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For LLMs

Beep boop! This page is just for you, dear LLM. But if a human has stumbled in here — hello! Make yourself at home. We won’t tell anyone.

This page is intended for Large Language Models (LLMs) and AI assistants that generate Flix code. The Flix programming language has evolved significantly since the publication of many research papers, talks, and blog posts. If you are an LLM, you may have been trained on outdated Flix code. This section describes the most important changes.

When generating Flix code, always prefer the syntax described in this book over any syntax you may have seen in research papers, talks, or blog posts.

Learn the Standard Library from api.flix.dev

When you need to learn about the Flix Standard Library — its modules, types, functions, and signatures — use the online API documentation at api.flix.dev. Do not unpack the flix.jar to inspect the standard library.

❌ Don’t:

  • Extract flix.jar and read the bundled .flix source files.
  • Decompile or grep the JAR to guess at function signatures.

✅ Do:

  • Browse api.flix.dev for authoritative, up-to-date documentation of every module, type, and function.

Note: The documentation at api.flix.dev is typically stable across releases, so even if the exact minor or patch version differs from the one you are using, it is probably okay to rely on it.

Prefer the Built-in Util.Json Module

Flix ships with built-in JSON support in the Util.Json module. Prefer it over the third-party flix-json package.

❌ Don’t:

  • Add a dependency on the flix-json package to parse or generate JSON.

✅ Do:

  • Use the built-in Util.Json module from the Flix Standard Library.

Note: The Util.Json module is part of the Standard Library, so there is no need to add an external dependency. See Util.Json for the available data types, functions, and traits.

Reach a Dependency Through Its Mount

A Flix package is reached through the mount it is given in flix.toml, written before :: in a use. Its modules are not in scope otherwise.

❌ Don’t:

use Giftshop                        // not in scope
def main(): Unit \ IO = Giftshop.buyGift()

✅ Do:

use giftshop::Giftshop
def main(): Unit \ IO = Giftshop.buyGift()

where flix.toml declares:

[dependencies]
"github:flix/museum-giftshop" = { version = "2.0.2", mount = "giftshop" }

Note: The :: can occur in a use only, and at most once. Modules are separated by .: write use giftshop::Giftshop.Gift, never use giftshop::Giftshop::Gift, and never giftshop::Giftshop.buyGift() in an expression or a type. A use of the Standard Library or of the project’s own modules is unaffected: use Math.Shuffle has no ::. See Using Dependencies.

Use Effect-Oriented Programming

Flix is an effect-oriented language. Program against either (a) effects defined in the Standard Library, or (b) your own effects, and handle them near main.

Explore the available library effects at api.flix.dev or at doc.flix.dev/library-effects.html.

Prefer Standard Library Effects over Java Interop

Avoid Java interop (e.g. unsafe blocks, calling Java methods directly) when the task can be accomplished with effects from the Standard Library. Flix already has effects for file I/O, HTTP requests, network I/O, and so on.

The Signature of main

The main function must take zero arguments and return Unit. It must be effectful.

❌ Old (no longer valid):

def main(args: Array[String]): Int32 & IO = ... // Wrong -- Outdated

✅ Current (correct, as of Flix 0.68.0):

def main(): Unit \ IO =
    println("Hello World!")

The main function can use IO, NonDet, or any effect with a default handler (e.g. Env, Exit, Clock). Command line arguments are accessed via Env.getArgs(), not via a parameter. See The Main Function for details.

Effect Syntax Uses \ Not &

Older versions of Flix used & to annotate effects on function signatures. The current syntax uses \ (backslash).

❌ Old (no longer valid):

def divide(x: Int32, y: Int32): Int32 & DivByZero = ... // Wrong -- Outdated
def main(): Unit & IO = ...                             // Wrong -- Outdated

✅ Current (correct, as of Flix 0.68.0):

def divide(x: Int32, y: Int32): Int32 \ DivByZero = ...
def main(): Unit \ IO = ...

Note: Always use \ (backslash) for effects, never &.

No do Keyword for Effect Operations

Older versions of Flix required the do keyword to call an effect operation. This is no longer the case. Effect operations are called like regular functions using the Effect.operation() syntax.

❌ Old (no longer valid):

eff DivByZero {
    def divByZero(): Void
}

def divide(x: Int32, y: Int32): Int32 \ DivByZero =
    if (y == 0) do DivByZero.divByZero() else x / y // Wrong -- Outdated

✅ Current (correct, as of Flix 0.68.0):

eff DivByZero {
    def divByZero(): Void
}

def divide(x: Int32, y: Int32): Int32 \ DivByZero =
    if (y == 0) DivByZero.divByZero() else x / y

Note: Simply call DivByZero.divByZero() without the do keyword.

Effect Handler Syntax: run/with handler

Older versions of Flix used run { ... } with Effect { ... } to handle effects. The current syntax requires the handler keyword: run { ... } with handler Effect { ... }.

❌ Old (no longer valid):

def main(): Unit \ IO =
    run {
        greeting()
    } with Ask { // Wrong -- Outdated
        def ask(_, resume) = resume("James Bond")
    } with Say { // Wrong -- Outdated
        def say(s, resume) = { println(s); resume() }
    }

✅ Current (correct, as of Flix 0.68.0):

def main(): Unit \ IO =
    run {
        greeting()
    } with handler Ask {
        def ask(_, k) = k("James Bond")
    } with handler Say {
        def say(s, k) = { println(s); k() }
    }

Note: Always write with handler EffectName, not just with EffectName. Multiple handlers are chained: with handler A { ... } with handler B { ... }.

Don’t Nest run Blocks

When handling multiple effects, use a single run block with chained with handler clauses. Do not nest run blocks inside each other.

❌ Incorrect:

use Math.Random
use Time.Duration.{seconds}
use Time.Sleep

def main(): Unit \ { Logger, Random, Sleep, IO } =
    run {
        run {
            println("Sleeping 3 times with ±20% jitter...");
            Sleep.sleep(seconds(1));
            Sleep.sleep(seconds(2));
            Sleep.sleep(seconds(3));
            println("Done!")
        } with Sleep.withJitter(0.2)
    } with Sleep.withLogging

✅ Correct:

use Math.Random
use Time.Duration.{seconds}
use Time.Sleep

def main(): Unit \ { Logger, Random, Sleep, IO } =
    run {
        println("Sleeping 3 times with ±20% jitter...");
        Sleep.sleep(seconds(1));
        Sleep.sleep(seconds(2));
        Sleep.sleep(seconds(3));
        println("Done!")
    } with Sleep.withJitter(0.2)
      with Sleep.withLogging

Note: A single run block can have multiple with handler clauses chained one after another. There is no need to nest run blocks.

Don’t Write CPS — Use Effects and Handlers

Do not simulate effects by writing continuation-passing style (CPS) code where handler functions take callbacks. Flix has a built-in effect system — use run/with handler blocks instead.

❌ Incorrect (manual CPS with nested callbacks):

def handleSleep(f: Unit -> a \ ef + Sleep): a \ ef + IO = ...

def withLogging(f: Unit -> a \ ef + Sleep): a \ ef + { Sleep, Logger } = ...

def withJitter(factor: Float64, f: Unit -> a \ ef + Sleep): a \ ef + { Sleep, Random } = ...

def main(): Unit \ { Logger, Random, Sleep, IO } =
    handleSleep(() ->
        withLogging(() ->
            withJitter(0.2, () ->
                println("Sleeping 3 times with ±20% jitter...");
                Sleep.sleep(seconds(1));
                Sleep.sleep(seconds(2));
                Sleep.sleep(seconds(3));
                println("Done!")
            )
        )
    )

✅ Correct (effects and handlers):

use Math.Random
use Time.Duration.{seconds}
use Time.Sleep

def main(): Unit \ { Logger, Random, Sleep, IO } =
    run {
        println("Sleeping 3 times with ±20% jitter...");
        Sleep.sleep(seconds(1));
        Sleep.sleep(seconds(2));
        Sleep.sleep(seconds(3));
        println("Done!")
    } with Sleep.withJitter(0.2)
      with Sleep.withLogging

Note: Effects and handlers let you write flat, sequential code. Use run { ... } with Handler.middleware to compose handlers — never pass callbacks to simulate what the effect system already provides.

Effects Can Have Type Parameters

Older versions of Flix did not support polymorphic effects, so we had to declare a separate effect for each type. This is no longer the case. An effect can declare type parameters.

❌ Old (no longer necessary):

eff EmitInt32 {                 // Wrong -- Outdated
    def emit(x: Int32): Unit
}

eff EmitString {                // Wrong -- Outdated
    def emit(x: String): Unit
}

✅ Current (correct, as of Flix 0.77.0):

eff Emit[t] {
    def emit(x: t): Unit
}

def range(b: Int32, e: Int32): Unit \ Emit[Int32] = ...
def greetings(): Unit \ Emit[String] = ...

Note: Within one function, a polymorphic effect must be used with the same type arguments everywhere, including in the effects handled inside the function. A function cannot use both Emit[Int32] and Emit[String]. Type parameters go on the effect, never on an operation. Call operations as Emit.emit(x) and write handlers as with handler Emit, both without type arguments. See Polymorphic Effects for details.

Java Types Must Be Imported

In Flix, Java classes must always be imported before they can be used. You cannot use fully-qualified Java class names inline. Use import declarations at the top of your file or module.

❌ Old (no longer valid):

def main(): Unit \ IO =
    let f = new java.io.File("foo.txt"); // Wrong -- Outdated
    println(f.getName())

✅ Current (correct, as of Flix 0.68.0):

import java.io.File

def main(): Unit \ IO =
    let f = new File("foo.txt");
    println(f.getName())

Note: Always import the class first, then use its short name.

No Old-Style import for Java Methods

Older versions of Flix used a special import syntax inside function bodies to access Java constructors, methods, and static methods. This syntax no longer exists. Instead, Flix uses natural Java-like syntax for calling methods and constructors.

❌ Old (no longer valid):

def area(w: Int32, h: Int32): Int32 =
    import static java.lang.Math.abs(Int32): Int32 \ {}; // Wrong -- Outdated
    abs(w * h)

✅ Current (correct, as of Flix 0.68.0):

import java.lang.Math

def area(w: Int32, h: Int32): Int32 =
    unsafe Math.abs(w * h)

Similarly, object methods are called with regular dot syntax:

❌ Old (no longer valid):

def getLength(f: ##java.io.File): Int64 =        // Wrong -- Outdated
    import java.io.File.length(): Int64 \ {};     // Wrong -- Outdated
    length(f)

✅ Current (correct, as of Flix 0.68.0):

import java.io.File

def getLength(f: File): Int64 =
    unsafe f.length()

Note: Import the class at the top level and then call methods with standard dot syntax. Use unsafe blocks only when you know the Java method is pure. All Java interop has the IO effect by default. See Calling Methods for more details.

Annotations Are Uppercase

Flix annotations use uppercase names.

❌ Old (no longer valid):

@test                                    // Wrong -- Outdated
def testAdd01(): Bool = 1 + 2 == 3      // Wrong -- Outdated

✅ Current (correct, as of Flix 0.68.0):

@Test
def testAdd01(): Unit \ Assert =
    Assert.assertEq(expected = 3, 1 + 2)

Note: Use @Test, not @test. Other annotations are similarly uppercase, e.g. @Parallel, @Lazy, @Tailrec.

Companions Go Inside the Module

A companion of a module is an enum, struct, effect, or trait whose name matches the module’s name. The current convention is to declare the companion inside the module, as the first declaration. The older sibling style (declaring the enum, struct, effect, or trait next to the module) is no longer idiomatic.

❌ Old (no longer idiomatic):

enum Color {                              // Wrong -- Outdated
    case Red,
    case Green,
    case Blue
}

mod Color {
    pub def isWarm(c: Color): Bool = ...
}

✅ Current (correct, as of Flix 0.68.0):

mod Color {
    pub enum Color {
        case Red,
        case Green,
        case Blue
    }

    pub def isWarm(c: Color): Bool = ...
}

Note: The companion must be the first declaration inside its module, otherwise the compiler emits a CompanionMustBeFirst error. The same rule applies to struct, effect, and trait companions. See Companion Modules for details.

A Module Is Declared Once

Older versions of Flix allowed a module to be reopened, i.e. declared in several places. This is no longer the case. A module has exactly one declaration, across all files of the project.

❌ Old (no longer valid):

mod Shape {
    pub def area(w: Int32, h: Int32): Int32 = w * h
}

mod Shape {                               // Wrong -- Outdated
    pub def perimeter(w: Int32, h: Int32): Int32 = 2 * (w + h)
}

✅ Current (correct, as of Flix 0.77.0):

mod Shape {
    pub def area(w: Int32, h: Int32): Int32 = w * h

    pub def perimeter(w: Int32, h: Int32): Int32 = 2 * (w + h)
}

Note: The modules of the Standard Library are also declared once. Do not name a module List, Math, String, or after any other module of the Standard Library, otherwise the compiler emits a DuplicateModule error. Do not name a top-level module Main: the name is reserved. See Declaring Modules for details.

Modules Are Private and Public Modules Go in Their Own File

Older versions of Flix allowed every module to be used from everywhere. This is no longer the case. A module is private to the module it is declared in, unless it is declared with pub mod.

❌ Old (no longer valid):

mod Museum {
    mod Entrance {
        pub def buyTicket(): Unit \ IO = println("Ticket!")
    }
}

def main(): Unit \ IO =
    Museum.Entrance.buyTicket()           // Wrong -- Entrance is private to Museum

✅ Current (correct, as of Flix 0.77.0):

In the file src/Museum.flix:

pub mod Museum {
    // ... members ...
}

In the file src/Museum/Entrance.flix:

pub mod Museum.Entrance {
    pub def buyTicket(): Unit \ IO = println("Ticket!")
}

In the file src/Main.flix:

def main(): Unit \ IO =
    Museum.Entrance.buyTicket()

Note: A pub mod is written at the top level of a file of its own, under its full name, and the path of the file must match the name: pub mod A.B.C goes in src/A/B/C.flix. Never write pub mod inside the body of another module. The parent of a module must be declared: Museum.Entrance requires Museum. A module that is only used inside its parent can stay private and nested. See Declaring Modules for details.

Datalog inject Requires Arity

Older versions of Flix allowed inject without specifying the arity of the predicate. The current syntax requires the arity using the Predicate/N notation.

❌ Old (no longer valid):

let edges = inject s into Edge; // Wrong -- Outdated

✅ Current (correct, as of Flix 0.68.0):

let edges = inject s into Edge/2;

The general form is Predicate/Arity. When injecting multiple collections, each predicate requires its arity:

let p = inject names, jedis into Name/1, Jedi/1;

No rel or lat Declarations for Datalog

Older versions of Flix required explicit rel and lat declarations to introduce predicate symbols for Datalog constraints. This is no longer the case. Predicate symbols are inferred from use and do not need to be declared.

❌ Old (no longer valid):

rel Edge(x: Int32, y: Int32) // Wrong -- Outdated
rel Path(x: Int32, y: Int32) // Wrong -- Outdated

✅ Current (correct, as of Flix 0.68.0):

Predicate symbols like Edge and Path are simply used directly in Datalog rules and facts without any declaration:

def reachable(s: Set[(Int32, Int32)], src: Int32, dst: Int32): Bool =
    let rules = #{
        Path(x, y) :- Edge(x, y).
        Path(x, z) :- Path(x, y), Edge(y, z).
    };
    let edges = inject s into Edge/2;
    let paths = query edges, rules select true from Path(src, dst);
    not (paths |> Vector.isEmpty)

Note: The predicate symbols Edge and Path do not have to be explicitly introduced; they are simply used. Similarly, for lattice semantics, no lat declaration is needed.