@ -17,8 +34,8 @@ import Effect.Postgres.Pool as X
 -- |  * If the pool is 'full' and all clients are currently checked out will wait in a FIFO queue until a client becomes available by it being released back to the pool.
 -- |
 -- | <https://node-postgres.com/apis/pool#poolconnect>
-connect :: Pool -> Aff Client
-connect = Promise.toAffE <<< __connect
+connect :: Pool -> E.Except Aff Client
+connect = E.Except.with E.Connecting <<< Promise.toAffE <<< __connect

 -- | Drain the pool of all active clients, disconnect them,
 -- | and shut down any internal timers in the pool.
@ -27,8 +44,8 @@ connect = Promise.toAffE <<< __connect
 -- | your process is attempting to shut down cleanly.
 -- |
 -- | <https://node-postgres.com/apis/pool#poolend>
-end :: Pool -> Aff Unit
-end = Promise.toAffE <<< __end
+end :: Pool -> E.Except Aff Unit
+end = E.Except.with E.Disconnecting <<< Promise.toAffE <<< __end

 -- | FFI binding to `Pool#end`
 foreign import __end :: Pool -> Effect (Promise Unit)
--- a/src/Effect.Postgres.Error.Common.purs
+++ b/src/Effect.Postgres.Error.Common.purs
@ -0,0 +1,30 @@
+module Effect.Postgres.Error.Common where
+
+import Prelude
+
+import Data.Array.NonEmpty (NonEmptyArray)
+import Data.Generic.Rep (class Generic)
+import Data.Postgres.Query (Query)
+import Data.Show.Generic (genericShow)
+import Effect.Exception as Effect
+import Foreign (MultipleErrors)
+
+type E = NonEmptyArray Error
+
+data Error
+  = Deserializing Query MultipleErrors
+  | Serializing MultipleErrors
+  | Executing Query Effect.Error
+  | Connecting Effect.Error
+  | Disconnecting Effect.Error
+  | Other Effect.Error
+
+derive instance Generic Error _
+instance Show Error where
+  show = genericShow
+
+toException' :: Error -> Effect.Error
+toException' = Effect.error <<< show
+
+toException :: E -> Effect.Error
+toException = Effect.error <<< show
--- a/src/Effect.Postgres.Error.Except.purs
+++ b/src/Effect.Postgres.Error.Except.purs
@ -0,0 +1,39 @@
+module Effect.Postgres.Error.Except where
+
+import Prelude
+
+import Control.Monad.Error.Class (class MonadError, class MonadThrow, liftEither, try)
+import Control.Monad.Except (ExceptT(..), runExceptT)
+import Data.Bifunctor (lmap)
+import Data.Either (Either)
+import Data.Postgres (RepT)
+import Data.Postgres.Query (Query)
+import Effect.Class (class MonadEffect, liftEffect)
+import Effect.Exception as Effect
+import Effect.Postgres.Error.Common (E, Error(..), toException)
+
+type Except m = ExceptT E m
+
+run :: forall m a. MonadThrow Effect.Error m => Except m a -> m a
+run m = liftEither =<< lmap toException <$> runExceptT m
+
+toEither :: forall m a. Except m a -> m (Either E a)
+toEither = runExceptT
+
+with :: forall e m a. MonadError e m => (e -> Error) -> m a -> Except m a
+with e m = ExceptT $ map (lmap $ pure <<< e) $ try m
+
+withEither :: forall e m a. Monad m => (e -> Error) -> m (Either e a) -> Except m a
+withEither e m = ExceptT $ map (lmap $ pure <<< e) $ m
+
+exception :: forall m a. MonadError Effect.Error m => m a -> Except m a
+exception = with Other
+
+parsing :: forall m a. MonadEffect m => Query -> RepT a -> Except m a
+parsing q = withEither (Deserializing q) <<< liftEffect <<< runExceptT
+
+printing :: forall m a. MonadEffect m => RepT a -> Except m a
+printing = withEither Serializing <<< liftEffect <<< runExceptT
+
+executing :: forall m a. MonadError Effect.Error m => Query -> m a -> Except m a
+executing q = with (Executing q)
--- a/src/Effect.Postgres.Error.RE.purs
+++ b/src/Effect.Postgres.Error.RE.purs
@ -0,0 +1,116 @@
+module Effect.Postgres.Error.RE where
+
+import Prelude hiding (join)
+
+import Control.Alt (class Alt)
+import Control.Monad.Base (class MonadBase)
+import Control.Monad.Error.Class (class MonadError, class MonadThrow, catchError, liftEither, throwError)
+import Control.Monad.Except (ExceptT, runExceptT)
+import Control.Monad.Fork.Class (class MonadBracket, class MonadFork, class MonadKill, BracketCondition, bracket, fork, join, kill, never, suspend, uninterruptible)
+import Control.Monad.Fork.Class as Bracket
+import Control.Monad.Morph (class MFunctor, class MMonad, embed, hoist)
+import Control.Monad.Reader (class MonadAsk, class MonadReader, ReaderT(..), runReaderT)
+import Control.Monad.Rec.Class (class MonadRec)
+import Control.Monad.Trans.Class (class MonadTrans, lift)
+import Control.Monad.Unlift (class MonadUnlift, withRunInBase)
+import Control.Parallel (class Parallel, parallel, sequential)
+import Data.Bifunctor (lmap)
+import Data.Either (Either)
+import Data.Functor.Compose (Compose)
+import Data.Newtype (class Newtype, unwrap, wrap)
+import Effect.Aff.Class (class MonadAff)
+import Effect.Aff.Unlift (class MonadUnliftAff, withRunInAff)
+import Effect.Class (class MonadEffect)
+import Effect.Exception as Effect
+import Effect.Postgres.Error.Common (E, Error(..), toException)
+import Effect.Postgres.Error.Except (Except)
+import Effect.Unlift (class MonadUnliftEffect, withRunInEffect)
+
+-- | `ReaderT` with `ExceptT E`
+-- |
+-- | `ReaderT r (ExceptT (NonEmptyList Effect.Postgres.Error.Error) m) a`
+newtype RE r m a = RE (ReaderT r (ExceptT E m) a)
+
+newtype ParRE r f a = ParRE (ReaderT r (Compose f (Either E)) a)
+
+finally :: forall r f m a. MonadBracket Effect.Error f m => RE r m Unit -> RE r m a -> RE r m a
+finally after m = (m <* after) `catchError` \e -> after *> throwError e
+
+run :: forall m r a. MonadThrow Effect.Error m => RE r m a -> r -> m a
+run m r = liftEither =<< lmap toException <$> runExceptT (runReaderT (unwrap m) r)
+
+toExcept :: forall m r a. RE r m a -> r -> Except m a
+toExcept m r = runReaderT (unwrap m) r
+
+toEither :: forall m r a. RE r m a -> r -> m (Either E a)
+toEither m r = runExceptT $ runReaderT (unwrap m) r
+
+liftExcept :: forall m r a. Monad m => Except m a -> RE r m a
+liftExcept = wrap <<< lift
+
+derive instance Newtype (ParRE r m a) _
+derive newtype instance Functor m => Functor (ParRE r m)
+derive newtype instance Apply m => Apply (ParRE r m)
+derive newtype instance Applicative m => Applicative (ParRE r m)
+
+derive instance Newtype (RE r m a) _
+derive newtype instance Monad m => MonadAsk r (RE r m)
+derive newtype instance Monad m => MonadReader r (RE r m)
+derive newtype instance Monad m => Functor (RE r m)
+derive newtype instance Monad m => Apply (RE r m)
+derive newtype instance Monad m => Applicative (RE r m)
+derive newtype instance Monad m => Bind (RE r m)
+derive newtype instance Monad m => Monad (RE r m)
+derive newtype instance Monad m => MonadError E (RE r m)
+derive newtype instance Monad m => MonadThrow E (RE r m)
+derive newtype instance MonadEffect m => MonadEffect (RE r m)
+derive newtype instance MonadAff m => MonadAff (RE r m)
+derive newtype instance MonadRec m => MonadRec (RE r m)
+
+instance Parallel p m => Parallel (ParRE r p) (RE r m) where
+  parallel = wrap <<< parallel <<< unwrap
+  sequential = wrap <<< sequential <<< unwrap
+
+instance MonadTrans (RE r) where
+  lift = wrap <<< lift <<< lift
+
+instance MFunctor (RE r) where
+  hoist mn (RE m) = RE $ hoist (hoist mn) m
+
+instance MMonad (RE r) where
+  embed :: forall n m b. Monad n => (forall a. m a -> RE r n a) -> RE r m b -> RE r n b
+  embed mtn (RE m) =
+    RE $ ReaderT $ \r -> embed (flip runReaderT r <<< unwrap <<< mtn) (runReaderT m r)
+
+instance Monad m => Alt (RE r m) where
+  alt a b = catchError b (\e -> catchError a (\e' -> throwError $ e <> e'))
+
+instance (MonadThrow Effect.Error m, MonadUnliftEffect m) => MonadUnliftEffect (RE r m) where
+  withRunInEffect f = RE $ ReaderT $ \r -> lift $ withRunInEffect @m $ \a -> f (a <<< flip run r)
+
+instance (MonadThrow Effect.Error m, MonadUnliftAff m) => MonadUnliftAff (RE r m) where
+  withRunInAff f = RE $ ReaderT $ \r -> lift $ withRunInAff @m $ \a -> f (a <<< flip run r)
+
+instance (MonadBase m (RE r m), MonadThrow Effect.Error m) => MonadUnlift m (RE r m) where
+  withRunInBase f = RE $ ReaderT $ \r -> lift $ f (flip run r)
+
+instance Monad m => MonadBase m (RE r m) where
+  liftBase = lift
+
+instance (MonadThrow Effect.Error m, MonadFork f m) => MonadFork f (RE r m) where
+  fork m = withRunInBase \f -> fork $ f m
+  suspend m = withRunInBase \f -> suspend $ f m
+  join f = lift $ join f
+
+instance (MonadKill Effect.Error f m) => MonadKill E f (RE r m) where
+  kill e f = lift $ kill (toException e) f
+
+instance (MonadBracket Effect.Error f m) => MonadBracket E f (RE r m) where
+  bracket acq rel m = withRunInBase \f -> bracket (f acq) (\c r -> f $ rel ((bracketCondError (pure <<< Other)) c) r) (f <<< m)
+  uninterruptible = hoist uninterruptible
+  never = lift never
+
+bracketCondError :: forall ea eb a. (ea -> eb) -> BracketCondition ea a -> BracketCondition eb a
+bracketCondError _ (Bracket.Completed a) = Bracket.Completed a
+bracketCondError f (Bracket.Failed a) = Bracket.Failed $ f a
+bracketCondError f (Bracket.Killed a) = Bracket.Killed $ f a
--- a/src/Effect.Postgres.Error.purs
+++ b/src/Effect.Postgres.Error.purs
@ -0,0 +1,5 @@
+module Effect.Postgres.Error (module X) where
+
+import Effect.Postgres.Error.Common as X
+import Effect.Postgres.Error.RE (RE(..), ParRE(..)) as X
+import Effect.Postgres.Error.Except (Except) as X
--- a/src/Effect.Postgres.Pool.purs
+++ b/src/Effect.Postgres.Pool.purs
@ -9,6 +9,8 @@ import Effect (Effect)
 import Effect.Exception (Error)
 import Effect.Postgres.Client (Client)
 import Effect.Postgres.Client as Client
+import Effect.Postgres.Error as E
+import Effect.Postgres.Error.Except as E.Except
 import Effect.Uncurried (EffectFn2, mkEffectFn1, mkEffectFn2)
 import Foreign (Foreign, unsafeToForeign)
 import Node.EventEmitter (EventHandle(..))
@ -46,12 +48,12 @@ make r = do
  __make $ __uncfg { unwrapMillis: unwrap } $ unsafeToForeign asClientConfig

 -- | <https://node-postgres.com/apis/pool#releasing-clients>
-release :: Pool -> Client -> Effect Unit
-release p c = __release p c false
+release :: Pool -> Client -> E.Except Effect Unit
+release p c = E.Except.with E.Disconnecting $ __release p c false

 -- | <https://node-postgres.com/apis/pool#releasing-clients>
-destroy :: Pool -> Client -> Effect Unit
-destroy p c = __release p c true
+destroy :: Pool -> Client -> E.Except Effect Unit
+destroy p c = E.Except.with E.Disconnecting $ __release p c true

 -- | <https://node-postgres.com/apis/pool#connect>
 connectE :: EventHandle1 Pool Client
--- a/src/Pipes.Postgres.purs
+++ b/src/Pipes.Postgres.purs
@ -2,8 +2,10 @@ module Pipes.Postgres where

 import Prelude

+import Control.Monad.Cont (lift)
 import Control.Monad.Error.Class (class MonadError, class MonadThrow, catchError, throwError)
-import Control.Monad.Postgres (class MonadPostgres)
+import Control.Monad.Morph (hoist)
+import Control.Monad.Postgres (PostgresT)
 import Control.Monad.Reader (class MonadAsk, ask)
 import Data.Maybe (Maybe(..))
 import Effect.Aff.Class (class MonadAff, liftAff)
@ -12,6 +14,7 @@ import Effect.Aff.Postgres.Pool (Pool)
 import Effect.Aff.Postgres.Pool as Pool
 import Effect.Class (liftEffect)
 import Effect.Exception (Error)
+import Effect.Postgres.Error.RE as RE
 import Node.Buffer (Buffer)
 import Node.Stream.Object as O
 import Pipes ((>->))
@ -20,46 +23,45 @@ import Pipes.Node.Stream (fromReadable, fromWritable)
 import Pipes.Prelude as Pipes

 stdin
-  :: forall m s c ct
+  :: forall m
   . MonadAff m
  => MonadError Error m
  => MonadAsk Pool m
-  => MonadPostgres m s c ct
  => String
-  -> Consumer (Maybe Buffer) m Unit
+  -> Consumer (Maybe Buffer) (PostgresT m) Unit
 stdin q = do
-  pool <- ask
-  client <- liftAff $ Pool.connect pool
-  stream <- liftEffect $ Client.execWithStdin q client
-  liftAff $ void $ Client.exec "begin" client
+  pool <- lift ask
+  client <- lift $ RE.liftExcept $ hoist liftAff $ Pool.connect pool
+  stream <- lift $ RE.liftExcept $ hoist liftEffect $ Client.execWithStdin q client
+  lift $ RE.liftExcept $ hoist liftAff $ void $ Client.exec "begin" client
  let
    releaseOnEOS Nothing = do
-      liftAff $ void $ Client.exec "commit" client
-      liftEffect $ Pool.release pool client
+      RE.liftExcept $ hoist liftAff $ void $ Client.exec "commit" client
+      RE.liftExcept $ hoist liftEffect $ Pool.release pool client
      pure Nothing
    releaseOnEOS (Just a) = pure (Just a)

-    pipe = Pipes.mapM releaseOnEOS >-> fromWritable (O.unsafeFromBufferWritable stream)
-    err e = do
-      liftAff $ void $ Client.exec "rollback" client
-      liftEffect $ Pool.release pool client
+    pipe = Pipes.mapM releaseOnEOS >-> hoist lift (fromWritable $ O.unsafeFromBufferWritable stream)
+    err e = lift do
+      RE.liftExcept $ hoist liftAff $ void $ Client.exec "rollback" client
+      RE.liftExcept $ hoist liftEffect $ Pool.release pool client
      throwError e

  catchError pipe err

 stdout
-  :: forall m s c ct
+  :: forall m
   . MonadAff m
  => MonadThrow Error m
-  => MonadAsk Pool m
-  => MonadPostgres m s c ct
  => String
-  -> Producer (Maybe Buffer) m Unit
+  -> Producer (Maybe Buffer) (PostgresT m) Unit
 stdout q = do
-  pool <- ask
-  client <- liftAff $ Pool.connect pool
-  stream <- liftEffect $ Client.queryWithStdout q client
+  pool <- lift ask
+  client <- lift $ RE.liftExcept $ hoist liftAff $ Pool.connect pool
+  stream <- lift $ RE.liftExcept $ hoist liftEffect $ Client.queryWithStdout q client
  let
-    releaseOnEOS Nothing = liftEffect $ Pool.release pool client $> Nothing
+    releaseOnEOS :: Maybe Buffer -> PostgresT m (Maybe Buffer)
+    releaseOnEOS Nothing = RE.liftExcept $ hoist liftEffect $ Pool.release pool client $> Nothing
    releaseOnEOS (Just a) = pure (Just a)
-  fromReadable (O.unsafeFromBufferReadable stream) >-> Pipes.mapM releaseOnEOS
+  hoist lift (fromReadable (O.unsafeFromBufferReadable stream))
+    >-> Pipes.mapM releaseOnEOS
--- a/test/Test.Common.purs
+++ b/test/Test.Common.purs
@ -5,7 +5,7 @@ import Prelude
 import Data.Either (Either(..))
 import Data.String.Regex (Regex)
 import Data.String.Regex as Regex
-import Data.String.Regex.Flags (RegexFlags(..))
+import Data.String.Regex.Flags (RegexFlags)
 import Data.Tuple.Nested (type (/\), (/\))
 import Effect (Effect)
 import Effect.Aff (Aff, bracket, makeAff)
@ -14,14 +14,13 @@ import Effect.Aff.Postgres.Client as Client
 import Effect.Aff.Postgres.Pool (Pool)
 import Effect.Aff.Postgres.Pool as Pool
 import Effect.Class (liftEffect)
+import Effect.Postgres.Error.Except as X
 import Effect.Unsafe (unsafePerformEffect)
-import Node.EventEmitter (EventHandle(..))
+import Node.EventEmitter (EventHandle)
 import Node.EventEmitter as EE
 import Node.Path as Path
 import Node.Process (cwd)
 import Partial.Unsafe (unsafePartial)
-import Record (insert)
-import Type.Prelude (Proxy(..))

 type Config =
  { database :: String
@ -41,16 +40,16 @@ withConfig :: (Config -> Aff Unit) -> Aff Unit
 withConfig f = f =<< liftEffect config

 withClient :: (Client -> Aff Unit) -> Aff Unit
-withClient = bracket (Client.connected =<< liftEffect config) Client.end
+withClient = bracket (X.run $ Client.connected =<< liftEffect config) (X.run <<< Client.end)

 pool :: Pool
 pool = unsafePerformEffect $ Pool.make =<< liftEffect config

 withPool :: (Pool -> Aff Unit) -> Aff Unit
-withPool = bracket (liftEffect $ Pool.make =<< config) Pool.end
+withPool = bracket (liftEffect $ Pool.make =<< config) (X.run <<< Pool.end)

 withPoolClient :: (Client -> Aff Unit) -> Aff Unit
-withPoolClient = bracket (Pool.connect pool) (liftEffect <<< Pool.release pool)
+withPoolClient = bracket (X.run $ Pool.connect pool) (liftEffect <<< X.run <<< Pool.release pool)

 unsafeFromRight :: forall a b. Either a b -> b
 unsafeFromRight e = unsafePartial $ case e of Right b -> b
--- a/test/Test.Control.Monad.Postgres.purs
+++ b/test/Test.Control.Monad.Postgres.purs
@ -2,23 +2,32 @@ module Test.Control.Monad.Postgres where

 import Prelude

-import Control.Monad.Error.Class (throwError)
+import Control.Monad.Cont (lift)
+import Control.Monad.Error.Class (catchError, throwError, try)
 import Control.Monad.Fork.Class (class MonadBracket, bracket)
-import Control.Monad.Postgres (PostgresT, exec_, query, runPostgres, session, transaction, cursor, fetch, fetchAll, fetchOne)
-import Control.Parallel (parTraverse_)
+import Control.Monad.Postgres (PostgresT, cursor, exec_, fetch, fetchAll, fetchOne, query, runPostgres, transaction)
+import Control.Monad.Reader (ask)
+import Control.Parallel (parTraverse)
 import Data.Array as Array
 import Data.Array.NonEmpty as Array.NonEmpty
 import Data.Maybe (Maybe(..), fromJust, maybe)
 import Data.String.Regex as Regex
 import Data.String.Regex.Flags as Regex.Flag
 import Data.Traversable (for_)
+import Data.Tuple.Nested ((/\))
 import Effect.Aff (Fiber)
-import Effect.Aff.Class (class MonadAff)
+import Effect.Aff.Class (class MonadAff, liftAff)
+import Effect.Aff.Unlift (UnliftAff(..), askUnliftAff)
+import Effect.Class (liftEffect)
+import Effect.Console (log)
 import Effect.Exception (Error, error)
+import Effect.Postgres.Error as E
+import Effect.Postgres.Error.Except as X
+import Effect.Postgres.Error.RE as RE
 import Partial.Unsafe (unsafePartial)
 import Test.Common (re, withConfig)
 import Test.Spec (Spec, around, describe, it)
-import Test.Spec.Assertions (expectError, shouldEqual)
+import Test.Spec.Assertions (shouldEqual)

 withTable :: forall a m. MonadBracket Error Fiber m => MonadAff m => String -> PostgresT m a -> PostgresT m a
 withTable s m =
@ -30,30 +39,41 @@ withTable s m =
 spec :: Spec Unit
 spec =
  around withConfig $ describe "Control.Monad.Postgres" do
-    it "empty works" \cfg -> runPostgres cfg $ pure unit
-    it "connects" \cfg -> runPostgres cfg $ shouldEqual 1 =<< query "select 1"
-    it "multiple sessions serially" \cfg -> runPostgres cfg do
-      shouldEqual 1 =<< query "select 1"
-      shouldEqual 2 =<< query "select 2"
-    it "multiple sessions concurrently" \cfg -> runPostgres cfg do
-      flip parTraverse_ [ 1, 2, 3 ] \_ -> shouldEqual 1 =<< query "select 1"
-    it "transaction commits" \cfg -> runPostgres cfg do
+    it "empty works" \cfg -> X.run $ runPostgres cfg $ pure unit
+    it "connects" \cfg -> shouldEqual 1 =<< X.run (runPostgres cfg $ query "select 1")
+    it "multiple sessions serially" \cfg -> do
+      a /\ b <- X.run $ runPostgres cfg do
+        a <- query "select 1"
+        b <- query "select 2"
+        pure $ a /\ b
+      a `shouldEqual` 1
+      b `shouldEqual` 2
+    it "multiple sessions concurrently" \cfg -> do
+      nums <- X.run $ runPostgres cfg $ parTraverse (\n -> query $ "select $1 :: int" /\ n) (Array.range 1 3)
+      Array.sort nums `shouldEqual` [1, 2, 3]
+    it "transaction commits" \cfg -> do
+      a <- X.run $ runPostgres cfg do
        exec_ "create temporary table test_txn_commits (id int);"
        transaction $ exec_ "insert into test_txn_commits values (1);"
-      shouldEqual [ 1 ] =<< query "select * from test_txn_commits"
-    it "transaction rolls back" \cfg -> runPostgres cfg do
+        query "select * from test_txn_commits"
+      a `shouldEqual` [ 1 ]
+    it "transaction rolls back" \cfg -> do
+      a <- X.run $ runPostgres cfg do
        exec_ "create temporary table test_txn_rolls_back (id int);"
        exec_ "insert into test_txn_rolls_back values (1);"
-      expectError $ transaction do
+        void $ try $ transaction do
          exec_ "insert into test_txn_rolls_back values (2);"
-        throwError $ error "foo"
-      shouldEqual [ 1 ] =<< query "select * from test_txn_rolls_back"
-    it "cursor" \cfg -> runPostgres cfg do
+          throwError $ pure $ E.Other $ error "foo"
+        query "select * from test_txn_rolls_back"
+      a `shouldEqual` [1]
+    it "cursor" \cfg ->
+      X.run $ runPostgres cfg do
        exec_ $ "create temporary table test_cursor_data (id int primary key generated always as identity)"
        for_ (Array.range 1 50) $ const $ exec_ "insert into test_cursor_data (id) values (default);"
        cursor @Int "test_cursor" "select id from test_cursor_data" do
-        shouldEqual (Just 1) =<< fetchOne
-        shouldEqual (Just 2) =<< fetchOne
-        shouldEqual (Just 3) =<< fetchOne
-        shouldEqual [ 4, 5, 6, 7, 8 ] =<< fetch 5
-        shouldEqual (Array.range 9 50) =<< fetchAll
+          UnliftAff unliftAff <- askUnliftAff
+          liftAff $ shouldEqual (Just 1) =<< unliftAff fetchOne
+          liftAff $ shouldEqual (Just 2) =<< unliftAff fetchOne
+          liftAff $ shouldEqual (Just 3) =<< unliftAff fetchOne
+          liftAff $ shouldEqual [ 4, 5, 6, 7, 8 ] =<< unliftAff (fetch 5)
+          liftAff $ shouldEqual (Array.range 9 50) =<< unliftAff fetchAll
--- a/test/Test.Data.Postgres.purs
+++ b/test/Test.Data.Postgres.purs
@ -31,6 +31,7 @@ import Effect.Aff (Aff)
 import Effect.Aff.Postgres.Client (exec, query)
 import Effect.Class (liftEffect)
 import Effect.Postgres.Client (Client)
+import Effect.Postgres.Error.Except as X
 import Effect.Unsafe (unsafePerformEffect)
 import Foreign (Foreign, unsafeToForeign)
 import Foreign.Object as Object
@ -206,13 +207,13 @@ spec =
              x' <- Q.param $ fromArb x
              let val = x' <> " :: " <> sql
              pure $ "select " <> val
-        void $ exec createtab c
+        void $ X.run $ exec createtab c
        seed <- liftEffect randomSeed
        let xs = sample seed 10 (arbitrary @x)
        flip parTraverse_ xs
          \x -> do
-            void $ exec (ser x) c
-            res <- query (de x) c
+            void $ X.run $ exec (ser x) c
+            res <- X.run $ query (de x) c
            let
              exp = fromArb x
              act = unsafePartial fromJust $ Array.head res
--- a/test/Test.Effect.Postgres.Client.purs
+++ b/test/Test.Effect.Postgres.Client.purs
@ -12,6 +12,7 @@ import Effect.Aff (forkAff, joinFiber)
 import Effect.Aff.Postgres.Client (query)
 import Effect.Aff.Postgres.Client as Client
 import Effect.Exception as Error
+import Effect.Postgres.Error.Except as X
 import Test.Common (onceAff, withClient)
 import Test.Spec (Spec, around, describe, it)
 import Test.Spec.Assertions (shouldEqual)
@ -23,40 +24,40 @@ spec =
      describe "events" do
        it "end" \c -> do
          expect <- forkAff $ onceAff Client.endE c
-          Client.end c
+          X.run $ Client.end c
          joinFiber expect
        it "notice" \c -> do
          expect <- forkAff do
            e <- onceAff Client.noticeE c
            Error.message e `shouldEqual` "hello"
-          void $ Client.exec "do language plpgsql $$ begin raise notice 'hello'; end; $$;" c
+          X.run $ void $ Client.exec "do language plpgsql $$ begin raise notice 'hello'; end; $$;" c
          joinFiber expect
        it "notification" \c -> do
-          void $ Client.exec "listen hello;" c
+          X.run $ void $ Client.exec "listen hello;" c
          expect <- forkAff do
            n <- onceAff Client.notificationE c
            n.payload `shouldEqual` (Just "world")
-          void $ Client.exec "notify hello, 'world';" c
+          X.run $ void $ Client.exec "notify hello, 'world';" c
          joinFiber expect
      it "connect & end do not throw" $ const $ pure unit
      describe "query" do
-        it "ok if connected" \c -> shouldEqual [ 1, 2, 3 ] =<< query "select unnest(array[1, 2, 3])" c
+        it "ok if connected" \c -> shouldEqual [ 1, 2, 3 ] =<< X.run (query "select unnest(array[1, 2, 3])" c)
        it "throws if ended" \c -> do
-          Client.end c
-          res :: Either _ (Array Int) <- try $ query "select 1" c
+          X.run $ Client.end c
+          res :: Either _ (Array Int) <- try $ X.run $ query "select 1" c
          isLeft res `shouldEqual` true
        it "rowsAffected is correct" \c -> do
-          void $ Client.exec "create temp table foo (bar int);" c
-          shouldEqual 1 =<< Client.exec "insert into foo values (1);" c
-          shouldEqual 3 =<< Client.exec "insert into foo values (1), (2), (3);" c
-          shouldEqual 4 =<< Client.exec "update foo set bar = 10;" c
+          X.run $ void $ Client.exec "create temp table foo (bar int);" c
+          shouldEqual 1 =<< X.run (Client.exec "insert into foo values (1);" c)
+          shouldEqual 3 =<< X.run (Client.exec "insert into foo values (1), (2), (3);" c)
+          shouldEqual 4 =<< X.run (Client.exec "update foo set bar = 10;" c)
        describe "timestamp" do
          it "unmarshals" \c -> do
            let exp = toDateTimeLossy <$> fromRFC3339String (wrap "2020-01-01T00:00:00Z")
-            shouldEqual exp =<< query "select '2020-01-01T00:00:00Z' :: timestamptz" c
-          it "is string" \c -> shouldEqual "2020-01-01 00:00:00+00" =<< query "select '2020-01-01T00:00:00Z' :: timestamptz" c
-          it "array is string" \c -> shouldEqual [ [ "2020-01-01 00:00:00+00" ] ] =<< query "select array['2020-01-01T00:00:00Z' :: timestamptz]" c
+            shouldEqual exp =<< X.run (query "select '2020-01-01T00:00:00Z' :: timestamptz" c)
+          it "is string" \c -> shouldEqual "2020-01-01 00:00:00+00" =<< X.run (query "select '2020-01-01T00:00:00Z' :: timestamptz" c)
+          it "array is string" \c -> shouldEqual [ [ "2020-01-01 00:00:00+00" ] ] =<< X.run (query "select array['2020-01-01T00:00:00Z' :: timestamptz]" c)
        describe "json" do
-          it "unmarshals" \c -> shouldEqual (JSON { foo: "bar" }) =<< query "select '{\"foo\": \"bar\"}' :: json" c
-          it "is string" \c -> shouldEqual "{\"foo\": \"bar\"}" =<< query "select '{\"foo\": \"bar\"}' :: json" c
-          it "array is string" \c -> shouldEqual [ [ "{\"foo\": \"bar\"}" ] ] =<< query "select array['{\"foo\": \"bar\"}' :: json]" c
+          it "unmarshals" \c -> shouldEqual (JSON { foo: "bar" }) =<< X.run (query "select '{\"foo\": \"bar\"}' :: json" c)
+          it "is string" \c -> shouldEqual "{\"foo\": \"bar\"}" =<< X.run (query "select '{\"foo\": \"bar\"}' :: json" c)
+          it "array is string" \c -> shouldEqual [ [ "{\"foo\": \"bar\"}" ] ] =<< X.run (query "select array['{\"foo\": \"bar\"}' :: json]" c)
--- a/test/Test.Effect.Postgres.Pool.purs
+++ b/test/Test.Effect.Postgres.Pool.purs
@ -7,6 +7,7 @@ import Effect.Aff (finally, forkAff, joinFiber)
 import Effect.Aff.Postgres.Client as Client
 import Effect.Aff.Postgres.Pool as Pool
 import Effect.Class (liftEffect)
+import Effect.Postgres.Error.Except as X
 import Test.Common (config, onceAff, withPool)
 import Test.Spec (Spec, around, describe, it)
 import Test.Spec.Assertions (expectError, shouldEqual)
@ -20,78 +21,78 @@ spec = describe "Pool" do
    void $ liftEffect $ Pool.make cfg
  around withPool do
    it "idleCount, totalCount" \p -> do
-      a <- Pool.connect p
-      b <- Pool.connect p
-      c <- Pool.connect p
-      liftEffect $ Pool.release p a
-      liftEffect $ Pool.release p b
-      finally (liftEffect $ Pool.release p c) do
+      a <- X.run $ Pool.connect p
+      b <- X.run $ Pool.connect p
+      c <- X.run $ Pool.connect p
+      liftEffect $ X.run $ Pool.release p a
+      liftEffect $ X.run $ Pool.release p b
+      finally (liftEffect $ X.run $ Pool.release p c) do
        Pool.clientIdleCount p `shouldEqual` 2
        Pool.clientCount p `shouldEqual` 3
      Pool.clientIdleCount p `shouldEqual` 3
      Pool.clientCount p `shouldEqual` 3
    it "waitingCount" \p -> do
-      a <- Pool.connect p
-      b <- Pool.connect p
-      c <- Pool.connect p
-      dFiber <- forkAff $ Pool.connect p
+      a <- X.run $ Pool.connect p
+      b <- X.run $ Pool.connect p
+      c <- X.run $ Pool.connect p
+      dFiber <- forkAff $ X.run $ Pool.connect p
      let
        rel =
          do
-            void $ liftEffect $ traverse (Pool.release p) [ a, b, c ]
+            void $ liftEffect $ X.run $ traverse (Pool.release p) [ a, b, c ]
            d <- joinFiber dFiber
-            liftEffect $ Pool.release p d
+            liftEffect $ X.run $ Pool.release p d
      finally rel $ Pool.clientWaitingCount p `shouldEqual` 1
    describe "events" do
      it "connect" \p -> do
        expect <- forkAff $ void $ onceAff Pool.connectE p
-        c <- Pool.connect p
-        finally (liftEffect $ Pool.release p c) $ joinFiber expect
+        c <- X.run $ Pool.connect p
+        finally (liftEffect $ X.run $ Pool.release p c) $ joinFiber expect
      it "acquire" \p -> do
-        c <- Pool.connect p
-        liftEffect $ Pool.release p c
+        c <- X.run$ Pool.connect p
+        liftEffect $ X.run$ Pool.release p c
        expect <- forkAff do
          c'' <- onceAff Pool.acquireE p
          refEq c c'' `shouldEqual` true
-        c' <- Pool.connect p
-        finally (liftEffect $ Pool.release p c') $ joinFiber expect
+        c' <- X.run $ Pool.connect p
+        finally (liftEffect $ X.run$ Pool.release p c') $ joinFiber expect
      it "release" \p -> do
-        c <- Pool.connect p
+        c <- X.run $ Pool.connect p
        expect <- forkAff do
          c' <- onceAff Pool.releaseE p
          refEq c c' `shouldEqual` true
-        liftEffect $ Pool.release p c
+        liftEffect $ X.run $ Pool.release p c
        joinFiber expect
      it "remove" \p -> do
-        c <- Pool.connect p
+        c <- X.run $ Pool.connect p
        expect <- forkAff do
          c' <- onceAff Pool.removeE p
          refEq c c' `shouldEqual` true
-        liftEffect $ Pool.destroy p c
+        liftEffect $ X.run $ Pool.destroy p c
        joinFiber expect
    it "connect" \p -> do
-      c <- Pool.connect p
-      let rel = liftEffect $ Pool.release p c
-      finally rel $ shouldEqual 1 =<< Client.query "select 1" c
+      c <- X.run $ Pool.connect p
+      let rel = liftEffect $ X.run $ Pool.release p c
+      finally rel $ shouldEqual 1 =<< X.run (Client.query "select 1" c)
    describe "destroy" do
      it "throws on query after destroy" \p -> do
-        c <- Pool.connect p
-        liftEffect $ Pool.destroy p c
-        expectError $ Client.exec "select 1" c
+        c <- X.run $ Pool.connect p
+        liftEffect $ X.run $ Pool.destroy p c
+        expectError $ X.run $ Client.exec "select 1" c
      it "different client yielded after destroy" \p -> do
-        a <- Pool.connect p
-        liftEffect $ Pool.destroy p a
-        b <- Pool.connect p
-        liftEffect $ Pool.destroy p b
+        a <- X.run $ Pool.connect p
+        liftEffect $ X.run $ Pool.destroy p a
+        b <- X.run $ Pool.connect p
+        liftEffect $ X.run $ Pool.destroy p b
        refEq a b `shouldEqual` false
    describe "release" do
      it "allows reuse" \p -> do
-        a <- Pool.connect p
-        liftEffect $ Pool.release p a
-        b <- Pool.connect p
-        liftEffect $ Pool.release p b
+        a <- X.run $ Pool.connect p
+        liftEffect $ X.run $ Pool.release p a
+        b <- X.run $ Pool.connect p
+        liftEffect $ X.run $ Pool.release p b
        refEq a b `shouldEqual` true
      it "throws when invoked twice" \p -> do
-        c <- Pool.connect p
-        liftEffect $ Pool.release p c
-        expectError $ liftEffect $ Pool.release p c
+        c <- X.run $ Pool.connect p
+        liftEffect $ X.run $ Pool.release p c
+        expectError $ liftEffect $ X.run $ Pool.release p c