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purescript-postgresql-client is a PostgreSQL client library for PureScript based on node-postgres.


To use this library, you need to add pg and decimal.js as an npm dependency. You can also find first of them on


This guide is a literate Purescript file which is extracted into testing module (using literate-purescript) so it is a little verbose.

Let's start with imports.

module Test.README where

import Prelude

import Control.Monad.Except.Trans (ExceptT, runExceptT)
import Database.PostgreSQL.PG (defaultPoolConfiguration, PGError, command, execute, newPool, Pool, Connection, query, Query(Query))
import Database.PostgreSQL.PG as PG
import Database.PostgreSQL.Row (Row0(Row0), Row3(Row3))
import Data.Decimal as Decimal
import Data.Maybe (Maybe(..))
import Data.Tuple.Nested ((/\))
import Effect.Aff (Aff)
import Effect.Class (liftEffect)
import Test.Assert (assert)

The whole API for interaction with PostgreSQL is performed asynchronously in Aff (the only function which runs in plain Effect is newPool). Core library functions usually results in somthing like Aff (Either PGError a) which can be easily wrapped by user into ExceptT or any other custom monad stack. To be honest we provide alternatives to functions in the Database.PostgreSQL.PG module that work on any stack m with MonadError PGError m and MonadAff m. The module contains two functions withConnection and withTransaction that require additional parameter - a transformation from a custom monad stack to Aff (Either PGError a). We are going to work with PG type in this tutorial but please don't consider it as the only option if you encounter any troubles integrating it into your own app monad stack.

type PG a = ExceptT PGError Aff a

withConnection :: forall a. Pool -> (Connection -> PG a) -> PG a
withConnection = PG.withConnection runExceptT

withTransaction :: forall a. Connection -> PG a -> PG a
withTransaction = PG.withTransaction runExceptT

We assume here that Postgres is running on a standard local port with ident authentication so configuration can be nearly empty (defaultPoolConfiguration). It requires only database name which we pass to newPool function. Additionally we pass idleTimeoutMillis value because this code is run by our test suite and we want to exit after its execution quickly ;-)

run  PG Unit
run = do

  pool <- liftEffect $ newPool
    ((defaultPoolConfiguration "purspg") { idleTimeoutMillis = Just 1000 })
  withConnection pool \conn -> do

We can now create our temporary table which we are going to query in this example. execute ignores result value which is what we want in this case. The last Row0 value indicates that this Query doesn't take any additional parameters.

    execute conn (Query """
        name text NOT NULL,
        delicious boolean NOT NULL,
        price NUMERIC(4,2) NOT NULL,
        PRIMARY KEY (name)
    """) Row0

There is withTransaction helper provided. You can wrap the whole piece of interaction with database in it. It will rollback if any exception is thrown during execution of a given Aff block. It excecutes COMMIT in the other case. We start our session with insert of some data. It is done by execute function with INSERT statement. Please notice that we are passing a tuple of the arguments to this query using dedicated constructor. In this case Row3. This library provides types from Row0 to Row19 and they are wrappers which provide instances for automatic conversions from and to SQL values. For details please investigate following classes ToSQLRow, ToSQLValue, FromSQLRow and FromSQLValue.

    withTransaction conn $ do
      execute conn (Query """
        INSERT INTO fruits (name, delicious, price)
        VALUES ($1, $2, $3)
      """) (Row3 "coconut" true (Decimal.fromString "8.30"))

We can also use nested tuples instead of Row* constructors. This can be a bit more verbose but is not restricted to limited and constant number of arguments. /\ is just an alias for the Tuple constructor from Data.Tuple.Nested.

      execute conn (Query """
        INSERT INTO fruits (name, delicious, price)
        VALUES ($1, $2, $3)
      """) ("lemon" /\ false /\ Decimal.fromString "3.30")

Of course Row* types and nested tuples can be also used when we are fetching data from db. query function processes db response and returns an Array of rows.

      names <- query conn (Query """
        SELECT name, delicious
        FROM fruits
        ORDER BY name ASC
      """) Row0
      liftEffect <<< assert $ names == ["coconut" /\ true, "lemon" /\ false]

There is also a command function at our disposal. Some postgres SQL expressions return a "command tag" which carries a value with a number of rows which were affected by a given query. For example we can have: DELETE rows, UPDATE rows, INSERT oid rows etc. This function should return rows value associated with given response.

      deleted <- command conn (Query """DELETE FROM fruits """) Row0
      liftEffect <<< assert $ deleted == 2


Currently tests are prepared to work with default and local setup for postgresql (ident authentication, standart port etc.). If you think that we should add configuration layer for our test runner please open an issue.

To run suite please:

  • $ npm install

  • $ createdb purspg

  • $ npm run test

Generating SQL Queries

The purspg preprocessor has been replaced by sqltopurs, which is a code generator instead of a preprocessor, and easier to use.