Hydra-Ext

Extension coders for Hydra: schema languages and data formats outside the core hydra-pg / hydra-rdf set. This package holds DSL sources only — generated output lands under dist/<lang>/hydra-ext/ for each target language that the package supports.

Hydra is a functional programming language based on the LambdaGraph data model. See the main Hydra README for project overview and use cases.

What's in this package

Coders, syntax models, and language metadata for:

• Schema languages and IDLs — Avro, Protobuf, GraphQL, Pegasus/PDL, JSON Schema, YAML, SQL (ANSI), Datalog, Kusto KQL.
• Generation targets — C++, C# (Csharp), Rust.
• Domain models — Workflow, Atlas, Azure DTDL, GeoJSON, IANA Relations, OSV (Open Source Vulnerability), STAC Items, Parquet/Delta Parquet.

The full list is maintained in src/main/haskell/Hydra/Sources/Ext/Manifest.hs.

Not in this package

A few originally-Ext members have moved out into their own packages as they matured into proper coders:

• Go → packages/hydra-go/ (issue #289).
• TypeScript → packages/hydra-typescript/ (issue #126).

Sync scope

hydra-ext is not part of the default bin/sync.sh host × target matrix. Most modules here are experimental schema and syntax models rather than full cross-language coders. The package's package.json declares:

"targetLanguages": ["python"]

which scopes regeneration to Python-side artifacts under dist/python/hydra-ext/. To regenerate, run from a worktree root:

bin/sync.sh --hosts haskell --targets python    # via the kernel manifest

The Haskell-side DSL sources are processed in Phase 1 of every bin/sync.sh invocation; the result lands in dist/json/hydra-ext/. Target-language emission beyond Python is currently opt-in and limited.

Demos using hydra-ext

Several demos exercise extension coders:

• Avro demo — bidirectional Avro schema reading and writing.
• GenPG demo — CSV → property graph (uses Avro for schemas).
• GraphQL JSON demo — GraphQL schema inference and JSON validation.
• SHACL demo — uses several ext models.

Adding a coder

For a new schema language or generation target:

1. Add the DSL source modules under src/main/haskell/Hydra/Sources/<Lang>/ (Coder.hs, Language.hs, Serde.hs, Syntax.hs at minimum).
2. Register them in src/main/haskell/Hydra/Sources/Ext/Manifest.hs's mainModules list.
3. Run heads/haskell/bin/sync-haskell.sh to regenerate the JSON kernel and validate that the new sources type-check against the rest of hydra-ext.

For the broader process, see Creating a new Hydra implementation (for full host languages) or Adding new primitives (for new functions in hydra.lib.*).

See also

• Concepts — the LambdaGraph data model.
• hydra-kernel README — the core types, terms, and primitives every extension coder builds on.
• Implementation overview — the coder framework and how it's organized.