haskell-pure-functional-programming

---
name: haskell-pure-functional-programming
description: Use this skill when authoring provably correct Haskell software using monads, applicative functors, algebraic data types, and type inference.
---
# Haskell Pure Functional Programming

## When to use
Use this skill when you need to authoring provably correct Haskell software using monads, applicative functors, algebraic data types, and type inference.

## Prerequisites
- Working knowledge of relevant programming language or database fundamentals.
- Access to testing environment or local runtime instance.

## Step-by-step workflow
1. **Analyze operational requirements**: Review performance boundaries, data integrity constraints, and query patterns.
2. **Design schema / interface**: Define explicit data contracts, types, or indexes supporting planned operations.
3. **Implement core routines**: Construct declarative, type-safe, or parameterized queries and functions.
4. **Validate performance**: Verify execution metrics using profilers, EXPLAIN plans, or benchmark test suites.
5. **Implement defensive controls**: Enforce timeout boundaries, connection pooling limits, and transaction safety.

## Commands / code examples
```haskell
module TransactionEngine where

data Currency = USD | EUR | GBP deriving (Show, Eq)
data Money = Money { amount :: Double, currency :: Currency } deriving (Show, Eq)

convert :: Double -> Money -> Money
convert rate (Money amt curr) = Money (amt * rate) curr

safeDivide :: Double -> Double -> Maybe Double
safeDivide _ 0 = Nothing
safeDivide x y = Just (x / y)
```

## Common mistakes & pitfalls
- Overlooking indexing strategies on foreign keys and frequently queried filter columns.
- Running unparameterized dynamic queries, creating severe SQL injection vulnerabilities.
- Holding open long-running database transactions during slow external network calls.

## Verification checklist
- [ ] Schema contracts and type signatures are strictly enforced.
- [ ] Queries are fully parameterized against injection vulnerabilities.
- [ ] Profiling or execution plans show optimal resource consumption.
- [ ] Error handling safely releases locks and rolls back aborted transactions.

## Related skills
- postgresql-query-optimization-explain
- sql-anti-patterns-remediation
- acid-transactions-isolation-levels
- secure-code-review-static-analysis-sast