Developers

This page is for developers of Hydra itself. See Contributors.

See also:

• Developer Recipes - Step-by-step guides for common development tasks, including creating new Hydra implementations

Source code organization

Haskell is Hydra's bootstrapping language. The Hydra kernel and much of its extended functionality is written in a Haskell-based domain-specific language (DSL), then code-generated into Haskell, Java, and Python. The source code is organized by functional concern into several categories:

Kernel sources (hydra-haskell)

The Hydra/Sources directory contains DSL-based definitions that are code-generated into all implementations:

• Kernel/Types: Core type system definitions

  • Core.hs - Fundamental types (Type, Term, Element, etc.)
  • Graph.hs - Graph and module definitions
  • Coders.hs - Bidirectional transformation types
  • And others for AST, constraints, workflows, etc.

• Kernel/Terms: Term-level operations

  • Inference.hs - Type inference
  • Rewriting.hs - Term rewriting and transformation
  • Reduction.hs - Term reduction/evaluation
  • Subdirectories for Adapt, Encode, Decode, Extract, etc.

• Haskell: Haskell language support (AST, coder, serialization)

• Json and Yaml: Data format support

• Test: Cross-language test suites

  • TestSuite.hs - Common test cases for all implementations
  • Inference - Type inference test cases

• Libraries.hs: Hydra standard library definitions

These sources are code-generated into:

• Haskell: src/gen-main/haskell
• Java: hydra-java/src/gen-main/java
• Python: hydra-python/src/gen-main/python

Extended sources (hydra-ext)

The Hydra/Ext/Sources directory contains DSL-based definitions for language coders, data format adapters, and domain-specific models:

• Language coders: Avro, Java, Python, Scala, Protobuf, and many others
• Property graphs: Pg (includes TinkerPop support)
• Semantic web: Rdf, Shacl, Owl
• Query languages: GraphQL, Cypher, SQL

DSLs and primitives

To write source definitions, use the DSLs in Hydra/Dsl:

• Types.hs - Type construction DSL
• Terms.hs - Term construction DSL
• Expect.hs - Term decoding DSL

Primitive functions (built-ins) are implemented natively in each language:

• Haskell: Hydra/Lib
• Java: hydra/lib
• Python: hydra/lib

Releases

See the Release process page for detailed instructions on releasing Hydra-Haskell, Hydra-Java, and Hydra-Python.