The CUE Language Specification


This is a reference manual for the CUE data constraint language. CUE, pronounced cue or Q, is a general-purpose and strongly typed constraint-based language. It can be used for data templating, data validation, code generation, scripting, and many other applications involving structured data. The CUE tooling, layered on top of CUE, provides a general purpose scripting language for creating scripts as well as simple servers, also expressed in CUE.

CUE was designed with cloud configuration, and related systems, in mind, but is not limited to this domain. It derives its formalism from relational programming languages. This formalism allows for managing and reasoning over large amounts of data in a straightforward manner.

The grammar is compact and regular, allowing for easy analysis by automatic tools such as integrated development environments.

This document is maintained by CUE has a lot of similarities with the Go language. This document draws heavily from the Go specification as a result.

CUE draws its influence from many languages. Its main influences were BCL/ GCL (internal to Google), LKB (LinGO), Go, and JSON. Others are Swift, Javascript, Prolog, NCL (internal to Google), Jsonnet, HCL, Flabbergast, JSONPath, Haskell, Objective-C, and Python.


The syntax is specified using Extended Backus-Naur Form (EBNF):

Production  = production_name "=" [ Expression ] "." .
Expression  = Alternative { "|" Alternative } .
Alternative = Term { Term } .
Term        = production_name | token [ "…" token ] | Group | Option | Repetition .
Group       = "(" Expression ")" .
Option      = "[" Expression "]" .
Repetition  = "{" Expression "}" .

Productions are expressions constructed from terms and the following operators, in increasing precedence:

|   alternation
()  grouping
[]  option (0 or 1 times)
{}  repetition (0 to n times)

Lower-case production names are used to identify lexical tokens. Non-terminals are in CamelCase. Lexical tokens are enclosed in double quotes "" or back quotes ``.

The form a … b represents the set of characters from a through b as alternatives. The horizontal ellipsis … is also used elsewhere in the spec to informally denote various enumerations or code snippets that are not further specified. The character … (as opposed to the three characters ...) is not a token of the Go language.

Source code representation

Source code is Unicode text encoded in UTF-8. Unless otherwise noted, the text is not canonicalized, so a single accented code point is distinct from the same character constructed from combining an accent and a letter; those are treated as two code points. For simplicity, this document will use the unqualified term character to refer to a Unicode code point in the source text.

Each code point is distinct; for instance, upper and lower case letters are different characters.

Implementation restriction: For compatibility with other tools, a compiler may disallow the NUL character (U+0000) in the source text.

Implementation restriction: For compatibility with other tools, a compiler may ignore a UTF-8-encoded byte order mark (U+FEFF) if it is the first Unicode code point in the source text. A byte order mark may be disallowed anywhere else in the source.


The following terms are used to denote specific Unicode character classes:

newline        = /* the Unicode code point U+000A */ .
unicode_char   = /* an arbitrary Unicode code point except newline */ .
unicode_letter = /* a Unicode code point classified as "Letter" */ .
unicode_digit  = /* a Unicode code point classified as "Number, decimal digit" */ .

In The Unicode Standard 8.0, Section 4.5 "General Category" defines a set of character categories. CUE treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo as Unicode letters, and those in the Number category Nd as Unicode digits.

Letters and digits

The underscore character _ (U+005F) is considered a letter.

letter        = unicode_letter | "_" .
decimal_digit = "0" … "9" .
octal_digit   = "0" … "7" .
hex_digit     = "0" … "9" | "A" … "F" | "a" … "f" .

Lexical elements


Comments serve as program documentation. There are two forms:

  1. Line comments start with the character sequence // and stop at the end of the line.
  2. General comments start with the character sequence /* and stop with the first subsequent character sequence */.

A comment cannot start inside string literal or inside a comment. A general comment containing no newlines acts like a space. Any other comment acts like a newline.


Tokens form the vocabulary of the CUE language. There are four classes: identifiers, keywords, operators and punctuation, and literals. White space, formed from spaces (U+0020), horizontal tabs (U+0009), carriage returns (U+000D), and newlines (U+000A), is ignored except as it separates tokens that would otherwise combine into a single token. Also, a newline or end of file may trigger the insertion of a comma. While breaking the input into tokens, the next token is the longest sequence of characters that form a valid token.


The formal grammar uses commas "," as terminators in a number of productions. CUE programs may omit most of these commas using the following two rules:

When the input is broken into tokens, a comma is automatically inserted into the token stream immediately after a line's final token if that token is

  • an identifier
  • null, true, false, bottom, or an integer, floating-point, or string literal
  • one of the characters ), ], or }

Although commas are automatically inserted, the parser will require explicit commas between two list elements.

To reflect idiomatic use, examples in this document elide commas using these rules.


Identifiers name entities such as fields and aliases. An identifier is a sequence of one or more letters and digits. It may not be _. The first character in an identifier must be a letter.

identifier = letter { letter | unicode_digit } .

Some identifiers are predeclared.


CUE has a limited set of keywords. All keywords may be used as labels (field names). They cannot, however, be used as identifiers to refer to the same name.


The following keywords are values.

null         true         false

These can never be used to refer to a field of the same name. This restriction is to ensure compatibility with JSON configuration files.


The following keywords are used at the preamble of a CUE file. After the preamble, they may be used as identifiers to refer to namesake fields.

package      import

Comprehension clauses

The following keywords are used in comprehensions.

for          in           if           let

The keywords for, if and let cannot be used as identifiers to refer to fields. All others can.


The following pseudo keywords can be used as operators in expressions.

div          mod          quo          rem

These may be used as identifiers to refer to fields in all other contexts.

Operators and punctuation

The following character sequences represent operators and punctuation:

+    div   &&    ==    !=    (    )
-    mod   ||    <     <=    [    ]
*    quo   !     >     >=    {    }
/    rem   &     :     <-    ;    ,
%    _|_   |     =           ...  .

Integer literals

An integer literal is a sequence of digits representing an integer value. An optional prefix sets a non-decimal base: 0 for octal, 0x or 0X for hexadecimal, and 0b for binary. In hexadecimal literals, letters a-f and A-F represent values 10 through 15. All integers allow interstitial underscores "_"; these have no meaning and are solely for readability.

Decimal integers may have a SI or IEC multiplier. Multipliers can be used with fractional numbers. When multiplying a fraction by a multiplier, the result is truncated towards zero if it is not an integer.

int_lit     = decimal_lit | octal_lit | binary_lit | hex_lit .
decimals  = ( "0" … "9" ) { [ "_" ] decimal_digit } .
decimal_lit = ( "1" … "9" ) { [ "_" ] decimal_digit } [ [ "." decimals ] multiplier ] |
            "." decimals multiplier.
binary_lit  = "0b" binary_digit { binary_digit } .
hex_lit     = "0" ( "x" | "X" ) hex_digit { [ "_" ] hex_digit } .
octal_lit   = "0o" octal_digit { [ "_" ] octal_digit } .
multiplier  = ( "K" | "M" | "G" | "T" | "P" | "E" | "Y" | "Z" ) [ "i" ]

Decimal floating-point literals

A decimal floating-point literal is a representation of a decimal floating-point value (a float). It has an integer part, a decimal point, a fractional part, and an exponent part. The integer and fractional part comprise decimal digits; the exponent part is an e or E followed by an optionally signed decimal exponent. One of the integer part or the fractional part may be elided; one of the decimal point or the exponent may be elided.

decimal_lit = decimals "." [ decimals ] [ exponent ] |
            decimals exponent |
            "." decimals [ exponent ] .
exponent  = ( "e" | "E" ) [ "+" | "-" ] decimals .
072.40  // == 72.40

String and byte sequence literals

A string literal represents a string constant obtained from concatenating a sequence of characters. Byte sequences are a sequence of bytes.

String and byte sequence literals are character sequences between, respectively, double and single quotes, as in "bar" and 'bar'. Within the quotes, any character may appear except newline and, respectively, unescaped double or single quote. String literals may only be valid UTF-8. Byte sequences may contain any sequence of bytes.

Several backslash escapes allow arbitrary values to be encoded as ASCII text in interpreted strings. There are four ways to represent the integer value as a numeric constant: \x followed by exactly two hexadecimal digits; \u followed by exactly four hexadecimal digits; \U followed by exactly eight hexadecimal digits, and a plain backslash \ followed by exactly three octal digits. In each case the value of the literal is the value represented by the digits in the corresponding base. Hexadecimal and octal escapes are only allowed within byte sequences (single quotes).

Although these representations all result in an integer, they have different valid ranges. Octal escapes must represent a value between 0 and 255 inclusive. Hexadecimal escapes satisfy this condition by construction. The escapes \u and \U represent Unicode code points so within them some values are illegal, in particular those above 0x10FFFF. Surrogate halves are allowed to be compatible with JSON, but are translated into their non-surrogate equivalent internally.

The three-digit octal (\nnn) and two-digit hexadecimal (\xnn) escapes represent individual bytes of the resulting string; all other escapes represent the (possibly multi-byte) UTF-8 encoding of individual characters. Thus inside a string literal \377 and \xFF represent a single byte of value 0xFF=255, while ÿ, \u00FF, \U000000FF and \xc3\xbf represent the two bytes 0xc3 0xbf of the UTF-8 encoding of character U+00FF.

After a backslash, certain single-character escapes represent special values:

\a   U+0007 alert or bell
\b   U+0008 backspace
\f   U+000C form feed
\n   U+000A line feed or newline
\r   U+000D carriage return
\t   U+0009 horizontal tab
\v   U+000b vertical tab
\/   U+002f slash (solidus)
\\   U+005c backslash
\'   U+0027 single quote  (valid escape only within single quoted literals)
\"   U+0022 double quote  (valid escape only within double quoted literals)

The escape \( is used as an escape for string interpolation. A \( must be followed by a valid CUE Expression, followed by a ).

All other sequences starting with a backslash are illegal inside literals.

escaped_char     = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .
unicode_value    = unicode_char | little_u_value | big_u_value | escaped_char .
byte_value       = octal_byte_value | hex_byte_value .
octal_byte_value = `\` octal_digit octal_digit octal_digit .
hex_byte_value   = `\` "x" hex_digit hex_digit .
little_u_value   = `\` "u" hex_digit hex_digit hex_digit hex_digit .
big_u_value      = `\` "U" hex_digit hex_digit hex_digit hex_digit
                           hex_digit hex_digit hex_digit hex_digit .

string_lit             = interpreted_string_lit |
                         interpreted_bytes_lit |
                         multiline_lit .

interpolation          = "\(" Expression ")" .
interpreted_string_lit = `"` { unicode_value | interpolation } `"` .
interpreted_bytes_lit  = `"` { unicode_value | interpolation | byte_value } `"` .
'\xa'        // illegal: too few hexadecimal digits
'Hello, world!\n'
"Hello, \( name )!"
"\uD800"             // illegal: surrogate half (TODO: probably should allow)
"\U00110000"         // illegal: invalid Unicode code point

These examples all represent the same string:

"日本語"                                 // UTF-8 input text
'日本語'                                 // UTF-8 input text as byte sequence
`日本語`                                 // UTF-8 input text as a raw literal
"\u65e5\u672c\u8a9e"                    // the explicit Unicode code points
"\U000065e5\U0000672c\U00008a9e"        // the explicit Unicode code points
"\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e"  // the explicit UTF-8 bytes

If the source code represents a character as two code points, such as a combining form involving an accent and a letter, the result will appear as two code points if placed in a string literal.

Each of the interpreted string variants have a multiline equivalent. Multiline interpreted strings are like their single-line equivalent, but allow newline characters. Carriage return characters (\r) inside raw string literals are discarded from the raw string value.

Multiline interpreted strings and byte sequences respectively start with a triple double quote (""") or triple single quote ('''), immediately followed by a newline, which is discarded from the string contents. The string is closed by a matching triple quote, which must be by itself on a newline, preceded by optional whitespace. The whitespace before a closing triple quote must appear before any non-empty line after the opening quote and will be removed from each of these lines in the string literal. A closing triple quote may not appear in the string. To include it is suffices to escape one of the quotes.

multiline_lit         = multiline_string_lit | multiline_bytes_lit .
multiline_string_lit  = `"""` newline
                        { unicode_char | interpolation | newline }
                        newline `"""` .
multiline_bytes_lit   = "'''" newline
                        { unicode_char | interpolation | newline | byte_value }
                        newline "'''" .
    out of the water
    out of itself

    picking bugs
    off the moon
        — Nick Virgilio, Selected Haiku, 1988

This represents the same string as:

"lily:\nout of the water\nout of itself\n\n" +
"bass\npicking bugs\noff the moon\n" +
"    — Nick Virgilio, Selected Haiku, 1988"


In addition to simple values like "hello" and 42.0, CUE has structs. A struct is a map from labels to values, like {a: 42.0, b: "hello"}. Structs are CUE's only way of building up complex values; lists, which we will see later, are defined in terms of structs.

All possible values are ordered in a lattice, a partial order where every two elements have a single greatest lower bound. A value a is an instance of a value b, denoted a ⊑ b, if b == a or b is more general than a, that is if a orders before b in the partial order ( is not a CUE operator). We also say that b subsumes a in this case. In graphical terms, b is "above" a in the lattice.

At the top of the lattice is the single ancestor of all values, called top, denoted _ in CUE. Every value is an instance of top.

At the bottom of the lattice is the value called bottom, denoted _|_. A bottom value usually indicates an error. Bottom is an instance of every value.

An atom is any value whose only instances are itself and bottom. Examples of atoms are 42.0, "hello", true, null.

A value is concrete if it is either an atom, or a struct all of whose field values are themselves concrete, recursively.

CUE's values also include what we normally think of as types, like string and float. But CUE does not distinguish between types and values; only the relationship of values in the lattice is important. Each CUE "type" subsumes the concrete values that one would normally think of as part of that type. For example, "hello" is an instance of string, and 42.0 is an instance of float. In addition to string and float, CUE has null, int, bool and bytes. We informally call these CUE's "basic types".

false ⊑ bool
true  ⊑ bool
true  ⊑ true
5.0   ⊑ float
bool  ⊑ _
_|_   ⊑ _
_|_   ⊑ _|_

_     ⋢ _|_
_     ⋢ bool
int   ⋢ bool
bool  ⋢ int
false ⋢ true
true  ⋢ false
float ⋢ 5.0
5     ⋢ 6


The unification of values a and b is defined as the greatest lower bound of a and b. (That is, the value u such that u ⊑ a and u ⊑ b, and for any other value v for which v ⊑ a and v ⊑ b it holds that v ⊑ u.) Since CUE values form a lattice, the unification of two CUE values is always unique.

These all follow from the definition of unification:

  • The unification of a with itself is always a.
  • The unification of values a and b where a ⊑ b is always a.
  • The unification of a value with bottom is always bottom.

Unification in CUE is a binary expression, written a & b. It is commutative and associative. As a consequence, order of evaluation is irrelevant, a property that is key to many of the constructs in the CUE language as well as the tooling layered on top of it.


The disjunction of values a and b is defined as the least upper bound of a and b. (That is, the value d such that a ⊑ d and b ⊑ d, and for any other value e for which a ⊑ e and b ⊑ e, it holds that d ⊑ e.) This style of disjunctions is sometimes also referred to as sum types. Since CUE values form a lattice, the disjunction of two CUE values is always unique.

These all follow from the definition of disjunction:

  • The disjunction of a with itself is always a.
  • The disjunction of a value a and b where a ⊑ b is always b.
  • The disjunction of a value a with bottom is always a.
  • The disjunction of two bottom values is bottom.

Disjunction in CUE is a binary expression, written a | b. It is commutative and associative.

The unification of a disjunction with another value is equal to the disjunction composed of the unification of this value with all of the original elements of the disjunction. In other words, unification distributes over disjunction.

(a_0 | ... |a_n) & b ==> a_0&b | ... | a_n&b.
Expression                Result
({a:1} | {b:2}) & {c:3}   {a:1, c:3} | {b:2, c:3}
(int | string) & "foo"    "foo"
("a" | "b") & "c"         _|_

Default values

One or more values in a disjunction can be marked by prefixing it with a * (a unary expression). A bottom value cannot be marked. When a marked value is unified, the result is also marked. (When unification results in a single value, the mark is dropped, as single values cannot be marked.)

A disjunction is normalized if there is no unmarked element a for which there is an element b such that a ⊑ b and no marked element c for which there is a marked element d such that c ⊑ d. A disjunction literal must be normalized.

If a disjunction appears where a concrete value is required (that is, as an operand or in a location where it will be emitted), the result is, after normalization and after dropping non-marked elements if some elements are marked, the resulting value itself if only a single value remains or bottom otherwise.

Expression                       Default
"tcp" | "udp"                    _|_ // more than one element remaining
*"tcp" | "udp"                   "tcp"
float | *1                       1
*string | 1.0                    string

(*"tcp"|"udp") & ("udp"|*"tcp")  "tcp"
(*"tcp"|"udp") & ("udp"|"tcp")   "tcp"
(*"tcp"|"udp") & "tcp"           "tcp"
(*"tcp"|"udp") & (*"udp"|"tcp")  _|_ // "tcp" & "udp"

(*true | false) & bool           true
(*true | false) & (true | false) true

{a: 1} | {b: 1}                  _|_ // more than one element remaining
{a: 1} | *{b: 1}                 {b:1}
*{a: 1} | *{b: 1}                _|_ // more than one marked element remaining
({a: 1} | {b: 1}) & {a:1}        {a:1} // after eliminating {a:1,b:1}
({a:1}|*{b:1}) & ({a:1}|*{b:1})  {b:1} // after eliminating *{a:1,b:1}

A disjunction always evaluates to the same default value, regardless of the context in which the value is used. For instance, [1, 3][*"a" | 1] will result in an error, as "a" will be selected as the default value.

[1, 2][*"a" | 1]          //  _|_ // "a" is not an integer value
[1, 2][(*"a" | 1) & int]  //  2, as "a" is eliminated before choosing a default.

Bottom and errors

Any evaluation error in CUE results in a bottom value, respresented by the token '|'. Bottom is an instance of every other value. Any evaluation error is represented as bottom.

Implementations may associate error strings with different instances of bottom; logically they all remain the same value.


Top is represented by the underscore character '_', lexically an identifier. Unifying any value v with top results v itself.

Expr        Result
_ &  5        5
_ &  _        _
_ & _|_      _|_
_ | _|_       _


The null value is represented with the keyword null. It has only one parent, top, and one child, bottom. It is unordered with respect to any other value.

null_lit   = "null"
null & 8     _|_
null & _     null
null & _|_   _|_

Boolean values

A boolean type represents the set of Boolean truth values denoted by the keywords true and false. The predeclared boolean type is bool; it is a defined type and a separate element in the lattice.

boolean_lit = "true" | "false"
bool & true          true
true & true          true
true & false         _|_
bool & (false|true)  false | true
bool & (true|false)  true | false

Numeric values

The integer type represents the set of all integral numbers. The decimal floating-point type represents the set of all decimal floating-point numbers. They are two distinct types. The predeclared integer and decimal floating-point types are int and float; they are defined types.

A decimal floating-point literal always has type float; it is not an instance of int even if it is an integral number.

An integer literal has both type int and float, with the integer variant being the default if no other constraints are applied. Expressed in terms of disjunction and type conversion, the literal 1, for instance, is defined as int(1) | float(1). Hexadecimal, octal, and binary integer literals are always of type int.

Numeric literals are exact values of arbitrary precision. If the operation permits it, numbers should be kept in arbitrary precision.

Implementation restriction: although numeric values have arbitrary precision in the language, implementations may implement them using an internal representation with limited precision. That said, every implementation must:

  • Represent integer values with at least 256 bits.
  • Represent floating-point values, with a mantissa of at least 256 bits and a signed binary exponent of at least 16 bits.
  • Give an error if unable to represent an integer value precisely.
  • Give an error if unable to represent a floating-point value due to overflow.
  • Round to the nearest representable value if unable to represent a floating-point value due to limits on precision. These requirements apply to the result of any expression except for builtin functions for which an unusual loss of precision must be explicitly documented.


The string type represents the set of all possible UTF-8 strings, not allowing surrogates. The predeclared string type is string; it is a defined type.

Strings are designed to be unicode-safe. Comparison is done using canonical forms ("é" == "e\u0301"). A string element is an extended grapheme cluster, which is an approximation of a human-readable character.

The length of a string s (its size in bytes) can be discovered using the built-in function len. A string's extended grapheme cluster can be accessed by integer index 0 through len(s)-1 for any byte that is part of that grapheme cluster.

To access the individual bytes of a string one should convert it to a sequence of bytes first.


A bound, syntactically_ a unary expression, defines an infinite disjunction of concrete values than can be represented as a single comparison.

For any comparison operator op except ==, op a is the disjunction of every x such that x op a.

2 & >=2 & <=5           // 2, where 2 is either an int or float.
2.5 & >=1 & <=5         // 2.5
2 & >=1.0 & <3.0        // 2.0
2 & >1 & <3.0           // 2.0
2.5 & int & >1 & <5     // _|_
2.5 & float & >1 & <5   // 2.5
int & 2 & >1.0 & <3.0   // _|_
2.5 & >=(int & 1) & <5  // _|_
>=0 & <=7 & >=3 & <=10  // >=3 & <=7
!=null & 1              // 1
>=5 & <=5               // 5


A struct is a set of elements called fields, each of which has a name, called a label, and value.

We say a label is defined for a struct if the struct has a field with the corresponding label. The value for a label f of struct a is denoted f.a. A struct a is an instance of b, or a ⊑ b, if for any label f defined for b, label f is also defined for a and a.f ⊑ b.f. Note that if a is an instance of b it may have fields with labels that are not defined for b.

The (unique) struct with no fields, written {}, has every struct as an instance. It can be considered the type of all structs.

The successful unification of structs a and b is a new struct c which has all fields of both a and b, where the value of a field f in c is a.f & b.f if f is in both a and b, or just a.f or b.f if f is in just a or b, respectively. Any references to a or b in their respective field values need to be replaced with references to c. The result of a unification is bottom (_|_) if any of its fields evaluates to bottom, recursively.

A field name may also be an interpolated string. Identifiers used in such strings are evaluated within the scope of the struct in which the label is defined.

Syntactically, a struct literal may contain multiple fields with the same label, the result of which is a single field with a value that is the unification of the values of those fields.

A TemplateLabel indicates a template value that is to be unified with the values of all fields within a struct. The identifier of a template label binds to the field name of each field and is visible within the template value.

StructLit     = "{" [ { Declaration "," } Declaration ] "}" .
Declaration   = FieldDecl | AliasDecl | ComprehensionDecl .
FieldDecl     = Label { Label } ":" Expression .

AliasDecl     = Label "=" Expression .
Label         = identifier | interpreted_string_lit | TemplateLabel .
TemplateLabel = "<" identifier ">" .
Tag           = "#" identifier [ ":" json_string ] .
{a: 1} ⊑ {}
{a: 1, b: 1} ⊑ {a: 1}
{a: 1} ⊑ {a: int}
{a: 1, b: 1} ⊑ {a: int, b: float}

{} ⋢ {a: 1}
{a: 2} ⋢ {a: 1}
{a: 1} ⋢ {b: 1}
Expression                             Result
{a: int, a: 1}                         {a: int(1)}
{a: int} & {a: 1}                      {a: int(1)}
{a: >=1 & <=7} & {a: >=5 & <=9}        {a: >=5 & <=7}
{a: >=1 & <=7, a: >=5 & <=9}           {a: >=5 & <=7}

{a: 1} & {b: 2}                        {a: 1, b: 2}
{a: 1, b: int} & {b: 2}                {a: 1, b: int(2)}

{a: 1} & {a: 2}                        _|_

In addition to fields, a struct literal may also define aliases. Aliases name values that can be referred to within the scope of their definition, but are not part of the struct: aliases are irrelevant to the partial ordering of values and are not emitted as part of any generated data. The name of an alias must be unique within the struct literal.

// The empty struct.

// A struct with 3 fields and 1 alias.
    alias = 3

    foo: 2
    bar: "a string"

    "not an ident": 4

A field whose value is a struct with a single field may be written as a sequence of the two field names, followed by a colon and the value of that single field.

job myTask replicas: 2

expands to

job: {
    myTask: {
        replicas: 2


A list literal defines a new value of type list. A list may be open or closed. An open list is indicated with a ... at the end of an element list, optionally followed by a value for the remaining elements.

The length of a closed list is the number of elements it contains. The length of an open list is the its number of elements as a lower bound and an unlimited number of elements as its upper bound.

ListLit       = "[" [ ElementList [ "," [ "..." [ Element ] ] ] "]" .
ElementList   = Element { "," Element } .
Element       = Expression | LiteralValue .

Lists can be thought of as structs:

List: *null | {
    Elem: _
    Tail: List

For closed lists, Tail is null for the last element, for open lists it is *null | List, defaulting to the shortest variant. For instance, the open list [ 1, 2, ... ] can be represented as:

open: List & { Elem: 1, Tail: { Elem: 2 } }

and the closed version of this list, [ 1, 2 ], as

closed: List & { Elem: 1, Tail: { Elem: 2, Tail: null } }

Using this representation, the subsumption rule for lists can be derived from those of structs. Implementations are not required to implement lists as structs. The Elem and Tail fields are not special and len will not work as expected in these cases.

Declarations and Scopes


A block is a possibly empty sequence of declarations. The braces of a struct literal { ... } form a block, but there are others as well:

  • The universe block encompasses all CUE source text.
  • Each package has a package block containing all CUE source text in that package.
  • Each file has a file block containing all CUE source text in that file.
  • Each for and let clause in a comprehension is considered to be its own implicit block.

Blocks nest and influence [scoping].

Declarations and scope

A declaration binds an identifier to a field, alias, or package. Every identifier in a program must be declared. Other than for fields, no identifier may be declared twice within the same block. For fields an identifier may be declared more than once within the same block, resulting in a field with a value that is the result of unifying the values of all fields with the same identifier.

TopLevelDecl   = Declaration | Emit .
Emit           = Operand .

The scope of a declared identifier is the extent of source text in which the identifier denotes the specified field, alias, or package.

CUE is lexically scoped using blocks:

  1. The scope of a predeclared identifier is the universe block.
  2. The scope of an identifier denoting a field or alias declared at top level (outside any struct literal) is the file block.
  3. The scope of the package name of an imported package is the file block of the file containing the import declaration.
  4. The scope of a field or alias identifier declared inside a struct literal is the innermost containing block.

An identifier declared in a block may be redeclared in an inner block. While the identifier of the inner declaration is in scope, it denotes the entity declared by the inner declaration.

The package clause is not a declaration; the package name does not appear in any scope. Its purpose is to identify the files belonging to the same package and to specify the default name for import declarations.

Predeclared identifiers

len       required  close     open

null      The null type and value
bool      All boolean values
int       All integral numbers
float     All decimal floating-point numbers
string    Any valid UTF-8 sequence
bytes     Any vallid byte sequence

Derived   Value
number    int | float
uint      >=0
uint8     >=0 & <=255
int8      >=-128 & <=127
uint16    >=0 & <=65536
int16     >=-32_768 & <=32_767
rune      >=0 & <=0x10FFFF
uint32    >=0 & <=4_294_967_296
int32     >=-2_147_483_648 & <=2_147_483_647
uint64    >=0 & <=18_446_744_073_709_551_615
int64     >=-9_223_372_036_854_775_808 & <=9_223_372_036_854_775_807
uint128   >=0 & <=340_282_366_920_938_463_463_374_607_431_768_211_455
int128    >=-170_141_183_460_469_231_731_687_303_715_884_105_728 &

Exported and manifested identifiers

An identifier of a package may be exported to permit access to it from another package. An identifier is exported if both: the first character of the identifier's name is not a Unicode lower case letter (Unicode class "Ll") or the underscore "_"; and the identifier is declared in the file block. All other identifiers are not exported.

An identifier that starts with the underscore "_" is not emitted in any data output. Quoted labels that start with an underscore are emitted, however.

Uniqueness of identifiers

Given a set of identifiers, an identifier is called unique if it is different from every other in the set, after applying normalization following Unicode Annex #31. Two identifiers are different if they are spelled differently.

Otherwise, they are the same.

Field declarations

A field declaration binds a label (the name of the field) to an expression. The name for a quoted string used as label is the string it represents. Tne name for an identifier used as a label is the identifier itself. Quoted strings and identifiers can be used used interchangeably, with the exception of identifiers starting with an underscore '_'. The latter represent hidden fields and are treated in a different namespace.

Alias declarations

An alias declaration binds an identifier to the given expression.

Within the scope of the identifier, it serves as an alias for that expression. The expression is evaluated in the scope as it was declared.


An expression specifies the computation of a value by applying operators and built-in functions to operands.


Operands denote the elementary values in an expression. An operand may be a literal, a (possibly qualified) identifier denoting field, alias, or a parenthesized expression.

Operand     = Literal | OperandName | ListComprehension | "(" Expression ")" .
Literal     = BasicLit | ListLit | StructLit .
BasicLit    = int_lit | float_lit | string_lit |
              null_lit | bool_lit | bottom_lit | top_lit .
OperandName = identifier | QualifiedIdent.

Qualified identifiers

A qualified identifier is an identifier qualified with a package name prefix.

QualifiedIdent = PackageName "." identifier .

A qualified identifier accesses an identifier in a different package, which must be [imported]. The identifier must be declared in the [package block] of that package.

math.Sin    // denotes the Sin function in package math

Primary expressions

Primary expressions are the operands for unary and binary expressions. A default expression is only valid as an operand to a disjunction.

PrimaryExpr =
	Operand |
	PrimaryExpr Selector |
	PrimaryExpr Index |
	PrimaryExpr Slice |
	PrimaryExpr Arguments .

Selector       = "." identifier .
Index          = "[" Expression "]" .
Slice          = "[" [ Expression ] ":" [ Expression ] "]"
Argument       = Expression .
Arguments      = "(" [ ( Argument { "," Argument } ) [ "..." ] [ "," ] ] ")" .
(s + ".txt")
f(3.1415, true)
s[i : j + 1]


For a [primary expression] x that is not a [package name], the selector expression


denotes the field f of the value x. The identifier f is called the field selector. The type of the selector expression is the type of f. If x is a package name, see the section on [qualified identifiers].

Otherwise, if x is not a struct, or if f does not exist in x, the result of the expression is bottom (an error).

T: {
    x: int
    y: 3

a: T.x  // int
b: T.y  // 3
c: T.z  // _|_ // field 'z' not found in T

Index expressions

A primary expression of the form


denotes the element of the list, string, bytes, or struct a indexed by x. The value x is called the index or field name, respectively. The following rules apply:

If a is not a struct:

  • the index x must be a concrete integer. If x is a disjunction, the default, if any will be selected without unifying x with int beforehand.
  • the index x is in range if 0 <= x < len(a), otherwise it is out of range

The result of a[x] is

for a of list type (including single quoted strings, which are lists of bytes):

  • the list element at index x, if x is within range, where only the explicitly defined values of an open-ended list are considered
  • bottom (an error), otherwise

for a of string type:

  • the grapheme cluster at the xth byte (type string), if x is within range where x may match any byte of the grapheme cluster
  • bottom (an error), otherwise

for a of struct type:

  • the value of the field named x of struct a, if this field exists
  • bottom (an error), otherwise
[ 1, 2 ][1]     // 2
[ 1, 2 ][2]     // _|_
[ 1, 2, ...][2] // _|_
"He\u0300?"[0]  // "H"
"He\u0300?"[1]  // "e\u0300"
"He\u0300?"[2]  // "e\u0300"
"He\u0300?"[3]  // "e\u0300"
"He\u0300?"[4]  // "?"
"He\u0300?"[5]  // _|_

Slice expressions

Slice expressions construct a substring or slice from a string or list.

For strings or lists, the primary expression

a[low : high]

constructs a substring or slice. The indices low and high must be concrete integers and select which elements of operand a appear in the result. The result has indices starting at 0 and length equal to high - low. After slicing the list a

a := [1, 2, 3, 4, 5]
s := a[1:4]

the list s has length 3 and elements

s[0] == 2
s[1] == 3
s[2] == 4

For convenience, any of the indices may be omitted. A missing low index defaults to zero; a missing high index defaults to the length of the sliced operand:

a[2:]  // same as a[2 : len(a)]
a[:3]  // same as a[0 : 3]
a[:]   // same as a[0 : len(a)]

Indices are in range if 0 <= low <= high <= len(a), otherwise they are out of range. For strings, the indices selects the start of the extended grapheme cluster at byte position indicated by the index. If any of the slice values is out of range or if low > high, the result of a slice is bottom (error).

"He\u0300?"[:2]  // "He\u0300"
"He\u0300?"[1:2] // "e\u0300"
"He\u0300?"[4:5] // "e\u0300?"

The result of a successful slice operation is a value of the same type as the operand.


Operators combine operands into expressions.

Expression = UnaryExpr | Expression binary_op Expression .
UnaryExpr  = PrimaryExpr | unary_op UnaryExpr .

binary_op  = "|" | "&" | "||" | "&&" | "==" | rel_op | add_op | mul_op  .
rel_op     = "!=" | "<" | "<=" | ">" | ">=" .
add_op     = "+" | "-" .
mul_op     = "*" | "/" | "%" | "div" | "mod" | "quo" | "rem" .
unary_op   = "+" | "-" | "!" | "*" | rel_op .

Comparisons are discussed elsewhere. For any binary operators, the operand types must unify.

Operator precedence

Unary operators have the highest precedence.

There are eight precedence levels for binary operators. Multiplication operators binds strongest, followed by addition operators, comparison operators, && (logical AND), || (logical OR), & (unification), and finally | (disjunction):

Precedence    Operator
    7             *  /  %  div mod quo rem
    6             +  -
    5             ==  !=  <  <=  >  >=
    4             &&
    3             ||
    2             &
    1             |

Binary operators of the same precedence associate from left to right. For instance, x / y * z is the same as (x / y) * z.

23 + 3*x[i]
x <= f()
f() || g()
x == y+1 && y == z-1
2 | int
{ a: 1 } & { b: 2 }

Arithmetic operators

Arithmetic operators apply to numeric values and yield a result of the same type as the first operand. The three of the four standard arithmetic operators (+, -, *) apply to integer and decimal floating-point types; + and * also apply to lists and strings. / and % only apply to decimal floating-point types and div, mod, quo, and rem only apply to integer types.

+    sum                    integers, floats, lists, strings, bytes
-    difference             integers, floats
*    product                integers, floats, lists, strings, bytes
/    quotient               floats
%    remainder              floats
div  division               integers
mod  modulo                 integers
quo  quotient               integers
rem  remainder              integers

Integer operators

For two integer values x and y, the integer quotient q = x div y and remainder r = x mod y implement Euclidean division and satisfy the following relationship:

r = x - y*q  with 0 <= r < |y|

where |y| denotes the absolute value of y.

 x     y    x div y   x mod y
 5     3       1         2
-5     3      -2         1
 5    -3      -1         2
-5    -3       2         1

For two integer values x and y, the integer quotient q = x quo y and remainder r = x rem y implement truncated division and satisfy the following relationship:

x = q*y + r  and  |r| < |y|

with x quo y truncated towards zero.

 x     y    x quo y   x rem y
 5     3       1         2
-5     3      -1        -2
 5    -3      -1         2
-5    -3       1        -2

A zero divisor in either case results in bottom (an error).

For integer operands, the unary operators + and - are defined as follows:

+x                          is 0 + x
-x    negation              is 0 - x

Decimal floating-point operators

For decimal floating-point numbers, +x is the same as x, while -x is the negation of x. The result of a floating-point division by zero is bottom (an error).

An implementation may combine multiple floating-point operations into a single fused operation, possibly across statements, and produce a result that differs from the value obtained by executing and rounding the instructions individually.

List operators

Lists can be concatenated using the + operator. For lists a and b,

a + b

will produce an open list if b is open. If list a is open, its default value, the shortest variant, is selected.

[ 1, 2 ]      + [ 3, 4 ]       // [ 1, 2, 3, 4 ]
[ 1, 2, ... ] + [ 3, 4 ]       // [ 1, 2, 3, 4 ]
[ 1, 2 ]      + [ 3, 4, ... ]  // [ 1, 2, 3, 4, ... ]

Lists can be multiplied with a positive int using the * operator to create a repeated the list by the indicated number.

3*[1,2]         // [1, 2, 1, 2, 1, 2]
3*[1, 2, ...]   // [1, 2, 1, 2, 1 ,2]
[byte]*4        // [byte, byte, byte, byte]

String operators

Strings can be concatenated using the + operator:

s := "hi " + name + " and good bye"

String addition creates a new string by concatenating the operands.

A string can be repeated by multiplying it:

s: "etc. "*3  // "etc. etc. etc. "
Comparison operators

Comparison operators compare two operands and yield an untyped boolean value.

==    equal
!=    not equal
<     less
<=    less or equal
>     greater
>=    greater or equal

In any comparison, the types of the two operands must unify.

The equality operators == and != apply to operands that are comparable. The ordering operators <, <=, >, and >= apply to operands that are ordered. These terms and the result of the comparisons are defined as follows:

  • Null is comparable with itself and any other type. Two null values are always equal, null is unequal with anything else.
  • Boolean values are comparable. Two boolean values are equal if they are either both true or both false.
  • Integer values are comparable and ordered, in the usual way.
  • Floating-point values are comparable and ordered, as per the definitions for binary coded decimals in the IEEE-754-2008 standard.
  • String values are comparable and ordered, lexically byte-wise after normalization to Unicode normal form NFC.
  • Struct are not comparable.
  • Lists are not comparable.
a: 3 < 4       // true
b: null == 2   // false
c: null != {}  // true
d: {} == {}    // _|_: structs are not comparable against structs

Logical operators

Logical operators apply to boolean values and yield a result of the same type as the operands. The right operand is evaluated conditionally.

&&    conditional AND    p && q  is  "if p then q else false"
||    conditional OR     p || q  is  "if p then true else q"
!     NOT                !p      is  "not p"


Calls can be made to core library functions, called builtins. Given an expression f of function type F,

f(a1, a2, … an)

calls f with arguments a1, a2, … an. Arguments must be expressions of which the values are an instance of the parameter types of F and are evaluated before the function is called.

a: math.Atan2(x, y)

In a function call, the function value and arguments are evaluated in the usual order. After they are evaluated, the parameters of the call are passed by value to the function and the called function begins execution. The return parameters of the function are passed by value back to the calling function when the function returns.


Lists and fields can be constructed using comprehensions.

Each define a clause sequence that consists of a sequence of for, if, and let clauses, nesting from left to right. The for and let clauses each define a new scope in which new values are bound to be available for the next clause.

The for clause binds the defined identifiers, on each iteration, to the next value of some iterable value in a new scope. A for clause may bind one or two identifiers. If there is one identifier, it binds it to the value, for instance a list element, a struct field value or a range element. If there are two identifiers, the first value will be the key or index, if available, and the second will be the value.

An if clause, or guard, specifies an expression that terminates the current iteration if it evaluates to false.

The let clause binds the result of an expression to the defined identifier in a new scope.

A current iteration is said to complete if the innermost block of the clause sequence is reached.

List comprehensions specify a single expression that is evaluated and included in the list for each completed iteration.

Field comprehensions follow a Field with a clause sequence, where the label and value of the field are evaluated for each iteration. The label must be an identifier or interpreted_string_lit, where the later may be a string interpolation that refers to the identifiers defined in the clauses. Values of iterations that map to the same label unify into a single field.

ComprehensionDecl   = Field [ "<-" ] Clauses .
ListComprehension   = "[" Expression [ "<-" ] Clauses "]" .

Clauses             = Clause { Clause } .
Clause              = ForClause | GuardClause | LetClause .
ForClause           = "for" identifier [ ", " identifier] "in" Expression .
GuardClause         = "if" Expression .
LetClause           = "let" identifier "=" Expression .
a: [1, 2, 3, 4]
b: [ x+1 for x in a if x > 1]  // [3, 4, 5]

c: { "\(x)": x + y for x in a if x < 4 let y = 1 }
d: { "1": 2, "2": 3, "3": 4 }

String interpolation

String interpolation allows constructing strings by replacing placeholder expressions with their string representation. String interpolation may be used in single- and double-quoted strings, as well as their multiline equivalent.

A placeholder consists of "(" followed by an expression and a ")". The expression is evaluated within the scope within which the string is defined.

a: "World"
b: "Hello \( a )!" // Hello World!

Builtin Functions

Built-in functions are predeclared. They are called like any other function.


The built-in function len takes arguments of various types and return a result of type int.

Argument type    Result

string            string length in bytes
bytes             length of byte sequence
list              list length, smallest length for an open list
struct            number of distinct fields
Expression           Result
len("Hellø")         6
len([1, 2, 3])       3
len([1, 2, ...])     2
len({a:1, b:2})      2


Implementations are required to interpret or reject cycles encountered during evaluation according to the rules in this section.

Reference cycles

A reference cycle occurs if a field references itself, either directly or indirectly.

// x references itself
x: x

// indirect cycles
b: c
c: d
d: b

Implementations should report these as an error except in the following cases:

Expressions that unify an atom with an expression

An expression of the form a & e, where a is an atom and e is an expression, always evaluates to a or bottom. As it does not matter how we fail, we can assume the result to be a and validate after the field in which the expression occurs has been evaluated that a == e.

// Config            Evaluates to
x: {                  x: {
    a: b + 100            a: _|_ // cycle detected
    b: a - 100            b: _|_ // cycle detected
}                     }

y: x & {              y: {
    a: 200                a: 200 // asserted that 200 == b + 100
                          b: 100
}                     }

Field values

A field value of the form r & v, where r evaluates to a reference cycle and v is a value, evaluates to v. Unification is idempotent and unifying a value with itself ad infinitum, which is what the cycle represents, results in this value. Implementations should detect cycles of this kind, ignore r, and take v as the result of unification.

Configuration    Evaluated
//    c           Cycles in nodes of type struct evaluate
//  ↙︎   ↖         to the fixed point of unifying their
// a  →  b        values ad infinitum.

a: b & { x: 1 }   // a: { x: 1, y: 2, z: 3 }
b: c & { y: 2 }   // b: { x: 1, y: 2, z: 3 }
c: a & { z: 3 }   // c: { x: 1, y: 2, z: 3 }

// resolve a             b & {x:1}
// substitute b          c & {y:2} & {x:1}
// substitute c          a & {z:3} & {y:2} & {x:1}
// eliminate a (cycle)   {z:3} & {y:2} & {x:1}
// simplify              {x:1,y:2,z:3}

This rule also applies to field values that are disjunctions of unification operations of the above form.

a: b&{x:1} | {y:1}  // {x:1,y:3,z:2} | {y:1}
b: {x:2} | c&{z:2}  // {x:2} | {x:1,y:3,z:2}
c: a&{y:3} | {z:3}  // {x:1,y:3,z:2} | {z:3}

// resolving a           b&{x:1} | {y:1}
// substitute b          ({x:2} | c&{z:2})&{x:1} | {y:1}
// simplify              c&{z:2}&{x:1} | {y:1}
// substitute c          (a&{y:3} | {z:3})&{z:2}&{x:1} | {y:1}
// simplify              a&{y:3}&{z:2}&{x:1} | {y:1}
// eliminate a (cycle)   {y:3}&{z:2}&{x:1} | {y:1}
// expand                {x:1,y:3,z:2} | {y:1}

Note that all nodes that form a reference cycle to form a struct will evaluate to the same value. If a field value is a disjunction, any element that is part of a cycle will evaluate to this value.

Structural cycles

CUE disallows infinite structures. Implementations must report an error when encountering such declarations.

// Disallowed: a list of infinite length with all elements being 1.
list: {
    head: 1
    tail: list

// Disallowed: another infinite structure (a:{b:{d:{b:{d:{...}}}}}, ...).
a: {
    b: c
c: {
    d: a

It is allowed for a value to define an infinite set of possibilities without evaluating to an infinite structure itself.

// List defines a list of arbitrary length (default null).
List: *null | {
    head: _
    tail: List

Modules, instances, and packages

CUE configurations are constructed combining instances. An instance, in turn, is constructed from one or more source files belonging to the same package that together declare the data representation. Elements of this data representation may be exported and used in other instances.

Source file organization

Each source file consists of an optional package clause defining collection of files to which it belongs, followed by a possibly empty set of import declarations that declare packages whose contents it wishes to use, followed by a possibly empty set of declarations.

SourceFile      = [ PackageClause "," ] { ImportDecl "," } { TopLevelDecl "," } .

Package clause

A package clause is an optional clause that defines the package to which a source file the file belongs.

PackageClause  = "package" PackageName .
PackageName    = identifier .

The PackageName must not be the blank identifier.

package math

Modules and instances

A module defines a tree of directories, rooted at the module root.

All source files within a module with the same package belong to the same package.

A module may define multiple packages.

An instance of a package is any subset of files belonging to the same package.

It is interpreted as the concatenation of these files.

An implementation may impose conventions on the layout of package files to determine which files of a package belongs to an instance. For example, an instance may be defined as the subset of package files belonging to a directory and all its ancestors.

Import declarations

An import declaration states that the source file containing the declaration depends on definitions of the imported package (§Program initialization and execution) and enables access to exported identifiers of that package. The import names an identifier (PackageName) to be used for access and an ImportPath that specifies the package to be imported.

ImportDecl       = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
ImportSpec       = [ "." | PackageName ] ImportPath .
ImportPath       = `"` { unicode_value } `"` .

The PackageName is used in qualified identifiers to access exported identifiers of the package within the importing source file. It is declared in the file block. If the PackageName is omitted, it defaults to the identifier specified in the package clause of the imported instance. If an explicit period (.) appears instead of a name, all the instances's exported identifiers declared in that instances's package block will be declared in the importing source file's file block and must be accessed without a qualifier.

The interpretation of the ImportPath is implementation-dependent but it is typically either the path of a builtin package or a fully qualifying location of an instance within a source code repository.

Implementation restriction: An interpreter may restrict ImportPaths to non-empty strings using only characters belonging to Unicode's L, M, N, P, and S general categories (the Graphic characters without spaces) and may also exclude the characters !"#$%&'()*,:;<=>?[]^`{|} and the Unicode replacement character U+FFFD.

Assume we have package containing the package clause "package math", which exports function Sin at the path identified by "lib/math". This table illustrates how Sin is accessed in files that import the package after the various types of import declaration.

Import declaration          Local name of Sin

import   "lib/math"         math.Sin
import m "lib/math"         m.Sin
import . "lib/math"         Sin

An import declaration declares a dependency relation between the importing and imported package. It is illegal for a package to import itself, directly or indirectly, or to directly import a package without referring to any of its exported identifiers.

An example package