pg-schema
pg-schema is a Haskell library that lifts a PostgreSQL schema description to the type level and gives you type-safe, compile-time-checked access to data.
The core idea is a type provider: from a live database (tables, primary and unique keys, foreign keys, column types—including arrays and enums) a schema representation is built that you then use with ordinary GHC types. You can have any number of such schemas; each may include any subset of tables.
To generate schemas you create a separate application that uses PgSchema.Generation.updateSchemaFile to produce .hs files describing each schema (sets of tables, relationships between them, and the types in use).
For reading and writing data, use the PgSchema.DML module. Queries are built from a typed EDSL, without hand-written SQL. You can describe data trees (nested records along relationships), including inserting an entire tree in one database round-trip (JSON is used internally), and fetching a tree with a single SELECT, with predicates, ordering, and limits at each level of the tree (WHERE, ORDER BY, LIMIT/OFFSET, and so on).
Inserts and reads use either ordinary Haskell records with a Generic instance or types of the form "fld1" := Int32 :. "fld2" := Maybe Text. Field names in Haskell records must match those in the database (with renaming supported) but you can work with any subset of the table’s fields. Navigation along relationships uses foreign-key constraint names. Types and nullability are checked against the data layout.
SELECT and INSERT/UPDATE/UPSERT are implemented for the nested structures. There are also versions of UPDATE and DELETE applied to a single table.
When reading data, an EDSL sets conditions, ordering, and grouping. All of these operations are type-safe.
For inserts and updates, additional compile-time checks derive from database constraints—for example, inserts verify that mandatory fields are present at every level.
Requirements
GHC ≥ 9.10. Theoretically, one could target older GHC version at the cost of a slightly worse API without RequiredTypeArguments.
