Interoperability Report: Integrating Custom Haskell Functions into SQL Databases

This report explores the integration of Haskell functions into SQL database systems, focusing on three popular engines: SQLite (embedded) and PostgreSQL and MySQL (standalone). It provides a high-level overview of the integration process, highlighting the differences between these database types. Designed for developers and stakeholders, this report aims to explain how the integration works and varies across database engines, without delving into technical implementation details.

Why Combine Haskell with SQL?

• Custom Logic: Haskell is great for writing precise, reusable functions—like adding randomness for privacy.
• Type Safety: Haskell ensures your logic is consistent and safe.
• Efficiency: SQL is optimized for working with large datasets; combining both brings the best of each.

General Integration Approach

1. Create the Haskell function (e.g., dpellaSampleRandom for generating noise).
2. Expose it in a way the database can call it (this varies by engine).
3. Use the function inside SQL queries like any built-in function.

Common Integration Elements

Regardless of the database engine:

• The function is written in Haskell and compiled.
• A connection is made between SQL and Haskell, either through embedded code (SQLite) or an extension/plugin (PostgreSQL, MySQL).
• Haskell’s random generator is managed to ensure consistent behavior.
• All databases call the same core function, ensuring consistent results across engines.

Database-Specific Details

1. SQLite

Type: Embedded (runs inside the Haskell program)

Integration Method: Direct registration using a Haskell library (sqlite-simple)

Strengths:

• Very easy to integrate
• Lightweight and ideal for prototyping or small-scale apps

Weaknesses:

• Does not scale well for high-concurrency or networked applications
• Tied closely to the Haskell application process

Use Case: Local tools or desktop apps needing custom logic without database server overhead

2. PostgreSQL

Type: Standalone server (runs independently of the Haskell app)

Integration Method: Uses a PostgreSQL extension written in C that connects to Haskell via FFI (Foreign Function Interface)

Strengths:

• Very scalable and robust
• Clean, structured lifecycle for the Haskell runtime

Weaknesses:

• Setup is complex and requires working with C code and database extensions
• Deployment may require administrative access

Use Case: Enterprise-level applications where performance, scalability, and integration quality matter

3. MySQL

Type: Standalone server

Integration Method: Loads Haskell via a User-Defined Function (UDF) written in C

Strengths:

• Flexible and easier to load dynamically at runtime
• Good balance of power and ease of use

Weaknesses:

• Runtime setup is less structured (lacks explicit shutdown process)
• Requires thread safety and careful handling of shared state

Use Case: Web apps or moderate-scale services where flexibility and ease of setup are desired

Comparison Table

Feature	SQLite	PostgreSQL	MySQL
Runs as	Embedded process	External server	External server
Integration	Direct API registration	C extension + FFI	C UDF + FFI
Complexity	Low	High	Medium
State Management	Per connection	Global (Haskell FFI)	Global (Haskell FFI)
Setup Requirements	Minimal	PostgreSQL admin	MySQL plugin loading
Lifecycle Control	Simple (app scope)	Explicit (_PG_init)	Lazy on first call
Best For	Lightweight domains	Large scale, complex queries	Moderate load

Conclusion

By integrating Haskell functions into SQL databases, developers can unlock powerful new workflows that combine Haskell’s expressiveness with SQL’s data handling. The approach varies by engine:

• SQLite: Best for lightweight, embedded apps.
• PostgreSQL: Ideal for complex, production-scale systems.
• MySQL: Balanced choice with dynamic flexibility.

With this setup, Haskell logic can enhance database queries–enabling advanced features like Differential Privacy–while remaining scalable and reusable.