Roc Library Development

Roc-specific patterns for library package and platform development. This skill extends

meta-library-dev

This Skill Extends

• meta-library-dev

  - Foundational library patterns (API design, versioning, testing strategies)

For general concepts like semantic versioning, module organization principles, and testing pyramids, see the meta-skill first.

This Skill Adds

• Roc platform system: Platform development, app/platform separation, I/O interfaces
• Package organization: Module structure, package declarations, visibility patterns
• Roc idioms: Pure functional patterns, platform interfaces, type inference
• Roc ecosystem: Standard library modules, builtin modules, platform conventions

This Skill Does NOT Cover

• General library patterns - see

  meta-library-dev

• Roc application development - focus is on reusable packages/platforms
• Roc-specific syntax basics - see language documentation
• CLI application development - see CLI-specific skills

Overview

Roc has a unique architecture that separates applications from platforms:

┌─────────────────────────────────────────────────────────────────┐
│                    Roc Package Architecture                     │
├─────────────────────────────────────────────────────────────────┤
│                                                                 │
│  ┌───────────────┐                                              │
│  │  Application  │  (your code)                                 │
│  │    (.roc)     │                                              │
│  └───────┬───────┘                                              │
│          │                                                      │
│          │ imports                                              │
│          ▼                                                      │
│  ┌───────────────┐                                              │
│  │    Package    │  (reusable modules)                          │
│  │  package {}   │                                              │
│  └───────┬───────┘                                              │
│          │                                                      │
│          │ depends on                                           │
│          ▼                                                      │
│  ┌───────────────┐         ┌──────────────┐                    │
│  │   Platform    │────────▶│  Runtime     │                    │
│  │  platform {}  │         │  (Rust, C)   │                    │
│  └───────────────┘         └──────────────┘                    │
│          │                                                      │
│          └─ Provides I/O primitives (no I/O in stdlib)         │
│                                                                 │
└─────────────────────────────────────────────────────────────────┘

Key Concepts:

This skill focuses on creating Packages and Platforms for reuse.

Quick Reference

File Types

app

app [main] { ... }

package

package [Module1, Module2] { ... }

platform

platform "name" requires {} exposes {} packages {} ...

module

module [export1, export2]

Common Commands

roc check

roc build

roc test

roc format

roc docs

Package Development

Package Structure

my-package/
├── package/
│   ├── main.roc           # Package entry point
│   ├── Parser.roc         # Module files
│   ├── Types.roc
│   └── Internal.roc
├── examples/
│   └── basic.roc          # Example apps
├── platform/              # Optional: custom platform
│   └── main.roc
└── README.md

Package Declaration

package/main.roc:

package [
    # Public modules (exposed to users)
    Parser,
    Types,
    # Can also expose specific values
    parseConfig,
    defaultConfig,
]

Key Principles:

1. Only expose what users need (minimal public API)
2. Keep implementation modules private
3. Re-export common types/functions at package level
4. Use descriptive module names

Module Structure

package/Parser.roc:

module [
    # Public API
    parse,
    parseWithConfig,
    Config,
    ParseError,
]

import Types exposing [Document, Metadata]
import Internal  # Private import, not exposed

## Parse a document string.
##
## ```roc
## result = parse "# Title\nContent"
## expect result == Ok { title: "Title", content: "Content" }
## ```
parse : Str -> Result Document ParseError
parse = \input ->
    parseWithConfig input defaultConfig

## Configuration for parsing.
Config : {
    strict : Bool,
    preserveWhitespace : Bool,
}

defaultConfig : Config
defaultConfig = {
    strict: Bool.false,
    preserveWhitespace: Bool.false,
}

## Parse with custom configuration.
parseWithConfig : Str, Config -> Result Document ParseError
parseWithConfig = \input, config ->
    # Implementation
    # ...

Visibility Patterns

Module-level visibility:

module [
    # Public: Anyone importing this module can use these
    publicFunction,
    PublicType,
]

# Private: Only visible within this module
privateHelper : Str -> Str
privateHelper = \x -> x

# Public functions can call private helpers
publicFunction : Str -> Str
publicFunction = \input ->
    privateHelper input

Package-level visibility:

# main.roc
package [
    # These modules are visible to package users
    Parser,
    Types,
]

# Internal is NOT in package exports = private to package
import Internal

Platform Development

Platform Structure

my-platform/
├── platform/
│   └── main.roc           # Platform declaration
├── host/
│   ├── Cargo.toml         # Rust host implementation
│   └── src/
│       ├── lib.rs
│       └── main.rs
└── examples/
    └── hello.roc

Platform Declaration

platform/main.roc:

platform "cli"
    requires { Model } { main : Task Model [] }
    exposes [
        Task,
        Stdout,
        Stdin,
        File,
        Path,
        Env,
    ]
    packages {}
    imports []
    provides [mainForHost]

import Task
import Internal.Task

mainForHost : Task.Task Model [] as Fx
mainForHost = main

Key Components:

requires

main

exposes

packages

imports

provides

Platform Interface Design

Good platform design:

1. Clear separation of concerns:

# Platform exposes pure types and Task-based I/O
module [
    # Pure types
    Request,
    Response,
    # Effectful operations
    serveHttp : Config, (Request -> Task Response []) -> Task {} []
]

2. Type-safe I/O:

# File module
read : Path -> Task Str [FileNotFound, PermissionDenied]
write : Path, Str -> Task {} [PermissionDenied, DiskFull]

# Stdin module
line : Task Str [EndOfInput]

3. Composable effects:

# Tasks can be chained
program : Task {} []
program =
    path <- Task.await (Env.var "CONFIG_PATH")
    contents <- Task.await (File.read path)
    config <- Task.await (parseConfig contents)
    runServer config

Module Organization Patterns

Single Responsibility Modules

# Types.roc - Just type definitions
module [Config, Document, Metadata]

Config : {
    timeout : U64,
    retries : U8,
}

Document : {
    title : Str,
    content : Str,
    metadata : Metadata,
}

Metadata : {
    author : Str,
    created : U64,
}

Extension Modules

# ConfigExt.roc - Operations on Config
module [default, withTimeout, validate]

import Types exposing [Config]

default : Config
default = {
    timeout: 30_000,
    retries: 3,
}

withTimeout : Config, U64 -> Config
withTimeout = \config, ms -> { config & timeout: ms }

validate : Config -> Result Config [InvalidTimeout, InvalidRetries]
validate = \config ->
    if config.timeout == 0 then
        Err InvalidTimeout
    else if config.retries > 10 then
        Err InvalidRetries
    else
        Ok config

Re-export Pattern

package/main.roc:

package [
    # Re-export commonly used types from multiple modules
    Config,
    Document,
    # Re-export main functions
    parse,
    parseWithConfig,
    # Expose modules for advanced usage
    Parser,
    Types,
]

# Import everything needed for re-exports
import Parser exposing [parse, parseWithConfig]
import Types exposing [Config, Document]

Type Design Patterns

Opaque Types

module [UserId, fromU64, toU64]

# Opaque: internals hidden from users
UserId := U64

## Create a UserId from a U64.
fromU64 : U64 -> UserId
fromU64 = @UserId

## Extract the U64 from a UserId.
toU64 : UserId -> U64
toU64 = \@UserId id -> id

Tagged Unions for Errors

module [ParseError, toStr]

ParseError : [
    InvalidSyntax Str,
    UnexpectedEnd,
    UnknownTag Str,
]

toStr : ParseError -> Str
toStr = \err ->
    when err is
        InvalidSyntax msg -> "Invalid syntax: $(msg)"
        UnexpectedEnd -> "Unexpected end of input"
        UnknownTag tag -> "Unknown tag: $(tag)"

Builder Pattern (Records with Defaults)

module [Config, default, build, withTimeout, withRetries]

Config : {
    timeout : U64,
    retries : U8,
    strict : Bool,
}

default : Config
default = {
    timeout: 30_000,
    retries: 3,
    strict: Bool.false,
}

build : {} -> Config
build = \{} -> default

withTimeout : Config, U64 -> Config
withTimeout = \config, ms -> { config & timeout: ms }

withRetries : Config, U8 -> Config
withRetries = \config, n -> { config & retries: n }

# Usage:
# config = build {} |> withTimeout 60_000 |> withRetries 5

Testing Patterns

Expect Blocks

## Parse a valid document.
parse : Str -> Result Document ParseError
parse = \input ->
    # Implementation
    # ...

# Test cases using expect
expect
    result = parse "# Title\nContent"
    result == Ok { title: "Title", content: "Content" }

expect
    result = parse ""
    result == Err UnexpectedEnd

expect
    result = parse "# Title\nContent"
    Result.isOk result

Test Modules

tests/ParserTests.roc:

module []

import Parser exposing [parse, ParseError]

expect
    # Happy path
    input = "# Hello\nWorld"
    result = parse input
    result == Ok { title: "Hello", content: "World" }

expect
    # Error case
    result = parse ""
    result == Err UnexpectedEnd

expect
    # Edge case
    input = Str.repeat "a" 10_000
    result = parse input
    Result.isOk result

Property-Based Testing Pattern

# Generate test cases
expectAll = \cases ->
    List.all cases \case ->
        expected = case.expected
        actual = parse case.input
        actual == expected

expect
    cases = [
        { input: "# A\nB", expected: Ok { title: "A", content: "B" } },
        { input: "", expected: Err UnexpectedEnd },
        { input: "###", expected: Err (InvalidSyntax "Too many #") },
    ]
    expectAll cases

Builtin Modules

Roc provides several builtin modules auto-imported into every file:

Str

concat

split

trim

to_utf8

Num

add

sub

is_even

abs

List

map

filter

fold_left

len

Dict

insert

get

keys

values

Set

insert

contains

union

Bool

true

false

not

Result

Ok

Err

map

try

Note: The standard library provides NO I/O operations. All I/O comes from platforms.

Breaking Changes (2025)

Snake Case Migration

Builtins are migrating from

camelCase

snake_case

# Old (deprecated)
Str.joinWith
List.keepIf

# New (current)
Str.join_with
List.keep_if

Action: Use new snake_case names in all new packages.

Task Deprecation (Purity Inference)

Task

# Old pattern
readConfig : Task Config [FileNotFound]
readConfig =
    path <- Task.await (Env.var "CONFIG")
    contents <- Task.await (File.read path)
    Task.ok (parse contents)

# New pattern (purity inferred)
readConfig : Config ![FileNotFound, EnvVarNotSet]
readConfig =
    path = Env.var! "CONFIG"
    contents = File.read! path
    parse contents

Action: For new platforms, use purity inference. For compatibility, check platform docs.

Publishing Packages

Pre-publish Checklist

• roc check

  passes on all modules

• roc test

  passes

• roc format

  applied to all files
• Documentation comments on all public functions
• Examples in

  examples/

  directory
• README.md with usage examples
• Version follows semantic versioning
• No dependencies on unreleased packages
• License file included

Package Metadata

Include in README.md:

# My Package

Brief description of what this package does.

## Installation

How to include this package in a Roc project.

## Quick Start

\```roc
import MyPackage exposing [parse]

main =
    result = parse "input"
    # ...
\```

## Modules

- `Parser` - Main parsing logic
- `Types` - Type definitions
- `Config` - Configuration utilities

## Examples

See `examples/` directory.

## License

MIT

Common Patterns

Error Handling

# Return Result for operations that can fail
parse : Str -> Result Document [InvalidSyntax Str, UnexpectedEnd]
parse = \input ->
    if Str.is_empty input then
        Err UnexpectedEnd
    else
        # Parse logic
        Ok document

# Chain Results
processDocument : Str -> Result ProcessedDoc [ParseError, ValidationError]
processDocument = \input ->
    parsed <- Result.try (parse input |> Result.map_err ParseError)
    validated <- Result.try (validate parsed |> Result.map_err ValidationError)
    Ok (process validated)

Polymorphic Functions

# Generic over any type
identity : a -> a
identity = \x -> x

# Constrained generics (via abilities - future feature)
compare : a, a -> [LT, EQ, GT] where a implements Ord

Pipeline Pattern

# Use |> for readable data transformations
processData : List Str -> List Str
processData = \items ->
    items
    |> List.map Str.trim
    |> List.keep_if (\s -> !(Str.is_empty s))
    |> List.map Str.to_lowercase
    |> List.sort_with Str.compare

Anti-Patterns

1. Leaking Implementation Details

# Bad: Exposing internal structure
module [Config, InternalState]

# Good: Only expose what's needed
module [Config, fromStr, toStr]

2. Overly Large Modules

# Bad: Single module with 1000+ lines

# Good: Split into focused modules
# - Types.roc (types)
# - Parser.roc (parsing)
# - Validator.roc (validation)
# - main.roc (re-exports)

3. Missing Documentation

# Bad: No docs
parse : Str -> Result Document ParseError

# Good: Documented
## Parse a document from a string.
##
## Returns a Document on success, or ParseError if the input is invalid.
##
## ```roc
## result = parse "# Title"
## expect result == Ok { title: "Title", ... }
## ```
parse : Str -> Result Document ParseError

4. Platform Doing Too Much

# Bad: Platform provides complex business logic
module [
    readUserFromDb,
    validateUser,
    sendEmail,
]

# Good: Platform provides primitives
module [
    dbQuery,
    httpRequest,
    # Let apps compose these into business logic
]

Troubleshooting

Module Not Found

Symptom:

Module 'MyModule' not found

Causes & Fixes:

package [...]

MyModule.roc

Type Inference Failures

Symptom:

Cannot infer type for this expression

Fixes:

1. Add explicit type annotation
2. Provide more context (often happens in empty lists)
3. Check for ambiguous number types (add

   U64

   ,

   I32

   , etc.)

# Problem: Ambiguous type
nums = []

# Fix: Provide type context
nums : List U64
nums = []

# Or use in context
nums = List.range { start: At 1, end: At 10 }

Circular Dependencies

Symptom:

Circular dependency detected

Fix: Restructure modules

# Before (circular):
# A.roc imports B.roc
# B.roc imports A.roc

# After: Extract shared types
# Types.roc (shared types)
# A.roc imports Types
# B.roc imports Types

Platform Examples

Basic CLI Platform

platform "basic-cli"
    requires { Model } { main : Task Model [] }
    exposes [
        Task,
        Stdout,
        Stdin,
        File,
        Path,
        Env,
        Arg,
    ]
    packages {}
    imports []
    provides [mainForHost]

mainForHost : Task Model [] as Fx
mainForHost = main

Web Platform

platform "basic-webserver"
    requires {} { main : Request -> Task Response [] }
    exposes [
        Request,
        Response,
        Task,
        Stdout,
    ]
    packages {}
    imports []
    provides [mainForHost]

Request : {
    method : [GET, POST, PUT, DELETE],
    url : Str,
    headers : Dict Str Str,
    body : List U8,
}

Response : {
    status : U16,
    headers : Dict Str Str,
    body : List U8,
}

References

• meta-library-dev

  - Foundational library patterns
• Roc Tutorial
• Roc Platforms
• GitHub - roc-lang/roc
• basic-cli Platform
• basic-webserver Platform

Sources

• The Roc Programming Language
• GitHub - roc-lang/roc
• Tutorial | Roc
• Platforms and Apps | Roc
• Understanding Roc: Functional and separate from the runtime | TechTarget