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 you have to work to get your storage layer to accept your application data types. 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. Haskellers have attempted a more revolutionary route of creating Haskell-specific data stores to get around these flaws that allow one to easily store any Haskell type. These options are great for certain use cases, but they constrain one to the storage techniques provided by the library, do not interface well with other languages, and the flexibility can also mean one must write reams of code for querying data. In contrast, Persistent allows us to choose among existing data....