Developing Web Apps with Haskell and Yesod, 2nd Edition

Chapter 10. Persistent

Forms deal with the boundary between the user and the application. Another boundary we need to deal with is between the application and the storage layer. Whether it be a SQL database, a YAML file, or a binary blob, odds are your storage layer does not natively understand your application’s data types, and you’ll need to perform some marshaling. Persistent is Yesod’s answer to data storage—a type-safe, universal data store interface for Haskell.

Haskell has many different database bindings available. However, most of these have little knowledge of a schema and therefore do not provide useful static guarantees. They also force database-dependent APIs and data types on the programmer.

Some Haskellers have attempted a more revolutionary route: creating Haskell-specific data stores that allow one to easily store any strongly typed Haskell data. These options are great for certain use cases, but they constrain one to the storage techniques provided by the library and do not interface well with other languages.

In contrast, Persistent allows us to choose among existing databases that are highly tuned for different data storage use cases, to interoperate with other programming languages, and to use a safe and productive query interface, while still keeping the type safety of Haskell data types.

Persistent follows the guiding principles of type safety and concise, declarative syntax. Some other nice features include the following:

Database agnosticity

There is ...