Language Details

This doc describes the Dark language. Dark is really a system - a combination of the language, editor, framework and infrastructure. In this doc, we'll focus on discussing the language itself, ignoring where possible the editor and infrastructure.

This discusses both the language and the intended language. We have taken many shortcuts to be able to ship Dark, and so many parts of the language are not currently at their end state. Here we discuss both what the language is, and what we intend it to be.

Dark is a statically-typed functional/imperative hybrid, based loosely on ML. It is a high-level language, with immutable values, garbage collection, and support for generics/polymorphic types.

Dark is somewhat similar to OCaml or Elm. It has many similarities to Rust and Haskell, and is also influenced by Clojure, Ruby, Python, CoffeeScript, as well as our experience with (alphabetically) Bash, C, Clojure, CoffeeScript, C++, Elm, Javascript, Java, Haskell, OCaml, Perl, PHP, Python, ReasonML, Ruby/Rails, React, and Rust.

Type system

Dark’s type system is most similar to Elm, Haskell, ReasonML, OCaml or Rust: based on records, enums, list, and built-in generics/polymorphism. All values in Dark are immutable, except refs.

Dark has some standard basic types: int, string, boolean, float, list, and dicts.

We support typical functional types: Option and Result.

Currently, Dark has a null type to support JSON values directly. In the future, we intend to deprecate null and replace it with Options.

Built-in types


Integers are signed 63-bit integer.

In the future, Dark will use infinite-precision integers. We also intend to add unsigned 8-bit integers and bit-manipulation functions.


Floats are double-precision 64-bit floating-point values (IEEE 754).

We intend for numeric operations to return Results, to handle situations which are undefined on the datatype. For example, integer division would return Result Error Int. Similarly, floating point values would never be NaN, and instead would return Result Error Float. See Error Handling in Dark for more.


Booleans are true or false.


Strings are unicode, and character are unicode “characters” (if it appears as one character on the screen, that’s a “character” in Dark).

Specifically, string are immutable UTF-8 encoded sequences of Unicode code points. Chars are “Extended Grapheme Clusters”. (A codepoint is some bytes that implement unicode characters, a grapheme is some codepoints forming a unicode entity, such as an emoji; an EGC is some graphemes, used to handle things like emojis which combine to form a single emoji).

Dark doesn't currently support string interpolation, but we plan to in the future.


In the future, Dark will support individual characters.


Lists and Arrays use the same datatype, called Lists. The Dark compiler will in the future optimize their implementation to support good algorithmic complexity and performance for whatever you use them for.

Lists should be used for all “I want a sequence of things” situations, including iterating across them, random access, push/pop, etc.

let x = [8]
let y = List::append x [6]
[8, 6]

List Example


Non-unicode sequences of bytes are supported as the Binary type.


Instead of allowing all values to potentially be null, as in most imperative languages, Dark uses an Option type:

Option a = Just a | Nothing

Option Example

This is intended to convert effortlessly to null in JSON, but we don't quite have enough of the type system to remove null, so they both exist right now.

Functions which return Option trigger the Error Rail.


Any functions which can have an error should use Results. We use results for Int division, Float operations, HTTP operations, etc.

Result a = Ok a | Err Error

Dark has no exceptions - all errors go through Results. See error handling for more details.

Result Example


Dicts are maps from a certain key type to a certain value type. The key must currently be a string. The value can be any type but all elements of the Dict are the same type (not currently enforced).

Dicts are different than records: dicts can have arbitrary keys.


Dark supports UUIDs directly.


As a temporary hack, Dark also supports null. This allows us handle JSON while we build out enough type-system support to allow them to be replaced by Option.

Null is mostly useful for comparing against incoming JSON and results of HttpClient calls. When returning JSON or making HttpClient calls, you can use Options instead and they will be converted properly to null in the JSON output.

User defined types

Dark currently has limited support for user-defined types. Currently, we support inline definition of records, but do not support defining record types explicitly.

Record types are actually implemented under the hood, and we intend to use them to support typed Datastores, API contracts, and static types.

Dark does not currently support user-defined enums.

User-defined types will be either records or enums, or combinations of other type expressions.

Types in Dark are out-of-line, meaning that they are not defined “on the canvas”, in a similar way to functions.

All types in Dark will be versioned.

In the future, we intend to support typeclasses or traits to allow ad-hoc polymorphism.


Records are most akin to Classes in an imperative language.

Records are a set of keys and values. Each key name and type are defined, and the types do not have to be the same.

let x = { name: "Robin"
age: 32
gender: "Other"

Records can not be accessed dynamically; they are not Maps/Hashtables/Dicts.

Records are structurally typed, and are equivalent to records of the same shape but a different name.

Note that at the moment, Dicts and Records share the same implementation and can be accessed and modified in the same way. We intend to break these apart in the future.


Enums are a set of “constructors”, each of which has a set of typed arguments.

Currently, Dark only supports Option and Result built-in enums. In the future, we will support user-defined enums.

type Person = Human {age: Int, name: String, itin: String }
| Corporation {age: Int, name: String, ein: String }
| Puppers Int String

Enums can be made by building on existing types, especially records and other enums.

Enums are nominally typed. (Two enums with the same field names and types are not equivalent).

Types unique to Dark


Programs in Dark start as a single empty expression. As they get built up in our structured editor, they can never become syntactically invalid. However, a program may be incomplete if any its expressions are empty.

An empty expression is incomplete. A developer may write programs where some parts are incomplete as they build out the code. As such, incomplete values appear dynamically when the code is executed. Structures containing incompletes are themselves incomplete. For example functions with incomplete arguments do not execute, and return incomplete; and records with an incomplete field are also incomplete.

Incompletes are never returned to end users, and cannot be stored in a datastore. Returning an incomplete via a HTTP handler causes a 500 error.

Error rail

You might occasionally see a value marked <ErrorRail>, this is used to indicate that a value is on the Error Rail. See Railway Oriented Programming for more details.

Sensitive Types

Some values are sensitive, for example, passwords or credit card numbers.

Currently Dark supports the Password type, which is never saved directly or sent to the editor.

In the future, Dark will allow you to specify types of sensitive values, preventing them from being stored in logs, and allowing a team to limit who has access to these values in the Dark editor.


All Dark language constructs are expressions. That means that they evaluate to a value, rather than being used to set state.


Lets creates a name with an immutable value, and a scope in which that is defined.

let name = "Stella"

We often refer to these names as variables, for simplicity. However, they never vary: once they are defined, they never have any other value.

Variable Scope

if age > 18
let height = 170
height + 12
let weight = 105
weight / 2

In the example above, the scope of height and weight only extends to within the then block and else block respectively. You cannot use either variable below the if expression.


Dark supports if/else statement. The argument to an if is a boolean. We currently support truthy types but intend to remove that ability.

We support && and || - they do not currently short-circuit but we intend them to in the future.

An if is not currently allowed without a corresponding else - we will relax this after we introduce statements.

If Example


Dark supports pattern matching, in particular, matching on Enums.

let introduction =
match name with
| Nothing -> "Hi!"
| Just name -> "Dear " ++ name

We expect to allow guards in the future. We also hope to add an if-let construct to support if statements that destructure from Enums.


Functions must have type declarations for inputs. We intend to support types on return values soon.

fetch url name =
{ url: url
, name: name

Functions in Dark are simple, and do not currently support functional language concepts, such as partial application, functions as first-class values, and defining functions in using points-free style.

Functions do not live in the “Canvas”, but rather are a little bit ethereal.

Built-in functions are all versioned: we frequently deprecate old functions and add updates. When we deprecate old versions, your code does not change, and you keep using the old ones. We intend to support automated refactoring and updating in the future.

In the future, we intend to support partial application/currying, and default/optional parameters.


Lambdas are anonymous functions. They are used to pass to functions which take Blocks, typically used for iteration.

List::map [5, 10, 11] \var -> var + 2
[7, 12, 13]

In the future, we intend to support a syntax for shorthands for creating lambdas to access fields: .fieldname. This can be included in a pipe or used as a first class function.

We also intend to support passing functions where blocks are expected.

There is a syntax for shorthands for creating lambda’s to call constructors: ConstructorName. This can be included in a pipe or used as a first class function.


Dark programs are intended to be written, as much as possible, as pipelines of data:

|> getFriends
|> (\f -> (f, getFriends f))
|> List.filter (\f -> == "Kevin Bacon")
|> (=) []

Feature Flags

Feature flags are similar to ifs:

flag myCondition
then 5
else 6

However, flags differ slightly from ifs in that any condition that is not true will cause the then block to activate. This is especially important around incompletes, allowing you to take working code and edit the feature flag without disturbing existing users. In an if statement, neither branch would execute.

Planned language features


Dark intends to support tuples: lists of defined length supporting heterogeneous types.

x = (1, "string", { name: "Sam" })


We intend for Dark to support Sets: unordered collections of a single type.


We intend to support the unit type, which indicates something that have no type, such as an imperative function that doesn't return anything.

x = ()

Imperative programming (Statements and refs)

It is intended that you write the program as you think it, not to shoehorn your program into a functional style. As such, Dark is planning to add a number of imperative concepts to allow you to easily write imperative algorithms, including statements, refs, and mutable data structures.