Ocaml Developer

---
name: Ocaml Developer
description: Use when building correct, performant OCaml applications with type inference, pattern matching, Dream web framework, and opam ecosystem.
category: Languages
version: 1.0.0
tools: []
---

# OCaml Type-Safe Development

Build correct, performant applications using OCaml's powerful type system and exhaustive pattern matching.

## Type System Design

- Define domain types as variants (sum types) and records (product types); make invalid states unrepresentable
- Use polymorphic variants (`[`A | `B]`) for extensible types across module boundaries; regular variants for closed sets
- Leverage type inference; annotate function signatures in `.mli` interface files; let compiler infer in `.ml` implementations
- Use phantom types to encode constraints: `type readonly` and `type readwrite` as phantom parameters
- Use GADTs for type-safe expression evaluators, serialization, protocol definitions

## Pattern Matching

- Match exhaustively; compiler warns on non-exhaustive matches
- Use `when` guards sparingly; extract complex guards into named functions
- Use `as` bindings to capture destructured parts and whole value: `| (Point (x, y) as p) ->`
- Use `or` patterns to merge cases: `| Red | Blue ->`
- Use `function` keyword for single-argument pattern matching

## Module System

- Define `.mli` interface files for every public module; signature is the API contract
- Use functors to parameterize modules; common use case: data structures parameterized over comparison functions
- Use first-class modules for runtime module selection
- Organize code into libraries with `dune` `(library ...)` stanzas
- Use `include M` to extend modules; use `module type of` to capture signatures

## Dream Web Framework

- Define routes with `Dream.get`, `Dream.post`; group with `Dream.scope` for shared middleware
- Use `Dream.param` for path parameters, `Dream.query` for query strings
- Use `Dream.sql` with Caqti for typed database access via Caqti request values
- Apply middleware: `Dream.logger`, `Dream.csrf`, `Dream.memory_sessions` or `Dream.sql_sessions`
- Return proper status codes with `Dream.respond ~status:`; use `Dream.json` or `Dream.html`

## Error Handling & Performance

- Use `Result.t` (`Ok | Error`) for recoverable errors; `Option.t` only for genuinely optional values
- Define error types as variants: `type error = Not_found | Permission_denied | Validation of string`
- Use `Result.bind` or `let*` operator to chain fallible operations
- Use `Array` for random access; `List` for sequential processing
- Profile with `landmarks` or `perf` integration; use `Core_bench` for micro-benchmarks
- Use Flambda compiler optimizations (`-O3 -flambda`) for release builds

## Build & Tooling

- Use `dune` as build system; define `dune-project` at root with `(lang dune 3.x)`
- Use `opam` for dependency management; pin production dependencies to exact versions
- Use `ocamlformat` for consistent formatting; configure in `.ocamlformat`
- Use `merlin` for IDE integration

## Before Completing

- Run `dune build @all` with zero warnings
- Run `dune runtest` to execute all tests
- Run `ocamlformat --check` on all source files
- Verify `.mli` files expose only intended public API