FP Complete

I’m pleased to announce the release of a Haskell library for connecting to SQL Server databases via ODBC.


The library is very simple, but what it does support should be high quality:

  1. Correct Unicode handling. All text is handled through the Text type. The test suite randomly generates Unicode to send to the server and query back in a roundtrip.
  2. A very simple API: simply connect, execute or query, and then close.
  3. Query formatting API (to protect against SQL injection).
  4. A streaming query function, so that you don’t have to load all data in memory at once.
  5. Comprehensive API documentation, including example code, pitfalls, considerations and warnings.
  6. SQL Server-specific support.
  7. Cross-platform: it should function the same on Windows, OS X or Linux. We have CI tests for this.

Tiny history

Haskell has lacked a stable, “drop in”, working package capable of connecting to SQL Server for years.

Existing implementations are pretty lacking, either they segfault, or they have weird bugs, and their code is difficult to correct.

This has been a bit of a sore point for people who just want to easily connect to an SQL Server database in Haskell. Many big businesses rely soley on many SQL Server databases, and this limitation excluded Haskell.

I needed an actual usable library for a client project. I have some good experience with FFI (foreign function interface) writing in Haskell, and knew that I could write something much more stable, provided I keep things simple.

How ODBC works

ODBC is a C API that is split into a manager and a driver.

On Windows, there is an ODBC manager that comes with the OS. On Linux and OS X, the unixODBC package provides the same functionality.

Separately, for each database type, you have driver packages. When you provide a connection string, like this:

ODBC_TEST_CONNECTION_STRING='DRIVER={ODBC Driver 13 for SQL Server};SERVER=;Uid=SA;Pwd=Passw0rd;Encrypt=no'

The DRIVER tells the ODBC API which library to use. In this case, it’s the recent SQL Server driver provided by Microsoft. Then, ODBC functions like SQLDriverConnectW will call that library.

Protective programming

I’ve put special effort into stability of the library. Talking to C can be perilous; we have to take special care in dealing with it.

Here are a number of steps taken to prevent common issues:

  1. Race conditions on Connection objects are blocked by wrapping the Connection in an MVar. This means only one thread can perform an operation on a connection at once.
  2. Double close on the connection is protected by putting the connection inside a Maybe: once freed, the Just becomes a Nothing. Future attempts to close just throw an exception to say you tried to double close.
  3. Garbage collected: the connection handle itself is wrapped in a ForeignPtr, whose finalizer (run when garbage collected) closes and frees the C object.
  4. It’s not possible to use an SQL statement separately from a connection; we don’t expose any functions to access statements directly. This prevents the user from being able to get a hold of a statement and use it on a closed (or freed) connection.
  5. It’s not possible to use an environment (which is a C object that the ODBC requires before making a connection) and a connection separately, they are held in the same C object. This prevents using a connection in a freed environment.

Comprehensive testing

There are several aspects to the testing on this project. The first is:

  1. We write tests that intentionally involve bad code (such as attempted double frees).
  2. We have QuickCheck tests that test roundtrips from the client to the server and back to the client again for all supported data types. This can take some care, as not all Haskell types roundtrip to SQL Server types without losing precision.
  3. On Windows, Linux and OS X, the library has been tested against Microsoft’s officially released SQL Server ODBC driver. I have CI setup for Windows and Linux (via Docker), on which we can run a real SQL Server to test against.


There is limited performance work done on this project, with its main focus on correctness. But there is a benchmark suite for space usage. This means we have a baseline for future improvements. Time usage tests can come later.

How to use it

It’s available on Hackage and on Stackage under the name odbc. The README explains how to install the Microsoft ODBC Driver and how to specify the right connection string.

The Travis CI and AppVeyor files should also be useful to see how a from-nothing setup is able to run a SQL Server and connect to it from Haskell.

We invite you to contribute any changes on GitHub and share any experiences you have using the library, especially if you are a heavy user of SQL Server!

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