persistwrap

A wrapper for the persistence layer in any application. Allows arbitrary dumb Haskell data to be easily stored and retrieved from a relational backend.

Advantanges to using persistwrap over other ORMs

• Naturally handles datastructures containing sum types, nested records, lists, and maps (but not recursive types).
  • Data which derives GHC.Generic is easily and automatically made persistable.
  • Note that this is achieved by not assuming a 1:1 relationship between tables and types. In general a type may require multiple tables to encode.
• Uses RankNTypes to define a monadic context with a phantom variable (similar to ST) in which the persistence layer is available. Keys from one context cannot be accidentally reused in another context. In this way, you can safely work with multiple persistent backends simultaneously without fear of mixing them up.
• Primary tables (associated to a top-level type) are named (associated to a unique GHC.TypeLits.Symbol) rather than keyed by type. You may have multiple tables each holding the same type of object.
• Distinguishes between MonadPersist m and MonadTransaction m where the latter is a monad in which a collection of persistent actions can occur atomically and the former provides a method atomicTransaction to drop into a Transaction monad.
• Complements a style in which data-structures which contain foreign keys are parameterized over the type of the foreign key fk. In this way you can easily switch out backends without having to make any changes to how your data is defined.
• Comes with a pure-Haskell in-memory STM-based backend for efficient unit testing.
• Can abstract over the set of datastructures which are available in a given context. This allows you to interact with the datastructures in the persistence layer that are relevant without having to have yet defined every datastructures you mean to persist. Ex.

  getFoo :: Persisted "foo" Foo m => ForeignKey m "foo" -> m Foo
  getFoo = lookupX
  

  Note how we don't need to know anything about what other types are persisted in order to define or call getFoo.

Examples

• Declaring with an EntityPart instance: Widget.hs. The generated schema for the Widget type can be found in widget_schemas.golden
• Working with persisted data: WidgetSpec.hs