diff --git a/bower.json b/bower.json
index 2e2a510..f2ebe90 100644
--- a/bower.json
+++ b/bower.json
@@ -2,19 +2,19 @@
   "name": "purescript-postgresql-client",
   "license": "BSD-3-Clause",
   "dependencies": {
-    "purescript-prelude": "^2.1.0",
-    "purescript-transformers": "^2.0.2",
-    "purescript-lists": "^3.2.1",
-    "purescript-foreign": "^3.0.1",
-    "purescript-tuples": "^3.0.0",
-    "purescript-aff": "^2.0.2",
-    "purescript-either": "^2.0.0",
-    "purescript-maybe": "^2.0.1",
-    "purescript-foldable-traversable": "^2.0.0",
-    "purescript-newtype": "^1.1.0",
-    "purescript-bytestrings": "^2.3.0",
-    "purescript-arrays": "^3.1.0",
-    "purescript-datetime": "^2.0.0"
+    "purescript-prelude": "^3.0.0",
+    "purescript-transformers": "^3.1.0",
+    "purescript-lists": "^4.0.1",
+    "purescript-foreign": "^4.0.0",
+    "purescript-tuples": "^4.0.0",
+    "purescript-aff": "^3.0.0",
+    "purescript-either": "^3.0.0",
+    "purescript-maybe": "^3.0.0",
+    "purescript-foldable-traversable": "^3.0.0",
+    "purescript-newtype": "^2.0.0",
+    "purescript-bytestrings": "^3.0.1",
+    "purescript-arrays": "^4.0.1",
+    "purescript-datetime": "^3.0.0"
   },
   "repository": {
     "type": "git",
diff --git a/src/Database/PostgreSQL.js b/src/Database/PostgreSQL.js
index 3b4c03f..01c1e65 100644
--- a/src/Database/PostgreSQL.js
+++ b/src/Database/PostgreSQL.js
@@ -59,3 +59,5 @@ exports.instantToString = function(i) {
 exports.unsafeIsBuffer = function(x) {
     return x instanceof Buffer;
 };
+
+exports['null'] = null;
diff --git a/src/Database/PostgreSQL.purs b/src/Database/PostgreSQL.purs
index 51273b8..e6fc19f 100644
--- a/src/Database/PostgreSQL.purs
+++ b/src/Database/PostgreSQL.purs
@@ -21,6 +21,7 @@ module Database.PostgreSQL
 ) where
 
 import Control.Monad.Aff (Aff)
+import Control.Monad.Eff (kind Effect)
 import Control.Monad.Eff.Exception (error)
 import Control.Monad.Error.Class (catchError, throwError)
 import Control.Monad.Except (runExcept)
@@ -30,7 +31,6 @@ import Data.ByteString (ByteString)
 import Data.DateTime.Instant (Instant)
 import Data.Either (Either(..))
 import Data.Foreign (Foreign, isNull, readArray, readBoolean, readChar, readInt, readNumber, readString, toForeign, unsafeFromForeign)
-import Data.Foreign.Null (writeNull)
 import Data.List (List)
 import Data.List as List
 import Data.Maybe (Maybe(..))
@@ -40,7 +40,7 @@ import Data.Tuple (fst, Tuple)
 import Data.Tuple.Nested ((/\))
 import Prelude
 
-foreign import data POSTGRESQL :: !
+foreign import data POSTGRESQL :: Effect
 
 -- | PostgreSQL connection pool configuration.
 type PoolConfiguration =
@@ -53,11 +53,13 @@ type PoolConfiguration =
     , idleTimeoutMillis :: Int
     }
 
+foreign import null :: Foreign
+
 -- | PostgreSQL connection pool.
-foreign import data Pool :: *
+foreign import data Pool :: Type
 
 -- | PostgreSQL connection.
-foreign import data Connection :: *
+foreign import data Connection :: Type
 
 -- | PostgreSQL query with parameter (`$1`, `$2`, …) and return types.
 newtype Query i o = Query String
@@ -143,7 +145,7 @@ instance toSQLValueInstant :: ToSQLValue Instant where
     toSQLValue = instantToString
 
 instance toSQLValueMaybe :: (ToSQLValue a) => ToSQLValue (Maybe a) where
-    toSQLValue Nothing = writeNull
+    toSQLValue Nothing = null
     toSQLValue (Just x) = toSQLValue x
 
 instance fromSQLValueMaybe :: (FromSQLValue a) => FromSQLValue (Maybe a) where
@@ -196,7 +198,8 @@ execute conn (Query sql) values =
 -- | Execute a PostgreSQL query and return its results.
 query
     :: ∀ i o eff
-     . (ToSQLRow i, FromSQLRow o)
+     . ToSQLRow i
+    => FromSQLRow o
     => Connection
     -> Query i o
     -> i
@@ -209,7 +212,8 @@ query conn (Query sql) values =
 
 scalar
     :: ∀ i o eff
-     . (ToSQLRow i, FromSQLValue o)
+     . ToSQLRow i
+    => FromSQLValue o
     => Connection
     -> Query i (Tuple o Unit)
     -> i