Integration of "hasql" with "postgresql-types" via the IsScalar typeclass from "postgresql-types-algebra". Provides automatic encoder and decoder generation for precise PostgreSQL scalar types.

The standard "hasql" codecs use common Haskell types like Text, DiffTime, UTCTime, etc. However these types do not always map precisely to PostgreSQL types. E.g., the PostgreSQL interval type carries information about months, years and microseconds, while the Haskell DiffTime type only represents a time difference in picoseconds. Such mismatches can lead to data loss or unexpected behavior. The "postgresql-types" library addresses such issues by providing precise Haskell representations for PostgreSQL types. This package integrates it with "hasql". It also provides a class-based polymorphic interface for defining "hasql" Value codecs.

import Hasql.PostgresqlTypes () -- for the instances
import qualified PostgresqlTypes as Pt
import qualified Hasql.Statement as Statement
import qualified Hasql.Encoders as Encoders
import qualified Hasql.Decoders as Decoders
import qualified Hasql.Mapping.IsScalar as IsScalar

myStatement :: Statement.Statement Pt.Timestamptz [Pt.Timestamptz]
myStatement = Statement.preparable sql enc dec
  where
    sql = "SELECT $1::timestamptz"
    enc = Encoders.param (Encoders.nonNullable IsScalar.encoder)
    dec = Decoders.rowList (Decoders.column (Decoders.nonNullable IsScalar.decoder))