Unison language reference


This document is an informal reference for the Unison language meant as an aid for Unison programmers as well as authors of implementations of the language. This isn't meant to be a tutorial or introductory guide to the language; it's more like a dry and unexciting tome you consult when you have questions about some aspect of the language. 🧐

This language reference, like the language it describes, is a work in progress and will be improved over time (GitHub link). Contributions and corrections are welcome!

A note on syntax

Unison is a language in which programs are not text. That is, the source of truth for a program is not its textual representation as source code, but its structured representation as an abstract syntax tree.

This document describes Unison in terms of its default (and currently, only) textual rendering into source code.

Top-level declarations

A top-level declaration can appear at the top level or outermost scope of a Unison file. It can be one of the following forms:

Term declarations

A Unison term declaration (or "term binding") consists of an optional type signature, and a term definition. For example:

timesTwo : Nat -> Nat
timesTwo x = x * 2

The first line in the above is a type signature. The type signature timesTwo : Nat -> Nat declares that the term named timesTwo is a function accepting an argument of type Nat and computes a value of type Nat. The type Nat is the type of 64-bit natural numbers starting from zero. See Unison types for details.

The second line is the term definition. The = sign splits the definition into a left-hand side, which is the term being defined, and the right-hand side, which is the definition of the term.

The general form of a term binding is:

name : Type
name p_1 p_2 … p_n = expression

Type signatures

name : Type is a type signature, where name is the name of the term being defined and Type is a type for that term. The name given in the type signature and the name given in the definition must be the same.

Type signatures are optional. In the absence of a type signature, Unison will automatically infer the type of a term declaration. If a type signature is present, Unison will verify that the term has the type given in the signature.

Term definition

A term definition has the form f p_1 p_2 … p_n = e where f is the name of the term being defined. The parameters p_1 through p_n are the names of parameters, if any (if the term is a function), separated by spaces. The right-hand side of the = sign is any Unison expression.

The names of the parameters as well as the name of the term are bound as local variables in the expression on the right-hand side (also known as the body of the function). When the function is called, the parameter names are bound to any arguments passed in the call. See function application for details on the call semantics of functions.

If the term takes no arguments, the term has the value of the fully evaluated expression on the right-hand side and is not a function.

The expression comprising the right-hand side can refer to the name given to the definition in the left-hand side. In that case, it’s a recursive definition. For example:

sumUpTo : Nat -> Nat
sumUpTo n =
  if n < 2 then n
  else n + sumUpto (drop n 1)

The above defines a function sumUpTo that recursively sums all the natural numbers less than some number n. As an example, sumUpTo 3 is 1 + 2 + 3, which is 6.

Note: The expression drop n 1 on line 4 above subtracts one from the natural number n. Since the natural numbers are not closed under subtraction (n - 1 is an Int), we use the operation drop which has the convention that drop 0 n = 0 for all natural numbers n. Unison's type system saves us from having to deal with negative numbers here.

Operator definitions

Operator identifiers are valid names for Unison definitions, but the syntax for defining them is slightly different. For example, we could define a binary operator |:

(|) x y = if x == 0 then y else x

Or we could define it using infix notation:

x | y = if x == 0 then y else x

If we want to give the operator a qualified name, we put the qualifier inside the parentheses:

(MyNamespace.|) x y = if x == 0 then y else x

Or if defining it infix:

x MyNamespace.| y = if x == 0 then y else x

The operator can be applied using either notation, no matter which way it's defined. See function application for details.

User-defined data types

A user-defined data type is introduced with the type keyword. (See Types for an informal description of Unison's type system.)

For example:

type Optional a = None | Some a

The = sign splits the definition into a left-hand side and a right-hand side, much like term definitions.

The left-hand side is the data type being defined. It gives a name for the data type and declares a new type constructor with that name (here it’s named Optional), followed by names for any type arguments (here there is one and it’s called a). These names are bound as type variables in the right-hand side. The right-hand side may also refer to the name given to the type in the left-hand side, in which case it is a recursive type declaration. Note that the fully saturated type construction Optional Nat is a type, whereas Optional by itself is a type constructor, not a type (it requires a type argument in order to construct a type).

The right-hand side consists of zero or more data constructors separated by |. These are data constructors for the type, or ways in which values of the type can be constructed. Each case declares a name for a data constructor (here the data constructors are None and Some), followed by the types of the arguments to the constructor.

When Unison compiles a type definition, it generates a term for each data constructor. Here they are the terms Optional.Some : a -> Optional a, and Optional.None : Optional a. It also generates patterns for matching on data (see Pattern Matching).

Note that these terms and patterns receive qualified names: if the type named x.y.Z has a data constructor C, the generated term and pattern for C will be named x.y.Z.C.

The general form of a type declaration is as follows:

<unique<[<regular-identifier>]?>?> type TypeConstructor p1 p2 … pn
  = DataConstructor_1
  | DataConstructor_2
  | DataConstructor_n

The optional unique keyword introduces a unique type, explained in the next section.

Unique types

A type declaration gives a name to a type, but Unison does not uniquely identify a type by its name. Rather, the hash of a type's definition identifies the type. The hash is based on the structure of the type definition, with all identifiers removed.

For example, Unison considers these type declarations to declare the exact same type, even though they give different names to both the type constructor and the data constructors:

type Optional a = Some a | None

type Maybe a = Just a | Nothing

So a value Some 10 and a value Just 10 are in fact the same value and these two expressions have the same type. Even though one nominally has the type Optional Nat and the other Maybe Nat, Unison understands that as the type #5isltsdct9fhcrvu ##Nat.

This is not always what you want. Sometimes you want to give meaning to a type that is more than just its structure. For example, it might be confusing that these two types are identical:

type Suit = Hearts | Spades | Diamonds | Clubs

type Direction = North | South | East | West

Unison will consider every unary type constructor with four nullary data constructors as identical to these declarations. So Unison will not stop us providing a Direction where a Suit is expected.

The unique keyword solves this problem:

unique type Suit = Hearts | Spades | Diamonds | Clubs

unique type Direction = North | South | East | West

When compiling these declarations, Unison will generate a universally unique identifier for the type and use that identifier when generating the hash for the type. As a result, the type gets a hash that is universally unique.

Record types

In the type declarations discussed above, the arguments to each data constructor are nameless. For example:

type Point = Point Nat Nat

Here, the data type Point has a constructor Point.Point, with two arguments, both of type Nat. The arguments have no name, so they are identified positionally, for example when creating a value of this type, like Point.Point 1 2.

Types with a single data constructor can also be defined in the following style, in which case they are called record types.

type Point = { x : Nat, y : Nat }

This assigns names to each argument of the constructor. The effect of this is to generate some accessor methods, to help get, set, and modify each field.

Point.x        : Point -> Nat
Point.x.modify : (Nat -> Nat) -> Point -> Point
Point.x.set    : Nat -> Point -> Point
Point.y        : Point -> Nat
Point.y.modify : (Nat -> Nat) -> Point -> Point
Point.y.set    : Nat -> Point -> Point

👉 Note that set and modify are returning new, modified copies of the input record - there's no mutation of values in Unison.

There's currently no special syntax for creating or pattern matching on records. That works the same as for regular data types:

p = Point.Point 1 2
px = case p of
       Point.Point x _ -> x

User-defined abilities

A user-defined ability declaration has the following general form:

ability A p_1 p_2 … p_n where
  Request_1 : Type_1
  Request_2 : Type_2
  Request_n : Type_n

This declares an ability type constructor A with type parameters p_1 through p_n, and request constructors Request_1 through Request_n.

See Abilities and Ability Handlers for more on user-defined abilities.


This section describes the syntax and informal semantics of Unison expressions.

Unison's evaluation strategy for expressions is Applicative Order Call-by-Value. See Function application for details.

Basic lexical forms

See the sections on:


Unison identifiers come in two flavors:

  1. Regular identifiers start with an alphabetic unicode character, emoji (which is any unicode character between 1F400 and 1FAFF inclusive), or underscore (_), followed by any number of alphanumeric characters, emoji, or the characters _, !, or '. For example, foo, _bar4, qux', and set! are valid regular identifiers.
  2. Operators consist entirely of the characters !$%^&*-=+<>.~\\/|:. For example, +, *, <>, and >>= are valid operators.

Namespace-qualified identifiers

The above describes unqualified identifiers. An identifier can also be qualified. A qualified identifier consists of a qualifier or namespace, followed by a ., followed by either a regular identifier or an operator. The qualifier is one or more regular identifiers separated by .. For example Foo.Bar.baz is a qualified identifier where Foo.Bar is the qualifier.

Absolutely qualified identifiers

Namespace-qualified identifiers described above are relative to a “current” namespace, which the programmer can set (and defaults to the root of the global namespace). To ignore the current namespace, an identifier can have an absolute qualifier. An absolutely qualified name begins with a .. For example, the name .base.List always refers to the name .base.List, regardless of the current namespace, whereas the name base.List will refer to foo.base.List if the current namespace is foo.

Note that operator identifiers may contain the character .. In order for this to not create ambiguity, the rule is as follows:

  1. . by itself is always an operator.
  2. Any other identifier beginning with . is an absolutely qualified identifier.
  3. A . immediately following a namespace is always a namespace separator.
  4. Otherwise a . is treated as part of an operator identifier.

if . is followed by whitespace or another operator character, the . is treated like an operator character. If it's followed by a regular identifier character, it's treated as a namespace separator.

Hash-qualified identifiers

Any identifier, including a namespace-qualified one, can appear hash-qualified. A hash-qualified identifier has the form x#h where x is an identifier and #h is a hash literal. The hash disambiguates names that may refer to more than one thing.

Reserved words

The following names are reserved by Unison and cannot be used as identifiers: =, :, ->, if, then, else, forall, handle, in, unique, where, use, and, or, true, false, type, ability, alias, let, namespace, case, of, with.

Name resolution and the environment

During typechecking, Unison substitutes free variables in an expression by looking them up in an environment populated from a codebase of available definitions. A Unison codebase is a database of term and type definitions, indexed by hashes and names.

A name in the environment can refer to either terms or types, or both (a type name can never be confused with a term name).

Suffix-based name resolution

If the list of segments of a name (base.List.map has the segments [base,List,map]) is a suffix of exactly one fully qualified name in the environment, Unison substitutes that name in the expression with a reference to the definition. For example, the fully qualified name .base.List.map could be referenced via base.List.map, List.map as long as no other definitions end in base.List.map or List.map. This reduces the number of imports needed and cuts down on needing to remember the fully qualified names for definitions. ("Was it base.List.map or util.List.map?")

Hash literals in the program are substituted with references to the definitions in the environment whose hashes they match.

If a free term variable in the program cannot be found in the environment and is not the name of another term in scope in the program itself, or if an free variable matches more than one name (it’s ambiguous), Unison tries type-directed name resolution.

Type-directed name resolution

During typechecking, if Unison encounters a free term variable that is not a term name in the environment, Unison attempts type-directed name resolution, which:

  1. Finds term definitions in the environment whose unqualified name is the same as the free variable.
  2. If exactly one of those terms has a type that conforms to the expected type of the variable (the type system has always inferred this type already at this point), perform that substitution and resume typechecking.

If name resolution is unable to find the definition of a name, or is unable to disambiguate an ambiguous name, Unison reports an error.

Blocks and statements

A block is an expression that has the general form:


A block can have zero or more statements, and the value of the whole block is the value of the final expression. A statement is either:

  1. A term definition which defines a term within the scope of the block. The definition is not visible outside this scope, and is bound to a local name. Unlike top-level definitions, a block-level definition does not result in a hash, and cannot be referenced with a hash literal.
  2. A Unison expression. In particular, blocks often contain action expressions, which are expressions evaluated solely for their effects. An action expression has type {A} T for some ability A (see Abilities and Ability Handlers) and some type T.
  3. A use clause.

An example of a block (this evaluates to 16):

x = 4
y = x + 2
f a = a + y
f 10

A number of language constructs introduce blocks. These are detailed in the relevant sections of this reference. Wherever Unison expects an expression, a block can be introduced with the let keyword:

let <block>

Where <block> denotes a block as described above.

The lexical syntax of blocks

The standard syntax expects statements to appear in a line-oriented layout, where whitespace is significant.

The opening keyword (let, if, then, or else, for example) introduces the block, and the position of the first character of the first statement in the block determines the top-left corner of the block. The beginning of each statement in the block must be lined up exactly with the left edge of the block. The first non-whitespace character that appears to the left of that edge (i.e. outdented) ends the block. Certain keywords also end blocks. For example, then ends the block introduced by if.

A statement or expression in a block can continue for more than one line as long as each line of the statement is indented further than the first character of the statement or expression.

For example, these are valid indentations for a block:

  x = 1
  y = 2
  x + y

let x = 1
    y = 2
    x + y

Whereas these are incorrect:

let x = 1
  y = 2
  x + y

let x = 1
     y = 2
       x + y
Syntactic precedence

Keywords that introduce blocks bind more tightly than function application. So f let x is the same as f (let x) and f if b then p else q is the same as f (if b then p else q).

Block keywords bind more tightly than delayed computations syntax. So 'let x is the same as _ -> let x and !if b then p else q is the same as (if b then p else q) ().

Blocks eagerly consume expressions, so if b then p else q + r is the same as if b then p else (q + r).


A literal expression is a basic form of Unison expression. Unison has the following types of literals:

  • A 64-bit unsigned integer of type .base.Nat (which stands for natural number) consists of digits from 0 to 9. The smallest Nat is 0 and the largest is 18446744073709551615.
  • A 64-bit signed integer of type .base.Int consists of a natural number immediately preceded by either + or -. For example, 4 is a Nat, whereas +4 is an Int. The smallest Int is -9223372036854775808 and the largest is +9223372036854775807.
  • A 64-bit floating point number of type .base.Float consists of an optional sign (+/-), followed by two natural numbers separated by .. Floating point literals in Unison are IEEE 754-1985 double-precision numbers. For example 1.6777216 is a valid floating point literal.
  • A text literal of type .base.Text is any sequence of Unicode characters between pairs of ". The escape character is \, so a " can be included in a text literal with the escape sequence \". The full list of escape sequences is given in the Escape Sequences section below. For example, "Hello, World!" is a text literal. A text literal can span multiple lines. Newlines do not terminate text literals, but become part of the literal text.
  • A character literal of type .base.Char consists of a ? character marker followed by a single Unicode character, or a single escape sequence. For example, ?a, ?🔥 or ?\t.
  • There are two Boolean literals: true and false, and they have type Boolean.
  • A hash literal begins with the character #. See the section Hashes for details on the lexical form of hash literals. A hash literal is a reference to a term or type. The type or term that it references must have a definition whose hash digest matches the hash in the literal. The type of a hash literal is the same as the type of its referent. #a0v829 is an example of a hash literal.
  • A literal list has the general form [v1, v2, ... vn] where v1 through vn are expressions. A literal list may be empty. For example, [], [x], and [1,2,3] are list literals. The expressions that form the elements of the list all must have the same type. If that type is T, then the type of the list literal is .base.List T or [T].
  • A function literal or lambda has the form p1 p2 ... pn -> e, where p1 through pn are regular identifiers and e is a Unison expression (the body of the lambda). The variables p1 through pn are local variables in e, and they are bound to any values passed as arguments to the function when it’s called (see the section Function Application for details on call semantics). For example x -> x + 2 is a function literal.
  • A tuple literal has the form (v1,v2, ..., vn) where v1 through vn are expressions. A value (a,b) has type (A,B) if a has type A and b has type B. The expression (a) is the same as the expression a. The nullary tuple () (pronounced “unit”) is of the trivial type (). See tuple types for details on these types and more ways of constructing tuples.

Documentation literals

Documentation blocks have type Doc (documentation is a first-class value in the language).[: starts a documentation block and :] finishes it. For example:

List.take.apiDocs : Doc
List.take.apiDocs = [: 
`@List.take n [1,2,3]` returns the first `n` elements of
a list. This is efficient and takes just `O(log n)`.

## Example

@[source]   examples.List.take.ex1
@[evaluate] examples.List.take.ex1

## Also see

* The `@List` type
* The `@[signature] List.drop` function
* More about finger trees (used to implement `@List`) here: `@docs.fingerTrees`

More specifically, within the block:

  • Links to definitions are done with @List. \@ is used to escape the @ symbol.
  • @[signature] List.take expands to the type signature of List.take
  • @[source] List.map expands to the full source of List.map
  • @[include] someOtherDoc, inserts a value someOtherDoc : Doc here.
  • @[evaluate] someDefinition expands to the result of evaluating someDefinition, which must be a pre-existing definition in the codebase (it can't be an arbitrary expression).

Escape sequences

Text literals can include the following escape sequences:

  • \0 = null character
  • \a = alert (bell)
  • \b = backspace
  • \f = form feed
  • \n = new line
  • \r = carriage return
  • \t = horizontal tab
  • \v = vertical tab
  • \\ = literal \ character
  • \' = literal ' character
  • \" = literal " character


A line comment starts with -- and is followed by any sequence of characters. A line that contains a comment can’t contain anything other than a comment and whitespace. Line comments are currently ignored by Unison.

A line starting with --- and containing no other characters is a fold. Any text below the fold is ignored by Unison.

Unison does not currently support block comments. A comment can span multiple lines by adding -- to the front of each line of the comment.

Type annotations

A type annotation has the form e:T where e is an expression and T is a type. This tells Unison that e should be of type T (or a subtype of type T), and Unison will check whether this is true. It's a type error for the actual type of e to be anything other than a type that conforms to T.

Parenthesized expressions

Any expression can appear in parentheses, and an expression (e) is the same as the expression e. Parentheses can be used to delimit where an expression begins and ends. For example (f : P -> Q) y is an application of the function f of type P -> Q to the argument y. The parentheses are needed to tell Unison that y is an argument to f, not a part of the type annotation expression.

Function application

A function application f a1 a2 an applies the function f to the arguments a1 through an.

The above syntax is valid where f is a regular identifier. If the function name is an operator such as *, then the syntax for application is infix : a1 * a2. Any operator can be used in prefix position by surrounding it in parentheses: (*) a1 a2. Any regular identifier can be used infix by surrounding it in backticks: a1 `f` a2.

All Unison functions are of arity 1. That is, they take exactly one argument. An n-ary function is modeled either as a unary function that returns a further function (a partially applied function) which accepts the rest of the arguments, or as a unary function that accepts a tuple.

Function application associates to the left, so the expression f a b is the same as (f a) b. If f has type T1 -> T2 -> Tn then f a is well typed only if a has type T1. The type of f a is then T2 -> Tn. The type constructor of function types, ->, associates to the right. So T1 -> T2 -> Tn parenthesizes as T1 -> (T2 -> TN).

The evaluation semantics of function application is applicative order Call-by-Value. In the expression f x y, x and y are fully evaluated in left-to-right order, then f is fully evaluated, then x and y are substituted into the body of f, and lastly the body is evaluated.

An exception to the evaluation semantics is Boolean expressions, which have non-strict semantics.

Unison supports proper tail calls so function calls in tail position do not grow the call stack.

Syntactic precedence

Prefix function application:

  • Binds more tightly than infix operators. So f x + g y is the same as (f x) + (g y).
  • Binds less tightly than keywords that introduce blocks. So f let x is the same as f (let x) and f if b then p else q is the same as f (if b then p else q)
  • Binds less tightly than ' and ! (see delayed computations), so 'f x y is the same as (_ -> f) x y and !f x y is the same as f () x y.

Boolean expressions

A Boolean expression has type Boolean which has two values, true and false.

Conditional expressions

A conditional expression has the form if c then t else f, where c is an expression of type Boolean, and t and f are expressions of any type, but t and f must have the same type.

Evaluation of conditional expressions is non-strict. The evaluation semantics of if c then t else f are:

  • The condition c is always evaluated.
  • If c evaluates to true, the expression t is evaluated and f remains unevaluated. The whole expression reduces to the value of t.
  • If c evaluates to false, the expression f is evaluated and t remains unevaluated. The whole expression reduces to the value of f.

The keywords if, then, and else each introduce a Block as follows:


Boolean conjunction and disjunction

A Boolean conjunction expression is a Boolean expression of the form a && b where a and b are Boolean expressions. Note that && is not a function, but built-in syntax.

The evaluation semantics of a && b are equivalent to if a then b else false.

A Boolean disjunction expression is a Boolean expression of the form a || b where a and b are Boolean expressions. Note that || is not a function, but built-in syntax.

The evaluation semantics of a || b are equivalent to if a then true else b.

Delayed computations

An expression can appear delayed as 'e, which is the same as _ -> e. If e has type T, then 'e has type forall a. a -> T.

If c is a delayed computation, it can be forced with !c, which is the same as c (). The expression c must conform to a type () -> t for some type t, in which case !c has type t.

Delayed computations are important for writing expressions that require abilities. For example:

use io

program : '{IO} ()
program = 'let
  printLine "What is your name?"
  name = !readLine
  printLine ("Hello, " ++ name)

This example defines a small I/O program. The type {IO} () by itself is not allowed as the type of a top-level definition, since the IO ability must be provided by a handler, see abilities and ability handlers). Instead, program has the type '{IO} () (note the ' indicating a delayed computation). Inside a handler for IO, this computation can be forced with !program.

Inside the program, !readLine has to be forced, as the type of io.readLine is '{IO} Text, a delayed computation which, when forced, reads a line from standard input.

Syntactic precedence

The reserved symbols ' and ! bind more tightly than function application, So 'f x is the same as (_ -> f) x and !x + y is the same as (x ()) + y.

These symbols bind less tightly than keywords that introduce blocks, so 'let x is the same as _ -> let x and !if b then p else q is the same as (if b then p else q) ().

Additional ' and ! combine in the obvious way:

  • ''x is the same as (_ -> (_ -> x)) or (_ _ -> x).
  • !!x is the same as x () ().
  • !'x and '!x are both the same as x.

You can of course use parentheses to precisely control how ' and ! get applied.

Case expressions and pattern matching

A case expression has the general form:

case e of
  pattern_1 -> block_1
  pattern_2 -> block_2
  pattern_n -> block_n

Where e is an expression, called the scrutinee of the case expression, and each case has a pattern to match against the value of the scrutinee and a block to evaluate in case it matches.

The evaluation semantics of case expressions are as follows:

  1. The scrutinee is evaluated.
  2. The first pattern is evaluated and matched against the value of the scrutinee.
  3. If the pattern matches, any variables in the pattern are substituted into the block to the right of its -> (called the match body) and the block is evaluated. If the pattern doesn’t match then the next pattern is tried and so on.

It's possible for Unison to actually evaluate cases in a different order, but such evaluation should always have the same observable behavior as trying the patterns in sequence.

It is an error if none of the patterns match. In this version of Unison, the error occurs at runtime. In a future version, this should be a compile-time error.

A pattern has one of the following forms:

Blank patterns

A blank pattern has the form _. It matches any expression without creating a variable binding.

For example:

case 42 of
  _ -> "Always matches"

Literal patterns

A literal pattern is a literal Boolean, Nat, Int, Float, or Text. A literal pattern matches if the scrutinee has that exact value.

For example:

case 2 + 2 of
  4 -> "Matches"
  _ -> "Doesn't match"

Variable patterns

A variable pattern is a regular identifier and matches any expression. The expression that it matches will be bound to that identifier as a variable in the match body.

For example, this expression evaluates to 3:

case 1 + 1 of
  x -> x + 1


An as-pattern has the form v@p where v is a regular identifier and p is a pattern. This pattern matches if p matches, and the variable v will be bound in the body to the value matching p.

For example, this expression evaluates to 3:

case 1 + 1 of
  x@4 -> x * 2
  y@2 -> y + 1
  _   -> 22

Constructor patterns

A constructor pattern has the form C p1 p2 ... pn where C is the name of a data constructor in scope, and p1 through pn are patterns such that n is the arity of C. Note that n may be zero. This pattern matches if the scrutinee reduces to a fully applied invocation of the data constructor C and the patterns p1 through pn match the arguments to the constructor.

For example, this expression uses Some and None, the constructors of the Optional type, to return the 3rd element of the list xs if present or 0 if there was no 3rd element.

case List.at 3 xs of
  None -> 0
  Some x -> x

List patterns

A list pattern matches a List t for some type t and has one of three forms:

  1. head +: tail matches a list with at least one element. The pattern head is matched against the first element of the list and tail is matched against the suffix of the list with the first element removed.
  2. init :+ last matches a list with at least one element. The pattern init is matched against the prefix of the list with the last element removed, and last is matched against the last element of the list.
  3. A literal list pattern has the form [p1, p2, ... pn] where p1 through pn are patterns. The patterns p1 through pn are matched against the elements of the list. This pattern only matches if the length of the scrutinee is the same as the number of elements in the pattern. The pattern [] matches the empty list.
  4. part1 ++ part2 matches a list which composed of the concatenation of part1 and part2. At least one of part1 or part2 must be a pattern with a known list length, otherwise it's unclear where the list is being split. For instance, [x,y] ++ rest is okay as is start ++ [x,y], but just a ++ b is not allowed.


first : [a] -> Optional a
first as = case as of
  h +: _ -> Some h
  [] -> None

last : [a] -> Optional a
last as = case as of
  _ :+ l -> Some l
  [] -> None

exactlyOne : [a] -> Boolean
exactlyOne a = case a of
  [_] -> true
  _   -> false

lastTwo : [a] -> Optional (a,a)
lastTwo a = case a of
  start ++ [a,a2] -> Some (a,a2)
  _ -> None
firstTwo : [a] -> Optional (a,a)
firstTwo a = case a of
  [a,a2] ++ rest -> Some (a,a2)
  _ -> None

Tuple patterns

A tuple pattern has the form (p1, p2, ... pn) where p1 through pn are patterns. The pattern matches if the scrutinee is a tuple of the same arity as the pattern and p1 through pn match against the elements of the tuple. The pattern (p) is the same as the pattern p, and the pattern () matches the literal value () of the trivial type () (both pronounced “unit”).

For example, this expression evaluates to 4:

case (1,2,3) of
  (a,_,c) -> a + c

Ability patterns (or Request patterns)

An ability pattern only appears in an ability handler and has one of two forms (see Abilities and ability handlers for details):

  1. {C p1 p2 ... pn -> k} where C is the name of an ability constructor in scope, and p1 through pn are patterns such that n is the arity of C. Note that n may be zero. This pattern matches if the scrutinee reduces to a fully applied invocation of the ability constructor C and the patterns p1 through pn match the arguments to the constructor. The scrutinee must be of type Request A T for some ability {A} and type T. The variable k will be bound to the continuation of the program. If the scrutinee has type Request A T and C has type X ->{A} Y, then k has type Y -> {A} T.
  2. {p} where p is a pattern. This matches the case where the computation is pure (the value of type Request A T calls none of the constructors of the ability {A}). A pattern match on an Request is not complete unless this case is handled.

See the section on abilities and ability handlers for examples of ability patterns.

Guard patterns

A guard pattern has the form p | g where p is a pattern and g is a Boolean expression that may reference any variables bound in p. The pattern matches if p matches and g evaluates to true.

For example, the following expression evaluates to 6:

case 1 + 2 of
  x | x == 4 -> 0
  x | x + 1 == 4 -> 6
  _ -> 42


A hash in Unison is a 512-bit SHA3 digest of a term or a type's internal structure, excluding all names. The textual representation of a hash is its base32Hex Unicode encoding.

Unison attributes a hash to every term and type declaration, and the hash may be used to unambiguously refer to that term or type in all contexts. As far as Unison is concerned, the hash of a term or type is its true name.

Literal hash references

A term, type, data constructor, or ability constructor may be unambiguously referenced by hash. Literal hash references have the following structure:

  • A term definition has a hash of the form #x where x is the base32Hex encoding of the hash of the term. For example #a0v829.
  • A term or type definition that’s part of a cycle of mutually recursive definitions hashes to the form #x.n where x is the hash of the cycle and n is the term or type’s index in its cycle. A cycle has a canonical order determined by sorting all the members of the cycle by their individual hashes (with the cycle removed).
  • A data constructor hashes to the form #x#c where x is the hash of the data type definition and c is the index of that data constructor in the type definition.
  • A data constructor in a cyclic type definition hashes to the form #x.n#c where #x.n is the hash of the data type and c is the data constructor’s index in the type definition.
  • A built-in reference to a Unison built-in term or type n has a hash of the form ##n. ##Nat is an example of a built-in reference.

Short hashes

A hash literal may use a prefix of the base32Hex encoded SHA3 digest instead of the whole thing. For example the programmer may use a short hash like #r1mtr0 instead of the much longer 104-character representation of the full 512-bit hash. If the short hash is long enough to be unambiguous given the environment, Unison will substitute the full hash at compile time. When rendering code as text, Unison may calculate the minimum disambiguating hash length before rendering a hash.


This section describes informally the structure of types in Unison. See also the section titled User-defined types for detailed information on how to define new data types.

Formally, Unison’s type system is an implementation of the system described by Joshua Dunfield and Neelakantan R. Krishnaswami in their 2013 paper Complete and Easy Bidirectional Typechecking for Higher-Rank Polymorphism.

Unison extends that type system with, pattern matching, scoped type variables, ability types (also known as algebraic effects). See the section titled Abilities and Ability Handlers for details on ability types.

Unison attributes a type to every valid expression. For example:

  • 4 < 5 has type Boolean
  • 42 + 3 has type Nat,
  • "hello" has type Text
  • the list [1,2,3] has type [Nat]
  • the function (x -> x) has type forall a. a -> a

The meanings of these types and more are explained in the sections below.

A full treatise on types is beyond the scope of this document. In short, types help enforce that Unison programs make logical sense. Every expression must be well typed, or Unison will give a compile-time type error. For example:

  • [1,2,3] is well typed, since lists require all elements to be of the same type.
  • 42 + "hello" is not well typed, since the type of + disallows adding numbers and text together.
  • printLine "Hello, World!" is well typed in some contexts and not others. It's a type error for instance to use I/O functions where an IO ability is not provided.

Types are of the following general forms.

Type variables

Type variables are regular identifiers beginning with a lowercase letter. For example a, x0, and foo are valid type variables.

Polymorphic types

A universally quantified or polymorphic type has the form forall v1 v2 vn. t, where t is a type. The type t may involve the variables v1 through vn.

The symbol is an alias for forall.

A type like forall x. F x can be written simply as F x (the forall x is implied) as long as x is free in F x (it is not bound by an outer scope; see Scoped type variables below).

A polymorphic type may be instantiated at any given type. For example, the empty list [] has type forall x. [x]. So it's a type-polymorphic value. Its type can be instantiated at Int, for example, which binds x to Int resulting in [Int] which is also a valid type for the empty list. In fact, we can say that the empty list [] is a value of type [x] for all choices of element type e, hence the type forall x. [x].

Likewise the identity function (x -> x), which simply returns its argument, has a polymorphic type forall t. t -> t. It has type t -> t for all choices of t.

Scoped type variables

Type variables introduced by a type signature for a term remain in scope throughout the definition of that term.

For example in the following snippet, the type annotation temp:x is telling Unison that temp has the type x which is bound in the type signature, so temp and a have the same type.

ex1 : x -> y -> x
ex1 a b =
  -- refers to the type x in the outer scope
  temp : x
  temp = a

To explicitly shadow a type variable in scope, the variable can be reintroduced in the inner scope by a forall binder, as follows:

ex2 : x -> y -> x
ex2 a b =
  -- doesn’t refer to x in outer scope
  id : ∀ x . x -> x
  id v = v
  temp = id 42
  id a

Note that here the type variable x in the type of id gets instantiated to two different types. First id 42 instantiates it to Nat, then id a, instantiates it to the outer scope's type x.

Type constructors

Just as values are built using data constructors, types are built from type constructors. Nullary type constructors like Nat, Int, Float are already types, but other type constructors like List and -> (see built-in type constructors) take type parameters in order to yield types. List is a unary type constructor, so it takes one type (the type of the list elements), and -> is a binary type constructor. List Nat is a type and Nat -> Int is a type.

Kinds of Types

Types are to values as kinds are to type constructors. Unison attributes a kind to every type constructor, which is determined by its number of type parameters and the kinds of those type parameters.

A type must be well kinded, just like an expression must be well typed, and for the same reason. However, there is currently no syntax for kinds and they do not appear in Unison programs (this will certainly change in a future version of Unison).

Unison’s kinds have the following forms:

  • A nullary type constructor or ordinary type has kind Type.
  • A type constructor has kind k1 -> k2 where k1 and k2 are kinds.

For example List, a unary type constructor, has kind Type -> Type as it takes a type and yields a type. A binary type constructor like -> has kind Type -> Type -> Type, as it takes two types (it actually takes a type and yields a partially applied unary type constructor that takes the other type). A type constructor of kind (Type -> Type) -> Type is a higher-order type constructor (it takes a unary type constructor and yields a type).

Type application

A type constructor is applied to a type or another type constructor, depending on its kind, similarly to how functions are applied to arguments at the term level. C T applies the type constructor C to the type T. Type application associates to the left, so the type A B C is the same as the type (A B) C.

Function types

The type X -> Y is a type for functions that take arguments of type X and yield results of type Y. Application of the binary type constructor -> associates to the right, so the type X -> Y -> Z is the same as the type X -> (Y -> Z).

Tuple types

The type (A,B) is a type for binary tuples (pairs) of values, one of type A and another of type B. The type (A,B,C) is a triple, and so on.

The type (A) is the same as the type A and is not considered a tuple.

The nullary tuple type () is the type of the unique value also written () and is pronouced “unit”.

In the standard Unison syntax, tuples of arity 2 and higher are actually of a type Tuple a b for some types a and b. For example, (X,Y) is syntactic shorthand for the type Tuple X (Tuple Y ()).

Tuples are either constructed with the syntactic shorthand (a,b) (see tuple literals) or with the built-in Tuple.Cons data constructor: Tuple.Cons a (Tuple.Cons b ()).

Built-in types

Unison provides the following built-in types:

  • .base.Nat is the type of 64-bit natural numbers, also known as unsigned integers. They range from 0 to 18,446,744,073,709,551,615.
  • .base.Int is the type of 64-bit signed integers. They range from -9,223,372,036,854,775,808 to +9,223,372,036,854,775,807.
  • .base.Float is the type of IEEE 754-1985 double-precision floating point numbers.
  • .base.Boolean is the type of Boolean expressions whose value is true or false.
  • .base.Bytes is the type of arbitrary-length 8-bit byte sequences.
  • .base.Text is the type of arbitrary-length strings of Unicode text.
  • The trivial type () (pronounced “unit”) is the type of the nullary tuple. There is a single data constructor of type () and it’s also written ().

See literals for more on how values of some of these types are constructed.

Built-in type constructors

Unison has the following built-in type constructors.

  • (->) is the constructor of function types. A type X -> Y is the type of functions from X to Y.
  • base.Tuple is the constructor of tuple types. See tuple types for details on tuples.
  • .base.List is the constructor of list types. A type List T is the type of arbitrary-length sequences of values of type T. The type [T] is an alias for List T.
  • .base.Request is the constructor of requests for abilities. A type Request A T is the type of values received by ability handlers for the ability A where current continuation requires a value of type T.

User-defined types

New types can be declared as described in detail in the User-defined types section. These include ordinary data types, unique types, and record types. A type declaration introduces a type, a corresponding type constructor, one or more data constructors that (collectively) construct all possible values of the type, and (in the case of record types) accessors for the named arguments of the type's single data constructor.

Abilities and ability handlers

Unison provides a system of abilities and ability handlers as a means of modeling computational effects in a purely functional language.

Unison is a purely functional language, so no expressions are allowed to have side effects, meaning they are evaluated to a result and nothing else. But we still need to be able to write programs that have effects, for example writing to disk, communicating over a network, generating randomness, looking at the clock, and so on. Ability types are Unison's way of allowing an expression to request effects it would like to have. Handlers then interpret those requests, often by translating them in turn to a computation that uses the built-in IO ability. Unison has a built-in handler for the IO ability which cannot be invoked in Unison programs (it can only be invoked by the Unison runtime). This allows Unison to provide I/O effects in a purely functional setting.

Unison's system of abilities is based on the Frank language by Sam Lindley, Conor McBride, and Craig McLaughlin. Unison diverges slightly from the scheme detailed in this paper. In particular, Unison's ability polymorphism is provided by ordinary polymorphic types, and a Unison type with an empty ability set explicitly disallows any abilities. In Frank, the empty ability set implies an ability-polymorphic type.

Abilities in function types

The general form for a function type in Unison is I ->{A} O, where I is the input type of the function, O is the output type, and A is the set of abilities that the function requires.

A function type in Unison like A -> B is really syntactic sugar for a type A ->{e} B where e is some set of abilities, possibly empty. A function that definitely requires no abilities has a type like A ->{} B (it has an empty set of abilities).

If a function f calls in its implementation another function requiring ability set {A}, then f will require A in its ability set as well. If f also calls a function requiring abilities {B}, then f will require abilities {A,B}.

Stated the other way around, f can only be called in contexts where the abilities {A,B} are available. Abilities are provided by handle blocks. See the Ability Handlers section below. The only exception to abilities being provided by handlers is the built-in provider of the IO ability in the Unison runtime.

User-defined abilities

A user-defined ability is declared with an ability declaration such as:

ability Store v where
  get : v
  put : v -> ()

This results in a new ability type constructor Store which takes a type argument v. It also create two value-level constructors named get and put. The idea is that get provides the ability to "get" a value of type v from somewhere, and put allows "putting" a value of type v somewhere. Where exactly these values of type v will be kept depends on the handler.

The Store constructors get and put have the following types:

  • get : forall v. {Store v} v
  • put : forall v. v ->{Store v} ()

The type {Store v} means that the computation which results in that type requires a Store v ability and cannot be executed except in the context of an ability handler that provides the ability.

Ability handlers

A constructor {A} T for some ability A and some type T (or a function which uses such a constructor), can only be used in a scope where the ability A is provided. Abilities are provided by handle expressions:

handle h in x

This expression gives x access to abilities handled by the function h which must have the type Request A T -> T if x has type {A} T. The type constructor Request is a special builtin provided by Unison which will pass arguments of type Request A T to a handler for the ability A.

The examples in the next section should help clarify how ability handlers work.

Pattern matching on ability constructors

Each constructor of an ability corresponds with a pattern that can be used for pattern matching in ability handlers. The general form of such a pattern is:

{A.c p_1 p_2 p_n -> k}

Where A is the name of the ability, c is the name of the constructor, p_1 through p_n are patterns matching the arguments to the constructor, and k is a continuation for the program. If the value matching the pattern has type Request A T and the constructor of that value had type X ->{A} Y, then k has type Y -> {A} T.

The continuation will always be a function accepting the return value of the ability constructor, and the body of this function is the remainder of the handle .. in block immediately following the call to the constructor. See below for an example.

A handler can choose to call the continuation or not, or to call it multiple times. For example, a handler can ignore the continuation in order to handle an ability that aborts the execution of the program:

ability Abort where
  aborting : ()

-- Returns `a` immediately if the program `e` calls `abort`
abortHandler : a -> Request Abort a -> a
abortHandler a e = case e of
   { Abort.aborting -> _ } -> a
   { x } -> x

p : Nat
p = handle abortHandler 0 in
  x = 4
  x + 2

The program p evaluates to 0. If we remove the Abort.aborting call, it evaluates to 6.

Note that although the ability constructor is given the signature aborting : (), its actual type is {Abort} ().

The pattern { Abort.aborting -> _ } matches when the Abort.aborting call in p occurs. This pattern ignores its continuation since it will not invoke it (which is how it aborts the program). The continuation at this point is the expression _ -> x + 2.

The pattern { x } matches the case where the computation is pure (makes no further requests for the Abort ability and the continuation is empty). A pattern match on a Request is not complete unless this case is handled.

When a handler calls the continuation, it needs describe how the ability is provided in the continuation of the program, usually with a recursive call, like this:

use .base Request

ability Store v where
  get : v
  put : v -> ()

storeHandler : v -> Request (Store v) a -> a
storeHandler storedValue s = case s of
  {Store.get -> k} ->
    handle storeHandler storedValue in k storedValue
  {Store.put v -> k} ->
    handle storeHandler v in k ()
  {a} -> a

Note that the storeHandler has a handle clause that uses storeHandler itself to handle the Requestss made by the continuation. So it’s a recursive definition. The initial "stored value" of type v is given to the handler in its argument named storedValue, and the changing value is captured by the fact that different values are passed to each recursive invocation of the handler.

In the pattern for Store.get, the continuation k expects a v, since the return type of get is v. In the pattern for Store.put, the continuation k expects (), which is the return type of put.

It's worth noting that this is a mutual recursion between storeHandler and the various continuations (all named k). This is no cause for concern, as they call each other in tail position and the Unison compiler performs tail call elimination.

An example use of the above handler:

modifyStore : (v -> v) ->{Store v} ()
modifyStore f =
  v = Store.get
  Store.put (f v)

Here, when the handler receives Store.get, the continuation is v -> Store.put (f v). When the handler receives Store.put, the continuation is _ -> ().

Use clauses

A use clause tells Unison to allow identifiers from a given namespace to be used unqualified in the lexical scope where the use clause appears.

In this example, the use .base.List clause allows the definition that follows it to refer to .base.List.take as simply take:

use .base.List

oneTwo = take 2 [1,2,3]

The general form of use clauses is as follows:

use namespace name_1 name_2 .. name_n

Where namespace is the namespace from which we want to use names unqualified, and name_1 through name_n are the names we want to use. If no names are given in the use clause, Unison allows all the names from the namespace to be used unqualified. There's no performance penalty for this, as use clauses are purely a syntactic convenience. When rendering code as text, Unison will insert precise use clauses that mention exactly the names it uses, even if the programmer omitted the list of names.

See the section on identifiers for more on namespaces as well as qualified and unqualified names.