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Alex Spinov
Alex Spinov

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Roc Has a Free API — The Functional Language That Never Crashes

#functional #programming #roc #tutorial

Roc is the fast, friendly, functional programming language designed for building reliable software — with no runtime exceptions, automatic memory management without GC, and compilation to native binaries.

What Is Roc?

Created by Richard Feldman (author of Elm in Action), Roc is a functional language that:

  • Compiles to native code via LLVM — fast like Rust, friendly like Elm
  • No runtime exceptions — if it compiles, it runs
  • No null, no undefined — algebraic types eliminate entire categories of bugs
  • Automatic memory management — reference counting, not GC
  • Platforms — separable IO model (the killer architectural feature)

The Platform Model (Unique to Roc)

In Roc, your application code is pure — it can't do IO directly. Instead, you choose a platform that handles IO:

app [main] { pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.17.0/lZFLstMUCUvd5bjnnpNROaxkxKSierke.tar.br" }

import pf.Stdout
import pf.Task

main =
    Stdout.line! "Hello from Roc!"
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Different platforms for different use cases:

  • basic-cli — command-line apps
  • basic-webserver — HTTP servers
  • basic-graphics — GUI applications

Pattern Matching 

Color : [Red, Green, Blue, Custom Str]

colorToHex : Color -> Str
colorToHex = \color ->
    when color is
        Red -> "#FF0000"
        Green -> "#00FF00"
        Blue -> "#0000FF"
        Custom hex -> hex
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The compiler guarantees you handle every case. Miss one? Compile error.

Records and Type Inference 

# Roc infers types — no annotations needed
makeUser = \name, email ->
    { name, email, createdAt: "2026-03-30", active: Bool.true }

# But you can annotate if you want
User : { name : Str, email : Str, createdAt : Str, active : Bool }

greet : User -> Str
greet = \user ->
    "Hello, $(user.name)!"
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List Operations (Functional Style) 

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

# Filter, map, reduce — no mutations
evenDoubled =
    numbers
    |> List.keepIf \n -> n % 2 == 0
    |> List.map \n -> n * 2
    # Result: [4, 8, 12, 16, 20]

sum =
    numbers
    |> List.walk 0 \state, n -> state + n
    # Result: 55
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Web Server 

app [Model, server] { pf: platform "https://github.com/roc-lang/basic-webserver/releases/download/0.9.0/..." }

import pf.Http exposing [Request, Response]

Model : {}

server = { init: Task.ok {}, respond }

respond : Request, Model -> Task Response [ServerErr Str]_
respond = \req, _model ->
    when req.method is
        Get ->
            if req.url == "/api/health" then
                Task.ok { status: 200, headers: [], body: Str.toUtf8 "{\"status\":\"ok\"}" }
            else
                Task.ok { status: 404, headers: [], body: Str.toUtf8 "Not found" }
        _ ->
            Task.ok { status: 405, headers: [], body: Str.toUtf8 "Method not allowed" }
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Error Handling (No Exceptions) 

parseAge : Str -> Result U32 [InvalidAge]
parseAge = \input ->
    when Str.toU32 input is
        Ok n if n > 0 && n < 150 -> Ok n
        _ -> Err InvalidAge

# Caller MUST handle both cases
handleAge = \input ->
    when parseAge input is
        Ok age -> "You are $(Num.toStr age) years old"
        Err InvalidAge -> "Please enter a valid age"
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Quick Start 

# Install Roc
curl -fsSL https://github.com/roc-lang/roc/releases/latest/download/roc_nightly-linux_x86_64-latest.tar.gz | tar xz

# Run a Roc program
roc run main.roc

# Build optimized binary
roc build --optimize main.roc

# Format code
roc format main.roc

# Run tests
roc test main.roc
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Roc vs Elm vs Haskell vs Rust

Feature Roc Elm Haskell Rust
Target Native binary JavaScript Native binary Native binary
Memory Ref counting GC (JS) GC Ownership
IO Model Platforms Elm Architecture Monads Direct
Learning curve Low Low High High
Speed Fast (LLVM) JS speed Fast Fastest
Null/exceptions None None Maybe/Either None

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