Convert Roc Fsharp

Convert Roc to F#

Convert Roc code to idiomatic F#. This skill extends meta-convert-dev with Roc-to-F# specific type mappings, idiom translations, and architectural guidance.

This Skill Extends

• meta-convert-dev - Foundational conversion patterns (APTV workflow, testing strategies)

For general concepts like the Analyze → Plan → Transform → Validate workflow, testing strategies, and common pitfalls, see the meta-skill first.

This Skill Adds

• Type mappings: Roc types → F# types
• Idiom translations: Roc patterns → idiomatic F#
• Error handling: Roc Result/tag unions → F# Result/Option
• Platform shift: Roc platform model → .NET runtime
• Paradigm alignment: Both functional-first, but different architectures

This Skill Does NOT Cover

• General conversion methodology - see meta-convert-dev
• Roc language fundamentals - see lang-roc-dev
• F# language fundamentals - see lang-fsharp-dev
• Reverse conversion (F# → Roc) - see convert-fsharp-roc

Quick Reference

When Converting Code

1. Analyze source thoroughly before writing target
2. Map types first - create type equivalence table
3. Preserve semantics over syntax similarity
4. Adopt F# idioms - leverage .NET ecosystem
5. Handle edge cases - error paths, effects, resource management
6. Test equivalence - same inputs → same outputs

Paradigm Translation

Mental Model Shift: Platform Model → .NET Runtime

Both Roc and F# are functional-first languages, but they differ fundamentally in how they handle effects:

Architecture Mental Model

Roc (Platform Model)              F# (.NET)
┌─────────────────────┐           ┌─────────────────────┐
│   Your Roc Code     │           │   Your F# Code      │
│   (pure only)       │           │  (can do I/O)       │
│   ↓                 │           │  ↓                  │
│   Platform API      │           │   .NET BCL          │
│   ↓                 │           │   ↓                 │
│   Platform Host     │           │   CLR Runtime       │
└─────────────────────┘           └─────────────────────┘
    Clear separation                   Everything in
    between pure & effects              same runtime

Key shift: In Roc, I/O goes through the platform's Task type. In F#, you can call Console.WriteLine or perform I/O anywhere.

Type System Mapping

Primitive Types

Collection Types

Composite Types

Roc Specific Types → F#

Idiom Translation

Pattern 1: Tag Unions to Discriminated Unions

Roc:

Color : [Red, Green, Blue, Custom(U8, U8, U8)]

describe : Color -> Str
describe = \color ->
    when color is
        Red -> "red"
        Green -> "green"
        Blue -> "blue"
        Custom(r, g, b) -> "rgb(\(Num.toStr(r)), \(Num.toStr(g)), \(Num.toStr(b)))"

F#:

type Color =
    | Red
    | Green
    | Blue
    | Custom of r: byte * g: byte * b: byte

let describe color =
    match color with
    | Red -> "red"
    | Green -> "green"
    | Blue -> "blue"
    | Custom (r, g, b) -> $"rgb({r}, {g}, {b})"

Why this translation:

• Roc tag unions → F# discriminated unions (nearly identical)
• Roc when → F# match (same exhaustiveness checking)
• Roc interpolation \(x) → F# interpolation $"{x}" or {x}
• Both enforce exhaustive pattern matching

Pattern 2: Optional Values

Roc:

findUser : I64 -> [Some User, None]
findUser = \id ->
    List.findFirst(users, \u -> u.id == id)
    |> Result.toOption

userName =
    when findUser(1) is
        Some(u) -> u.name
        None -> "Unknown"

F#:

let findUser id =
    users |> List.tryFind (fun u -> u.Id = id)

let userName =
    match findUser 1 with
    | Some u -> u.Name
    | None -> "Unknown"

Why this translation:

• Roc [Some a, None] → F# Option<'a> (built-in type)
• Roc pattern matching → F# pattern matching (direct mapping)
• F# has Option.map, Option.bind helpers not shown in Roc example
• Both achieve same null-safety

Pattern 3: Result Error Handling

Roc:

divide : I64, I64 -> Result I64 [DivByZero]
divide = \x, y ->
    if y == 0 then
        Err(DivByZero)
    else
        Ok(x // y)

calculate : I64, I64, I64 -> Result I64 [DivByZero]
calculate = \a, b, c ->
    x = divide!(a, b)
    y = divide!(x, c)
    Ok(y)

F#:

type DivisionError = DivByZero

let divide x y =
    if y = 0 then
        Error DivByZero
    else
        Ok (x / y)

let calculate a b c =
    result {
        let! x = divide a b
        let! y = divide x c
        return y
    }

Why this translation:

• Roc Result ok err → F# Result<'ok,'err> (built-in)
• Roc ! try operator → F# let! in computation expression
• Roc tag errors [DivByZero] → F# DU type DivisionError = DivByZero
• F# computation expressions provide cleaner syntax than nested matches

Pattern 4: List Operations

Roc:

result =
    items
    |> List.keepIf(\x -> x.active)
    |> List.map(\x -> x.value)
    |> List.walk(0, Num.add)

F#:

let result =
    items
    |> List.filter (fun x -> x.Active)
    |> List.map (fun x -> x.Value)
    |> List.sum

Why this translation:

• Roc keepIf → F# filter (different naming)
• Roc walk(0, Num.add) → F# sum (built-in helper)
• Both use pipeline operator idiomatically
• F# has more list helpers (sum, average, etc.)

Pattern 5: Record Updates

Roc:

Person : {
    firstName : Str,
    lastName : Str,
    age : U32,
}

person = { firstName: "Alice", lastName: "Smith", age: 30 }
olderPerson = { person & age: 31 }

F#:

type Person = {
    FirstName: string
    LastName: string
    Age: int
}

let person = { FirstName = "Alice"; LastName = "Smith"; Age = 30 }
let olderPerson = { person with Age = 31 }

Why this translation:

• Roc & operator → F# with keyword (copy-and-update)
• Roc uses camelCase → F# uses PascalCase (convention)
• Both create new records (immutable)
• F# uses semicolons ; for field separators, Roc uses commas

Pattern 6: Pipeline Operator

Roc:

result =
    userId
    |> fetchUser
    |> validateUser
    |> saveUser

F#:

// Same pipeline style
let result =
    userId
    |> fetchUser
    |> validateUser
    |> saveUser

// Or with composition
let processUser =
    fetchUser
    >> validateUser
    >> saveUser

let result = processUser userId

Why this translation:

• Both use |> for pipeline (identical)
• F# also has >> composition operator (Roc doesn't)
• Same left-to-right data flow
• F# provides more composition options

Error Handling

Roc Result Model → F# Result/Exception Model

Roc only has Result. F# supports both Result<'a,'e> and exceptions.

Roc:

divide : I64, I64 -> Result I64 [DivByZero]
divide = \x, y ->
    if y == 0 then
        Err(DivByZero)
    else
        Ok(x // y)

when divide(10, 0) is
    Ok(result) -> Stdout.line!("Result: \(Num.toStr(result))")
    Err(DivByZero) -> Stdout.line!("Cannot divide by zero")

F# (Result style - preferred):

type DivisionError = DivByZero

let divide x y =
    if y = 0 then
        Error DivByZero
    else
        Ok (x / y)

match divide 10 0 with
| Ok result -> printfn $"Result: {result}"
| Error DivByZero -> printfn "Cannot divide by zero"

F# (Exception style - for interop):

let divide x y =
    if y = 0 then
        raise (System.DivideByZeroException())
    else
        x / y

try
    let result = divide 10 0
    printfn $"Result: {result}"
with
| :? System.DivideByZeroException -> printfn "Cannot divide by zero"

Migration strategy:

1. Prefer F# Result type for functional code (matches Roc semantics)
2. Use exceptions when integrating with .NET libraries
3. Convert Roc when ... is to F# match ... with
4. Tag unions become discriminated unions

Multiple Error Types

Roc:

ValidationError : [EmptyName, InvalidAge, InvalidEmail]

validatePerson : Str, I64, Str -> Result Person ValidationError
validatePerson = \name, age, email ->
    if Str.isEmpty(name) then
        Err(EmptyName)
    else if age < 0 || age > 120 then
        Err(InvalidAge)
    else if !(Str.contains(email, "@")) then
        Err(InvalidEmail)
    else
        Ok({ name, age, email })

F#:

type ValidationError =
    | EmptyName
    | InvalidAge
    | InvalidEmail

type Person = {
    Name: string
    Age: int
    Email: string
}

let validatePerson name age email =
    if System.String.IsNullOrWhiteSpace(name) then
        Error EmptyName
    elif age < 0 || age > 120 then
        Error InvalidAge
    elif not (email.Contains("@")) then
        Error InvalidEmail
    else
        Ok { Name = name; Age = age; Email = email }

Why this translation:

• Roc tag unions → F# discriminated unions (direct mapping)
• Roc if/else if → F# if/elif (same control flow)
• Both use Result with typed errors
• Both have exhaustive pattern matching

Async and Effects

Roc Task → F# Async

This is a significant paradigm shift. Roc Task is platform-provided; F# Async is built into the language.

Roc:

import pf.Http
import pf.Task exposing [Task]

fetchData : Str -> Task Str [HttpErr]
fetchData = \url ->
    Http.get!(url)

processMultiple : List Str -> Task (List Str) [HttpErr]
processMultiple = \urls ->
    urls
    |> List.map(fetchData)
    |> Task.sequence

main : Task {} []
main =
    results = processMultiple!(urls)
    Stdout.line!("Done")

F#:

open System.Net.Http

type HttpError = HttpErr of string

let httpClient = new HttpClient()

let fetchData url = async {
    try
        let! response = httpClient.GetStringAsync(url) |> Async.AwaitTask
        return Ok response
    with
    | ex -> return Error (HttpErr ex.Message)
}

let processMultiple urls = async {
    let! results =
        urls
        |> List.map fetchData
        |> Async.Parallel
    return Array.toList results |> List.choose id  // Extract Ok values
}

[<EntryPoint>]
let main argv =
    let urls = ["url1"; "url2"; "url3"]
    processMultiple urls
    |> Async.RunSynchronously
    |> ignore
    printfn "Done"
    0

Why this translation:

• Roc Task a err → F# Async<Result<'a, 'err>> (effects + errors)
• Roc ! operator → F# let! in async { } block
• Roc platform handles execution → F# needs Async.RunSynchronously
• Roc main is a Task → F# main returns int (exit code)

Pure vs Effectful Code

Roc:

# Pure computation
add : I64, I64 -> I64
add = \x, y -> x + y

# Effectful computation (must return Task)
greet : Str -> Task Str []
greet = \name ->
    Stdout.line!("Hello, \(name)!")
    Task.ok(name)

F#:

// Pure computation
let add x y = x + y

// Effectful computation (no special type required)
let greet name =
    printfn $"Hello, {name}!"
    name  // Can return pure value directly

// Or as Async if needed
let greetAsync name = async {
    printfn $"Hello, {name}!"
    return name
}

Migration strategy:

1. Roc Task functions → F# Async or direct I/O (depends on context)
2. Roc pure functions → F# pure functions (direct mapping)
3. Roc ! try operator → F# let! or do!
4. Separate pure logic from effects for clarity

Platform Architecture

Roc Application + Platform → .NET Application

Roc (Platform-based):

app [main] {
    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.10.0/vNe6s9hWzoTZtFmNkvEICPErI9ptji_ySjicO6CkucY.tar.br"
}

import pf.Stdin
import pf.Stdout
import pf.Task exposing [Task]

main : Task {} []
main =
    input = Stdin.line!
    processed = processInput(input)
    Stdout.line!(processed)

processInput : Str -> Str
processInput = \input ->
    Str.toUpper(input)

F# (.NET Console App):

[<EntryPoint>]
let main argv =
    let input = System.Console.ReadLine()
    let processed = processInput input
    System.Console.WriteLine(processed)
    0  // Return exit code

let processInput input =
    input.ToUpper()

Key differences:

• Roc entry point is a Task that platform executes
• F# entry point is a function that returns int (exit code)
• Roc separates pure from effectful code; F# can mix them
• Roc uses platform imports; F# uses .NET BCL directly

Common Pitfalls

1. Forgetting F# allows mutability

   • Roc has no mutable variables
   • F# allows mutable keyword and ref cells
   • Benefit: Can use mutable state when performance-critical

2. Not leveraging F# exceptions

   • Roc only has Result type
   • F# has both Result and exceptions
   • Strategy: Use Result for domain errors, exceptions for unexpected failures

3. Missing .NET BCL libraries

   • Roc only has what the platform provides
   • F# has access to entire .NET ecosystem
   • Benefit: Rich library support (LINQ, JSON.NET, Entity Framework, etc.)

4. Not using F# computation expressions

   • Roc uses pattern matching and ! operator
   • F# has async { }, result { }, seq { }, etc.
   • Strategy: Use computation expressions for cleaner code

5. Ignoring F# type providers

   • Roc has no metaprogramming
   • F# has type providers for compile-time code generation
   • Benefit: Can generate types from SQL, JSON, CSV at compile time

6. Assuming strict platform/application split

   • Roc strictly separates pure (app) from effects (platform)
   • F# mixes pure and impure code freely
   • Strategy: Maintain separation for clarity, but leverage flexibility

7. Not using F# units of measure

   • Roc has no built-in units
   • F# has [<Measure>] for type-safe calculations
   • Benefit: Compile-time dimension checking

8. Missing F# Interactive (REPL)

   • Roc has limited REPL support
   • F# has FSI for interactive development
   • Benefit: Rapid prototyping and exploration

Module System

Roc Interfaces → F# Modules/Namespaces

Roc:

# User.roc
interface User
    exposes [User, create, getName]
    imports []

User : {
    id : I64,
    name : Str,
    email : Str,
}

create : Str, Str -> User
create = \name, email -> {
    id: generateId(),
    name,
    email,
}

getName : User -> Str
getName = \user -> user.name

F#:

// User.fs
namespace MyApp

module User =
    type User = {
        Id: int64
        Name: string
        Email: string
    }

    let create name email = {
        Id = generateId()
        Name = name
        Email = email
    }

    let getName user = user.Name

Migration notes:

• Roc interfaces → F# modules or namespaces
• Roc exposes is explicit → F# exports everything by default
• Roc file-based modules → F# file order matters in .fsproj

Build System

Roc Application → .NET Project

Roc:

# main.roc - single file or multiple interfaces
app [main] {
    pf: platform "https://..."
}

import Types
import Logic

main : Task {} []
main =
    Logic.run

Build commands:

roc build main.roc
roc run main.roc

F# (.fsproj):

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="Types.fs" />
    <Compile Include="Logic.fs" />
    <Compile Include="Program.fs" />
  </ItemGroup>

  <ItemGroup>
    <PackageReference Include="FSharp.Data" Version="6.3.0" />
  </ItemGroup>
</Project>

Build commands:

dotnet build
dotnet run

Key differences:

• Roc infers dependencies from imports
• F# needs .fsproj and explicit file ordering
• Roc uses platform URLs; F# uses NuGet packages
• F# has richer build tooling (watch mode, publish, etc.)

Testing

Roc Expect → F# Testing Frameworks

Roc:

add : I64, I64 -> I64
add = \x, y -> x + y

expect add(2, 2) == 4

divide : I64, I64 -> Result I64 [DivByZero]
divide = \x, y ->
    if y == 0 then
        Err(DivByZero)
    else
        Ok(x // y)

expect divide(10, 0) == Err(DivByZero)
expect divide(10, 2) == Ok(5)

Run tests:

roc test main.roc

F# (Expecto):

module Tests

open Expecto

let add x y = x + y

[<Tests>]
let tests =
    testList "Math tests" [
        testCase "addition" <| fun () ->
            Expect.equal (add 2 2) 4 "2 + 2 = 4"

        testCase "division by zero" <| fun () ->
            let result = divide 10L 0L
            Expect.equal result (Error DivByZero) "should error"

        testCase "division success" <| fun () ->
            let result = divide 10L 2L
            Expect.equal result (Ok 5L) "10 / 2 = 5"
    ]

[<EntryPoint>]
let main args =
    runTestsWithCLIArgs [] args tests

Run tests:

dotnet test

Migration strategy:

• Convert Roc expect statements to test framework assertions
• Group related expects into test lists
• F# has richer testing tools (Expecto, xUnit, FsUnit, FsCheck)

Examples

Example 1: Simple - Record and Pattern Matching

Before (Roc):

User : { id : I64, name : Str, email : Str }

users = [
    { id: 1, name: "Alice", email: "alice@example.com" },
    { id: 2, name: "Bob", email: "bob@example.com" },
]

findUserById : I64 -> [Some User, None]
findUserById = \id ->
    when List.findFirst(users, \u -> u.id == id) is
        Ok(user) -> Some(user)
        Err(_) -> None

getUserName : I64 -> Str
getUserName = \id ->
    when findUserById(id) is
        Some(u) -> u.name
        None -> "Unknown"

After (F#):

type User = {
    Id: int64
    Name: string
    Email: string
}

let users = [
    { Id = 1L; Name = "Alice"; Email = "alice@example.com" }
    { Id = 2L; Name = "Bob"; Email = "bob@example.com" }
]

let findUserById id =
    users |> List.tryFind (fun u -> u.Id = id)

let getUserName id =
    match findUserById id with
    | Some u -> u.Name
    | None -> "Unknown"

Example 2: Medium - Result with Multiple Errors

Before (Roc):

ValidationError : [InvalidName, InvalidAge]

Person : { name : Str, age : I64 }

validateName : Str -> Result Str [InvalidName]
validateName = \name ->
    if Str.isEmpty(name) then
        Err(InvalidName)
    else
        Ok(name)

validateAge : I64 -> Result I64 [InvalidAge]
validateAge = \age ->
    if age < 0 || age > 120 then
        Err(InvalidAge)
    else
        Ok(age)

createPerson : Str, I64 -> Result Person [InvalidName, InvalidAge]
createPerson = \name, age ->
    validName = validateName!(name)
    validAge = validateAge!(age)
    Ok({ name: validName, age: validAge })

After (F#):

type ValidationError =
    | InvalidName
    | InvalidAge

type Person = {
    Name: string
    Age: int
}

let validateName name =
    if System.String.IsNullOrWhiteSpace(name) then
        Error InvalidName
    else
        Ok name

let validateAge age =
    if age < 0 || age > 120 then
        Error InvalidAge
    else
        Ok age

let createPerson name age =
    result {
        let! validName = validateName name
        let! validAge = validateAge age
        return { Name = validName; Age = validAge }
    }

Example 3: Complex - Task-based File Processing

Before (Roc):

app [main] {
    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.10.0/vNe6s9hWzoTZtFmNkvEICPErI9ptji_ySjicO6CkucY.tar.br"
}

import pf.File
import pf.Path
import pf.Task exposing [Task]
import pf.Stdout

ProcessingError : [FileNotFound Str, InvalidFormat Str]

readFile : Str -> Task Str [FileReadErr Path.ReadErr]*
readFile = \path ->
    File.readUtf8(Path.fromStr(path))

processContent : Str -> Result Str [InvalidFormat Str]
processContent = \content ->
    if Str.contains(content, "error") then
        Err(InvalidFormat("Content contains error"))
    else
        Ok(Str.toUpper(content))

writeFile : Str, Str -> Task {} [FileWriteErr Path.WriteErr]*
writeFile = \path, content ->
    File.writeUtf8(Path.fromStr(path), content)

processFile : Str, Str -> Task {} [FileReadErr Path.ReadErr, InvalidFormat Str, FileWriteErr Path.WriteErr]*
processFile = \inputPath, outputPath ->
    content = readFile!(inputPath)
    processed = processContent!(content)
    writeFile!(outputPath, processed)

main : Task {} []
main =
    when processFile("input.txt", "output.txt") is
        Ok({}) -> Stdout.line!("File processed successfully")
        Err(FileReadErr(_)) -> Stdout.line!("Error reading file")
        Err(InvalidFormat(msg)) -> Stdout.line!("Invalid format: \(msg)")
        Err(FileWriteErr(_)) -> Stdout.line!("Error writing file")

After (F#):

open System.IO

type ProcessingError =
    | FileNotFound of string
    | InvalidFormat of string

let readFile path = async {
    try
        let! content = File.ReadAllTextAsync(path) |> Async.AwaitTask
        return Ok content
    with
    | :? FileNotFoundException ->
        return Error (FileNotFound path)
    | ex ->
        return Error (FileNotFound $"Error: {ex.Message}")
}

let processContent content =
    if content.Contains("error") then
        Error (InvalidFormat "Content contains error")
    else
        Ok (content.ToUpper())

let writeFile path content = async {
    try
        do! File.WriteAllTextAsync(path, content) |> Async.AwaitTask
        return Ok ()
    with
    | ex ->
        return Error (FileNotFound $"Write error: {ex.Message}")
}

let processFile inputPath outputPath = async {
    let! contentResult = readFile inputPath
    match contentResult with
    | Error e -> return Error e
    | Ok content ->
        match processContent content with
        | Error e -> return Error e
        | Ok processed ->
            return! writeFile outputPath processed
}

[<EntryPoint>]
let main argv =
    let result =
        processFile "input.txt" "output.txt"
        |> Async.RunSynchronously

    match result with
    | Ok () -> printfn "File processed successfully"
    | Error (FileNotFound msg) -> printfn $"File error: {msg}"
    | Error (InvalidFormat msg) -> printfn $"Invalid format: {msg}"

    0

Key conversions:

• Roc Task a err → F# Async<Result<'a, 'err>>
• Roc platform I/O → F# direct file I/O with .NET APIs
• Roc ! operator → F# let! in async blocks
• Roc main returns Task → F# main returns int

See Also

For more examples and patterns, see:

• meta-convert-dev - Foundational patterns with cross-language examples
• convert-fsharp-roc - Reverse conversion (F# → Roc)
• lang-roc-dev - Roc development patterns
• lang-fsharp-dev - F# development patterns

Cross-cutting pattern skills:

• patterns-concurrency-dev - Task model vs Async workflows
• patterns-serialization-dev - JSON, validation across languages
• patterns-metaprogramming-dev - No metaprogramming vs type providers