Multi-backend, high-level EDSL for interacting with SQL databases.

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MIT licensed by Anton Ekblad
Maintained by
This version can be pinned in stack with:selda-,3743


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What is Selda?

Selda is a Haskell library for interacting with SQL-based relational databases. It was inspired by LINQ and Opaleye.


  • Monadic interface.
  • Portable: backends for SQLite and PostgreSQL.
  • Generic: easy integration with your existing Haskell types.
  • Creating, dropping and querying tables using type-safe database schemas.
  • Typed query language with products, filtering, joins and aggregation.
  • Inserting, updating and deleting rows from tables.
  • Conditional insert/update.
  • Transactions, uniqueness constraints and foreign keys.
  • Seamless prepared statements.
  • Configurable, automatic, consistent in-process caching of query results.
  • Lightweight and modular: few dependencies, and non-essential features are optional or split into add-on packages.

Getting started

Install the selda package from Hackage, as well as at least one of the backends:

$ cabal update
$ cabal install selda selda-sqlite selda-postgresql

Then, read the tutorial. The API documentation will probably also come in handy.


Selda requires GHC 7.10+, as well as SQLite 3.7.11+ or PostgreSQL 9.6+. To build the SQLite backend, you need a C compiler installed. To build the PostgreSQL backend, you need the libpq development libraries installed (libpq-dev on Debian-based Linux distributions).



All forms of contributions are welcome!

If you have a bug to report, please try to include as much information as possible, preferably including:

  • A brief description (one or two sentences) of the bug.
  • The version of Selda+backend where the bug was found.
  • A step-by-step guide to reproduce the bug.
  • The expected result from following these steps.
  • What actually happens when following the steps.
  • Which component contains the bug (selda, selda-sqlite or selda-postgresql), if you’re reasonably sure about where the bug is.

Bonus points for a small code example that illustrates the problem.

If you want to contribute code, please consult the following checklist before sending a pull request:

  • Does the code build with a recent version of GHC?
  • Do all the tests pass?
  • Have you added any tests covering your code?

Setting up the build environment

From the repository root:

  • Install libpq-dev from your package manager. This is required to build the PostgreSQL backend.
  • Run make sandbox followed by make deps. This will set up a local sandbox for your Selda hacking, and install all Haskell necessary dependencies.
  • Familiarise yourself with the various targets in the makefile. The dependencies between Selda, the backends and the tests are slightly complex, so straight-up cabal is too quirky for day to day hacking.

Setting up a VM for PostgreSQL testing

While the SQLite backend is completely self-contained, the PostgreSQL backend needs an appropriate server for testing. Setting this up in a virtual machine is definitely less intrusive than setting up a server on your development machine. To set up a VM for the PostgreSQL backend tests:

  • Install your favourite hypervisor, such as VMWare player or VirtualBox.
  • Download a pre-built PostgreSQL VM from Bitnami.
  • Import the OVA file into your hypervisor.
  • Change the network settings of your newly imported VM to NAT, and make sure that port 5432 is forwarded. Note that this will conflict with any PostgreSQL server running on your machine while the VM is running.
  • Boot your VM and note the password displayed on the login screen.
  • Create the file selda-tests/PGConnectInfo.hs with the following content:
    {-# LANGUAGE OverloadedStrings #-}
    module PGConnectInfo where
    import Database.Selda.PostgreSQL
    pgConnectInfo = "test" `on` "localhost" `auth` ("postgres", "$PASSWORD")
    Where $PASSWORD is the password from the VM’s login screen.
  • Log in to the VM and disable the built-in firewall by running sudo systemctl disable ufw ; sudo systemctl stop ufw.
  • From your host machine, create the test database:
    $ psql -h -U postgres -W
    [password from login screen]
    # CREATE TABLE test;
    # \q
  • Run make pgtest to check that everything works.


Features that would be nice to have but are not yet implemented.

  • Monadic if/else.
  • Streaming
  • MySQL/MariaDB backend.
  • MSSQL backend.


Revision history for Selda – 2018-09-29

  • Added convenience functions for working with nullable columns. – 2018-09-04

  • DISTINCT should now always return distinct results.
  • DISTINCT can no longer produce ill-scoped queries. – 2018-09-01

  • Ad hoc selectors using OverloadedLabels.
  • Shorter build times.
  • Minor API updates and simplifications. – 2018-08-07

  • Some aggregates are now nullable.
  • sum_ on an empty table doesn’t crash anymore.
  • Aggregating over an empty selectValues doesn’t crash anymore. – 2018-08-06

  • Minor API fix when defining table attributes. – 2018-08-05

  • Support for Stack and GHC 8.4.
  • Precedence fix for selector index (!) operator.
  • Accept INT and SMALLINT columns in user-created PostgreSQL tables.
  • Add combinator for turning off foreign key checking.
  • Rename unsafeRowId/unsafeId to toRowId/rowId.
  • Add typed row identifiers.
  • More generic type for sum_.
  • Table validation against current database.
  • Basic migration support.
  • Basic index support.
  • Remove ad hoc tables; only generic tables from now on. – 2018-04-02

  • Support custom column names for generic tables.
  • Scope safety fix for inner queries.
  • Better type errors on GHC 8+ for inner queries. – 2018-02-27

  • New PPConfig hook for more flexibility when compiling types. – 2018-01-11

  • Allow recursive and optional foreign keys.
  • Allow arbitrary enums in tables, represented as text.
  • Fix RowID issues for PostgreSQL.
  • Fix auto-incrementing primary keys for generic tables. – 2017-12-14

  • Fix treatment of booleans in PostgreSQL backend. – 2017-10-10

  • Fix rare infinite loop bug in in-process cache. – 2017-09-08

  • Fix name generation in the presence of isIn over queries.
  • SELECT DISTINCT support.
  • Conditional expressions and matchNull. – 2017-08-11

  • Fix name generation in the presence of multiple aggregates. – 2017-08-01

  • Async exception safety.
  • Allow MonadSelda instances not built on SeldaT.
  • Chunk very large insertions on backends that request it (i.e. SQLite).
  • GHC 8.2 support. – 2017-06-16

  • Properly document semantics of order.
  • Export conditional inserts.
  • Fix Haste build for backends. – 2017-06-10

  • Move SQL pretty-printing config into a single type.
  • Support for binary blobs.
  • Support for prepared statements.
  • Support for connection reuse across Selda computations.
  • Cleaner and more robust backend API.
  • Stricter type constraints on comparisons.
  • Allow limit on inner queries.
  • Allow inspecting row identifiers. – 2017-05-17

  • Add specialized insertUnless upsert variant.
  • Fix potential race condition in upserts.
  • Use abstract row identifier type for auto-incrementing primary keys.
  • Less strict version bounds on dependencies. – 2017-05-07

  • Conditional insert (“upsert”) support.
  • Support SELECT x IN (SELECT ...) and SELECT x IN (a, b, ...) queries.
  • Explicit inner queries.
  • Rename inner to innerJoin, more intuitive behavior for suchThat.
  • Add from shorthand for \s q -> fmap (!s) q.
  • Unique and foreign key constraints for generics. – 2017-05-05

  • Inner join support.
  • More sensible names in backend API.
  • Fix rounding and casts. – 2017-05-04

  • Fix cache consistency bug in the presence of multiple databases. – 2017-05-04

  • Add uniqueness constraints and foreign keys. – 2017-05-04

  • Fix cache invalidation race when using transactions. – 2017-05-01

  • Only throw well-documented, Selda-specific exceptions. – 2017-05-01

  • More Hackage-friendly README. – 2017-04-30

  • Add selectors for non-generic tables.
  • Allow default insertions on all columns.
  • More sensible API for LIMIT.
  • Fix broken SQL being generated for pathological corner cases.
  • Documentation fixes. – 2017-04-20

  • Replace ¤ with :*: in table definitions. – 2017-04-20

  • Minor documentation fixes. – 2017-04-20

  • Generic tables, queries and mutation.
  • Select from inline tables.
  • Tutorial updates.
  • Minor bugfixes. – 2017-04-14

  • Initial release.