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hasql Generator

The haskell-hasql.gen generator produces a Haskell library that exposes each SQL query as a typed Statement value from the hasql library.

Repository: github.com/pgenie-io/haskell-hasql.gen

Setup

In your project1.pgn.yaml:

artifacts:
  hasql: https://raw.githubusercontent.com/pgenie-io/haskell-hasql.gen/v0.1.0/gen/Gen.dhall

The output is written to artifacts/hasql/.

What is Generated

For each query, the generator produces a Statement value with fully typed parameters and results. The generated library includes:

  • A Cabal package file (*.cabal) ready to add as a dependency.
  • One Haskell module per query, plus a top-level module that re-exports everything.
  • Data types for composite types and enumerations defined in your schema.
  • Proper nullability mapping (Maybe for nullable columns, direct types for NOT NULL).

Example

Given the query:

-- queries/select_album_by_name.sql

select id, name, released, format, recording
from album
where name = $name

The generator produces a Statement with:

  • Parameter: Maybe Text (nullable text)
  • Result row: a record with id :: Int64, name :: Text, released :: Maybe Day, format :: Maybe AlbumFormat, recording :: Maybe RecordingInfo

Type Mappings

Primitive Types

PostgreSQL Haskell
bool Bool
bytea ByteString
char Char
cidr / inet IPRange / IP
date Day
float4 Float
float8 Double
int2 Int16
int4 Int32
int8 Int64
interval DiffTime
json / jsonb Value (aeson)
numeric Scientific
text Text
time TimeOfDay
timestamp LocalTime
timestamptz UTCTime
timetz TimeOfDay
uuid UUID

Composite Types

Composite types are mapped to Haskell record types. Each field in the composite becomes a record field, with nullability respected.

Enumerations

PostgreSQL enumerations become Haskell newtype wrappers or sum types (depending on generator version).

Arrays

Arrays of supported element types are mapped to Vector from the vector library.

Nullability

  • NOT NULL columns / parameters → direct Haskell type (e.g. Text)
  • Nullable columns / parameters → Maybe (e.g. Maybe Text)

Versioning

Haskell uses PVP versioning. The generator adapts the version field from your project1.pgn.yaml (SemVer) to PVP by prepending 0.:

SemVer PVP
1.0.0 0.1.0.0
2.3.1 0.2.3.1

Further Documentation

Visit the haskell-hasql.gen repository for the full documentation, including:

  • Complete generated output examples (see demo-output/)
  • Advanced configuration
  • Changelog