leo/docs/grammar/abnf-grammar.txt

; Copyright (C) 2019-2022 Aleo Systems Inc.
; This file is part of the Leo library.

; The Leo library is free software: you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation, either version 3 of the License, or
; (at your option) any later version.

; The Leo library is distributed in the hope that it will be useful,
; but WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
; GNU General Public License for more details.

; You should have received a copy of the GNU General Public License
; along with the Leo library. If not, see <https://www.gnu.org/licenses/>.

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; Introduction
; ------------

; This file contains an ABNF (Augmented Backus-Naur Form) grammar of Leo.
; Background on ABNF is provided later in this file.

; This grammar provides an official definition of the syntax of Leo
; that is both human-readable and machine-readable.
; It will be part of an upcoming Leo language reference.
; It may also be used to generate parser tests at some point.

; We are also using this grammar
; as part of a mathematical formalization of the Leo language,
; which we are developing in the ACL2 theorem prover
; and which we plan to publish at some point.
; In particular, we have used a formally verified parser of ABNF grammars
; (at https://github.com/acl2/acl2/tree/master/books/kestrel/abnf;
; also see the paper at https://www.kestrel.edu/people/coglio/vstte18.pdf)
; to parse this grammar into a formal representation of the Leo concrete syntax
; and to validate that the grammar satisfies certain consistency properties.

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; Background on ABNF
; ------------------

; ABNF is an Internet standard:
; see RFC 5234 at https://www.rfc-editor.org/info/rfc5234
; and RFC 7405 at https://www.rfc-editor.org/info/rfc7405.
; It is used to specify the syntax of JSON, HTTP, and other standards.

; ABNF adds conveniences and makes slight modifications
; to Backus-Naur Form (BNF),
; without going beyond context-free grammars.

; Instead of BNF's angle-bracket notation for nonterminals,
; ABNF uses case-insensitive names consisting of letters, digits, and dashes,
; e.g. `HTTP-message` and `IPv6address`.
; ABNF includes an angle-bracket notation for prose descriptions,
; e.g. `<host, see [RFC3986], Section 3.2.2>`,
; usable as last resort in the definiens of a nonterminal.

; While BNF allows arbitrary terminals,
; ABNF uses only natural numbers as terminals,
; and denotes them via:
; (i) binary, decimal, or hexadecimal sequences,
; e.g. `%b1.11.1010`, `%d1.3.10`, and `%x.1.3.A`
; all denote the sequence of terminals [1, 3, 10];
; (ii) binary, decimal, or hexadecimal ranges,
; e.g. `%x30-39` denotes any singleton sequence of terminals
; [_n_] with 48 <= _n_ <= 57 (an ASCII digit);
; (iii) case-sensitive ASCII strings,
; e.g. `%s"Ab"` denotes the sequence of terminals [65, 98];
; and (iv) case-insensitive ASCII strings,
; e.g. `%i"ab"`, or just `"ab"`, denotes
; any sequence of terminals among
; [65, 66],
; [65, 98],
; [97, 66], and
; [97, 98].
; ABNF terminals in suitable sets represent ASCII or Unicode characters.

; ABNF allows repetition prefixes `n*m`,
; where `n` and `m` are natural numbers in decimal notation;
; if absent,
; `n` defaults to 0, and
; `m` defaults to infinity.
; For example,
; `1*4HEXDIG` denotes one to four `HEXDIG`s,
; `*3DIGIT` denotes up to three `DIGIT`s, and
; `1*OCTET` denotes one or more `OCTET`s.
; A single `n` prefix
; abbreviates `n*n`,
; e.g. `3DIGIT` denotes three `DIGIT`s.

; Instead of BNF's `|`, ABNF uses `/` to separate alternatives.
; Repetition prefixes have precedence over juxtapositions,
; which have precedence over `/`.
; Round brackets group things and override the aforementioned precedence rules,
; e.g. `*(WSP / CRLF WSP)` denotes sequences of terminals
; obtained by repeating, zero or more times,
; either (i) a `WSP` or (ii) a `CRLF` followed by a `WSP`.
; Square brackets also group things but make them optional,
; e.g. `[":" port]` is equivalent to `0*1(":" port)`.

; Instead of BNF's `::=`, ABNF uses `=` to define nonterminals,
; and `=/` to incrementally add alternatives
; to previously defined nonterminals.
; For example, the rule `BIT = "0" / "1"`
; is equivalent to `BIT = "0"` followed by `BIT =/ "1"`.

; The syntax of ABNF itself is formally specified in ABNF
; (in Section 4 of the aforementioned RFC 5234,
; after the syntax and semantics of ABNF
; are informally specified in natural language
; (in Sections 1, 2, and 3 of the aforementioned RFC 5234).
; The syntax rules of ABNF prescribe the ASCII codes allowed for
; white space (spaces and horizontal tabs),
; line endings (carriage returns followed by line feeds),
; and comments (semicolons to line endings).

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; Structure
; ---------

; This ABNF grammar consists of two (sub-)grammars:
; (i) a lexical grammar that describes how
; sequence of characters are parsed into tokens, and
; (ii) a syntactic grammar that describes how
; tokens are parsed into expressions, statements, etc.
; The adjectives 'lexical' and 'syntactic' are
; the same ones used in the Java language reference,
; for instance;
; alternative terms may be used in other languages,
; but the separation into these two components is quite common
; (the situation is sometimes a bit more complex, with multiple passes,
; e.g. Unicode escape processing in Java).

; This separation enables
; concerns of white space, line endings, etc.
; to be handled by the lexical grammar,
; with the syntactic grammar focused on the more important structure.
; Handling both aspects in a single grammar may be unwieldy,
; so having two grammars provides more clarity and readability.

; ABNF is a context-free grammar notation, with no procedural interpretation.
; The two grammars conceptually define two subsequent processing phases,
; as detailed below.
; However, a parser implementation does not need to perform
; two strictly separate phases (in fact, it typically does not),
; so long as it produces the same final result.

; The grammar is accompanied by some extra-grammatical requirements,
; which are not conveniently expressible in a context-free grammar like ABNF.
; These requirements are needed to make the grammar unambiguous,
; i.e. to ensure that, for each sequence of terminals,
; there is exactly one parse tree for that sequence terminals
; that satisfies not only the grammar rules
; but also the extra-grammatical requirements.
; These requirements are expressed as comments in this file.

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; Operator Precedence
; -------------------

; We formulate the grammar rules for expressions
; in a way that describes the relative precedence of operators,
; as often done in language syntax specifications.

; For instance, consider the rules
;
;  multiplicative-expression =
;          exponential-expression
;        / multiplicative-expression "*" exponential-expression
;        / multiplicative-expression "/" exponential-expression
;
;  additive-expression =
;          multiplicative-expression
;        / additive-expression "+" multiplicative-expression
;        / additive-expression "-" multiplicative-expression
;
; These rules tell us
; that the additive operators `+` and `-` have lower precedence
; than the multiplicative operators `*` and `/`,
; and that both the additive and multiplicative operators associate to the left.
; This may be best understood via the examples given below.

; According to the rules, the expression
;
;  x + y * z
;
; can only be parsed as
;
;    +
;   / \
;  x   *
;     / \
;    y   z
;
; and not as
;
;      *
;     / \
;    +   z
;   / \
;  x   y
;
; because a multiplicative expression cannot have an additive expression
; as first sub-expression, as it would in the second tree above.

; Also according to the rules, the expression
;
;  x + y + z
;
; can only be parsed as
;
;      +
;     / \
;    +   z
;   / \
;  x   y
;
; and not as
;
;    +
;   / \
;  x   +
;     / \
;    y   z
;
; because an additive expression cannot have an additive expression
; as second sub-expression, as it would in the second tree above.

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; Naming Convention
; -----------------

; This ABNF grammar uses nonterminal names
; that consist of complete English words, separated by dashes,
; and that describe the construct the way it is in English.
; For instance, we use the name `conditional-statement`
; to describe conditional statements.

; At the same time, this grammar establishes
; a precise and official nomenclature for the Leo constructs,
; by way of the nonterminal names that define their syntax.
; For instance, the rule
;
;  group-literal = product-group-literal
;                / affine-group-literal
;
; tells us that there are two kinds of group literals,
; namely product group literals and affine group literals.
; This is more precise than describing them as
; integers (which are not really group elements per se),
; or points (they are all points, just differently specified),
; or being singletons vs. pairs (which is a bit generic).

; The only exception to the nomenclature-establishing role of the grammar
; is the fact that, as discussed above,
; we write the grammar rules in a way that determines
; the relative precedence and the associativity of expression operators,
; and therefore we have rules like
;
;  unary-expression = postfix-expression
;                   / "!" unary-expression
;                   / "-" unary-expression
;
; In order to allow the recursion of the rule to stop,
; we need to regard, in the grammar, a primary expression as a unary expression
; (i.e. a primary expression is also a unary expression in the grammar;
; but note that the opposite is not true).
; However, this is just a grammatical artifact:
; ontologically, a primary expression is not really a unary expression,
; because a unary expression is one that consists of
; a unary operator and an operand sub-expression.
; These terminological exceptions should be easy to identify in the rules.

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; Lexical Grammar
; ---------------

; A Leo file is a finite sequence of Unicode characters,
; represented as Unicode code points,
; which are numbers in the range from 0 to 10FFFF.
; These are captured by the ABNF rule `character` below.

; The lexical grammar defines how, at least conceptually,
; the sequence of characters is turned into
; a sequence of tokens, comments, and whitespaces:
; these entities are all defined by the grammar rules below.

; As stated, the lexical grammar alone is ambiguous.
; For example, the sequence of characters `**` (i.e. two stars)
; could be equally parsed as two `*` symbol tokens or one `**` symbol token
; (see rule for `symbol` below).
; As another example, the sequence or characters `<CR><LF>`
; (i.e. carriage return followed by line feed)
; could be equally parsed as two line terminators or one
; (see rule for `line-terminator`).

; Thus, as often done in language syntax definitions,
; the lexical grammar is disambiguated by
; the extra-grammatical requirement that
; the longest possible sequence of characters is always parsed.
; This way, `**` must be parsed as one `**` symbol token,
; and `<CR><LF>` must be parsed as one line terminator.

; As mentioned above, a character is any Unicode code point.
; This grammar does not say how those are encoded in files (e.g. UTF-8):
; it starts with a decoded sequence of Unicode code points.
; Note that we allow any value,
; even though some values may not be used according to the Unicode standard.

character = %x0-10FFFF   ; any Unicode code point

; We give names to certain ASCII characters.

horizontal-tab = %x9   ; <HT>

line-feed = %xA   ; <LF>

carriage-return = %xD   ; <CR>

space = %x20   ; <SP>

double-quote = %x22   ; "

single-quote = %x27   ; '

; We give names to complements of certain ASCII characters.
; These consist of all the Unicode characters except for one or two.

not-star = %x0-29 / %x2B-10FFFF   ; anything but *

not-star-or-slash = %x0-29 / %x2B-2E / %x30-10FFFF   ; anything but * or /

not-line-feed-or-carriage-return = %x0-9 / %xB-C / %xE-10FFFF
                                   ; anything but <LF> or <CR>

not-double-quote-or-backslash = %x0-21 / %x23-5B / %x5D-10FFFF
                                ; anything but " or \

not-single-quote-or-backslash = %x0-26 / %x28-5B / %x5D-10FFFF
                                ; anything but ' or \

; Lines in Leo may be terminated via
; a single carriage return,
; a line feed,
; or a carriage return immediately followed by a line feed.
; Note that the latter combination constitutes a single line terminator,
; according to the extra-grammatical requirement of the longest sequence,
; described above.

line-terminator = line-feed / carriage-return / carriage-return line-feed

; Line terminators form whitespace, along with spaces and horizontal tabs.

whitespace = space / horizontal-tab / line-terminator

; There are two kinds of comments in Leo, as in other languages.
; One is block comments of the form `/* ... */`,
; and the other is end-of-line comments of the form `// ...`.
; The first kind start at `/*` and end at the first `*/`,
; possibly spanning multiple (partial) lines;
; these do no nest.
; The second kind start at `//` and extend till the end of the line.
; The rules about comments given below are similar to
; the ones used in the Java language reference.

comment = block-comment / end-of-line-comment

block-comment = "/*" rest-of-block-comment

rest-of-block-comment = "*" rest-of-block-comment-after-star
                      / not-star rest-of-block-comment

rest-of-block-comment-after-star = "/"
                                 / "*" rest-of-block-comment-after-star
                                 / not-star-or-slash rest-of-block-comment

end-of-line-comment = "//" *not-line-feed-or-carriage-return

; Below are the keywords in the Leo language.
; They cannot be used as identifiers.

keyword = %s"address"
        / %s"as"
        / %s"bool"
        / %s"char"
        / %s"circuit"
        / %s"console"
        / %s"const"
        / %s"else"
        / %s"field"
        / %s"for"
        / %s"function"
        / %s"group"
        / %s"i8"
        / %s"i16"
        / %s"i32"
        / %s"i64"
        / %s"i128"
        / %s"if"
        / %s"import"
        / %s"in"
        / %s"input"
        / %s"let"
        / %s"return"
        / %s"Self"
        / %s"self"
        / %s"static"
        / %s"type"
        / %s"u8"
        / %s"u16"
        / %s"u32"
        / %s"u64"
        / %s"u128"

; The following rules define (ASCII) letters.

uppercase-letter = %x41-5A   ; A-Z

lowercase-letter = %x61-7A   ; a-z

letter = uppercase-letter / lowercase-letter

; The following rules defines (ASCII) decimal, octal, and hexadecimal digits.
; Note that the latter are case-insensitive.

decimal-digit = %x30-39   ; 0-9

octal-digit = %x30-37   ; 0-7

hexadecimal-digit = decimal-digit / "a" / "b" / "c" / "d" / "e" / "f"

; An identifier is a non-empty sequence of
; letters, (decimal) digits, and underscores,
; starting with a letter.
; It must not be a keyword: this is an extra-grammatical requirement.
; It must also not be or start with `aleo1`,
; because that is used for address literals:
; this is another extra-grammatical requirement.

identifier = letter *( letter / decimal-digit / "_" )
             ; but not a keyword or a boolean literal or aleo1...

; Annotations have names, which are identifiers immediately preceded by `@`.

annotation-name = "@" identifier

; A natural (number) is a sequence of one or more decimal digits.
; We allow leading zeros, e.g. `007`.

natural = 1*decimal-digit

; Unsigned literals are naturals followed by unsigned types.

unsigned-literal = natural ( %s"u8" / %s"u16" / %s"u32" / %s"u64" / %s"u128" )

; Signed literals are naturals followed by signed types.

signed-literal = natural ( %s"i8" / %s"i16" / %s"i32" / %s"i64" / %s"i128" )

; Field literals are naturals followed by the type of field elements.

field-literal = natural %s"field"

; There are two kinds of group literals.
; One is a single natural followed by the type of group elements,
; which denotes the scalar product of the generator point by the natural.
; The other kind is not a token because it allows some whitespace inside;
; therefore, it is defined in the syntactic grammar.

product-group-literal = natural %s"group"

; Boolean literals are the usual two.

boolean-literal = %s"true" / %s"false"

; An address literal starts with `aleo1`
; and continues with exactly 58 lowercase letters and (decimal) digits.
; Thus an address always consists of 63 characters.

address-literal = %s"aleo1" 58( lowercase-letter / decimal-digit )

; A character literal consists of an element surrounded by single quotes.
; The element is any character other than single quote or backslash,
; or an escape, which starts with backslash.
; There are simple escapes with a single character,
; ASCII escapes with an octal and a hexadecimal digit
; (which together denote a number between 0 and 127),
; and Unicode escapes with one to six hexadecimal digits
; (which must not exceed 10FFFF).

character-literal = single-quote character-literal-element single-quote

character-literal-element = not-single-quote-or-backslash
                          / simple-character-escape
                          / ascii-character-escape
                          / unicode-character-escape

single-quote-escape = "\" single-quote   ; \'

double-quote-escape = "\" double-quote   ; \"

backslash-escape = "\\"

line-feed-escape = %s"\n"

carriage-return-escape = %s"\r"

horizontal-tab-escape = %s"\t"

null-character-escape = "\0"

simple-character-escape = single-quote-escape
                        / double-quote-escape
                        / backslash-escape
                        / line-feed-escape
                        / carriage-return-escape
                        / horizontal-tab-escape
                        / null-character-escape

ascii-character-escape = %s"\x" octal-digit hexadecimal-digit

unicode-character-escape = %s"\u{" 1*6hexadecimal-digit "}"

; A string literal consists of one or more elements surrounded by double quotes.
; Each element is any character other than double quote or backslash,
; or an escape among the same ones used for elements of character literals.

string-literal = double-quote *string-literal-element double-quote

string-literal-element = not-double-quote-or-backslash
                       / simple-character-escape
                       / ascii-character-escape
                       / unicode-character-escape

; The ones above are all the atomic literals
; (in the sense that they are tokens, without whitespace allowed in them),
; as defined by the following rule.

atomic-literal = unsigned-literal
               / signed-literal
               / field-literal
               / product-group-literal
               / boolean-literal
               / address-literal
               / character-literal
               / string-literal

; After defining the (mostly) alphanumeric tokens above,
; it remains to define tokens for non-alphanumeric symbols such as `+` and `(`.
; Different programming languages used different terminologies for these,
; e.g. operators, separators, punctuators, etc.
; Here we use `symbol`, for all of them.
; We also include a token consisting of
; a closing parenthesis `)` immediately followed by `group`:
; as defined in the syntactic grammar,
; this is the final part of an affine group literal;
; even though it includes letters,
; it seems appropriate to still consider it a symbol,
; particularly since it starts with a proper symbol.

symbol = "!" / "&" / "&&" / "||"
       / "==" / "!="
       / "<" / "<=" / ">" / ">="
       / "+" / "-" / "*" / "/" / "**"
       / "=" / "+=" / "-=" / "*=" / "/=" / "**="
       / "(" / ")"
       / "[" / "]"
       / "{" / "}"
       / "," / "." / ".." / "..." / ";" / ":" / "::" / "?"
       / "->" / "_"
       / %s")group"

; Everything defined above, other than comments and whitespace,
; is a token, as defined by the following rule.

token = keyword
      / identifier
      / atomic-literal
      / annotation-name
      / symbol

; Tokens, comments, and whitespace are lexemes, i.e. lexical units.

lexeme = token / comment / whitespace

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; Syntactic Grammar
; -----------------

; The processing defined by the lexical grammar above
; turns the initial sequence of characters
; into a sequence of tokens, comments, and whitespaces.
; The purpose of comments and whitespaces, from a syntactic point of view,
; is just to separate tokens:
; they are discarded, leaving a sequence of tokens.
; The syntactic grammar describes how to turn
; a sequence of tokens into concrete syntax trees.

; There are unsigned and signed integer types, for five sizes.

unsigned-type = %s"u8" / %s"u16" / %s"u32" / %s"u64" / %s"u128"

signed-type = %s"i8" / %s"i16" / %s"i32" / %s"i64" / %s"i128"

integer-type = unsigned-type / signed-type

; The integer types, along with the field and group types,
; for the arithmetic types, i.e. the ones that support arithmetic operations.

field-type = %s"field"

group-type = %s"group"

arithmetic-type = integer-type / field-type / group-type

; The arithmetic types, along with the boolean, address, and character types,
; form the scalar types, i.e. the ones whose values do not contain (sub-)values.

boolean-type = %s"bool"

address-type = %s"address"

character-type = %s"char"

scalar-type =  boolean-type / arithmetic-type / address-type / character-type

; A tuple type consists of zero, two, or more component types.

tuple-type = "(" [ type 1*( "," type ) ] ")"

; An array type consists of an element type
; and an indication of dimensions.
; There is either a single dimension,
; or a tuple of one or more dimensions.
; Each dimension is a natural.

array-type = "[" type ";" array-dimensions "]"

array-dimensions = natural / "(" natural *( "," natural ) ")"

; The keyword `Self` denotes the enclosing circuit type.
; It is only allowed inside a circuit type declaration.

self-type = %s"Self"

; Circuit types are denoted by identifiers and by `Self`.
; Identifiers may also be type aliases;
; syntactically (i.e. without a semantic analysis),
; they cannot be distinguished from circuit types.

; Scalar types, tuple types, array types,
; identifiers (which may be circuit types or type aliases),
; and the `Self` type
; form all the types.

type = scalar-type / tuple-type / array-type / identifier / self-type

; It is convenient to introduce a rule for types that are
; either identifiers or `Self`, as this is used in other rules.

identifier-or-self-type = identifier / self-type

; A named type is an identifier, the `Self` type, or a scalar type.
; These are types that are named by identifiers or keywords.

named-type = identifier / self-type / scalar-type

; The lexical grammar given earlier defines product group literals.
; The other kind of group literal is a pair of integer coordinates,
; which are reduced modulo the prime to identify a point,
; which must be on the elliptic curve.
; It is also allowed to omit one coordinate (not both),
; with an indication of how to fill in the missing coordinate
; (i.e. sign high, sign low, or inferred).
; This is an affine group literal,
; because it consists of affine point coordinates.

group-coordinate = [ "-" ] natural / "+" / "-" / "_"

affine-group-literal = "(" group-coordinate "," group-coordinate %s")group"

; A literal is either an atomic one or an affine group literal.

literal = atomic-literal / affine-group-literal

; The following rule is not directly referenced in the rules for expressions
; (which reference `literal` instead),
; but it is useful to establish terminology:
; a group literal is either a product group literal or an affine group literal.

group-literal = product-group-literal / affine-group-literal

; As often done in grammatical language syntax specifications,
; we define rules for different kinds of expressions,
; which also defines the relative precedence
; of operators and other expression constructs,
; and the (left or right) associativity of binary operators.

; The primary expressions are self-contained in a way,
; i.e. they have clear delimitations:
; Some consist of single tokens,
; while others have explicit endings.
; Primary expressions also include parenthesized expressions,
; i.e. any expression may be turned into a primary one
; by putting parentheses around it.

primary-expression = identifier
                   / %s"self"
                   / %s"input"
                   / literal
                   / "(" expression ")"
                   / tuple-expression
                   / array-expression
                   / circuit-expression

; Tuple expressions construct tuples.
; Each consists of zero, two, or more component expressions.

tuple-construction = "(" [ expression 1*( "," expression ) ] ")"

tuple-expression = tuple-construction

; Array expressions construct arrays.
; There are two kinds:
; one lists the element expressions (at least one),
; including spreads (via `...`) which are arrays being spliced in;
; the other repeats (the value of) a single expression
; across one or more dimensions.

array-inline-construction = "["
                            array-inline-element
                            *( "," array-inline-element )
                            "]"

array-inline-element = expression / "..." expression

array-repeat-construction = "[" expression ";" array-dimensions "]"

array-construction = array-inline-construction / array-repeat-construction

array-expression = array-construction

; Circuit expressions construct circuit values.
; Each lists values for all the member variables (in any order);
; there may be zero or more member variables.
; A single identifier abbreviates
; a pair consisting of the same identifier separated by colon;
; note that, in the expansion, the left one denotes a member name,
; while the right one denotes an expression (a variable),
; so they are syntactically identical but semantically different.

circuit-construction = identifier-or-self-type "{"
                       [ circuit-inline-element
                         *( "," circuit-inline-element ) [ "," ] ]
                       "}"

circuit-inline-element = identifier ":" expression / identifier

circuit-expression = circuit-construction

; After primary expressions, postfix expressions have highest precedence.
; They apply to primary expressions, and recursively to postfix expressions.

; There are postfix expressions to access parts of aggregate values.
; A tuple access selects a component by index (zero-based).
; There are two kinds of array accesses:
; one selects a single element by index (zero-based);
; the other selects a range via two indices,
; the first inclusive and the second exclusive --
; both are optional,
; the first defaulting to 0 and the second to the array length.
; A circuit access selects a member variable by name.

; Function calls are also postfix expressions.
; There are three kinds of function calls:
; top-level function calls,
; instance (i.e. non-static) member function calls, and
; static member function calls.
; What changes is the start, but they all end in an argument list.

; Accesses to static constants are also postfix expressions.
; They consist of a named type followed by the constant name,
; as static constants are associated to named types.

function-arguments = "(" [ expression *( "," expression ) ] ")"

postfix-expression = primary-expression
                   / postfix-expression "." natural
                   / postfix-expression "." identifier
                   / identifier function-arguments
                   / postfix-expression "." identifier function-arguments
                   / named-type "::" identifier function-arguments
                   / named-type "::" identifier
                   / postfix-expression "[" expression "]"
                   / postfix-expression "[" [expression] ".." [expression] "]"

; Unary operators have the highest operator precedence.
; They apply to postfix expressions,
; and recursively to unary expressions.

unary-expression = postfix-expression
                 / "!" unary-expression
                 / "-" unary-expression

; Next in the operator precedence is exponentiation,
; following mathematical practice.
; The current rule below makes exponentiation right-associative,
; i.e. `a ** b ** c` must be parsed as `a ** (b ** c)`.

exponential-expression = unary-expression
                       / unary-expression "**" exponential-expression

; Next in precedence come multiplication and division, both left-associative.

multiplicative-expression = exponential-expression
                          / multiplicative-expression "*" exponential-expression
                          / multiplicative-expression "/" exponential-expression

; Then there are addition and subtraction, both left-assocative.

additive-expression = multiplicative-expression
                    / additive-expression "+" multiplicative-expression
                    / additive-expression "-" multiplicative-expression

; Next in the precedence order are ordering relations.
; These are not associative, because they return boolean values.

ordering-expression = additive-expression
                    / additive-expression "<" additive-expression
                    / additive-expression ">" additive-expression
                    / additive-expression "<=" additive-expression
                    / additive-expression ">=" additive-expression

; Equalities return booleans but may also operate on booleans;
; the rule below makes them left-associative.

equality-expression = ordering-expression
                    / ordering-expression "==" ordering-expression
                    / ordering-expression "!=" ordering-expression

; Next come conjunctive expressions, left-associative.

conjunctive-expression = equality-expression
                       / conjunctive-expression "&&" equality-expression

; Next come disjunctive expressions, left-associative.

disjunctive-expression = conjunctive-expression
                       / disjunctive-expression "||" conjunctive-expression

; Finally we have conditional expressions.

conditional-expression = disjunctive-expression
                       / disjunctive-expression "?" expression ":" expression

; Those above are all the expressions.
; Recall that conditional expressions
; may be disjunctive expressions,
; which may be conjunctive expressions,
; and so on all the way to primary expressions.

expression = conditional-expression

; There are various kinds of statements, including blocks.
; Blocks are possibly empty sequences of statements surrounded by curly braces.

statement = expression-statement
          / return-statement
          / variable-declaration
          / constant-declaration
          / conditional-statement
          / loop-statement
          / assignment-statement
          / console-statement
          / block

block = "{" *statement "}"

; An expression (that must return the empty tuple, as semantically required)
; can be turned into a statement by appending a semicolon.

expression-statement = expression ";"

; A return statement always takes an expression, and ends with a semicolon.

return-statement = %s"return" expression ";"

; There are variable declarations and constant declarations,
; which only differ in the starting keyword.
; These declarations are also statements.
; The names of the variables or constants are
; either a single one or a tuple of two or more;
; in all cases, there is just one optional type
; and just one initializing expression.

variable-declaration = %s"let" identifier-or-identifiers ":" type
                       "=" expression ";"

constant-declaration = %s"const" identifier-or-identifiers ":" type
                       "=" expression ";"

identifier-or-identifiers = identifier
                          / "(" identifier 1*( "," identifier ) ")"

; A conditional statement always starts with a condition and a block
; (which together form a branch).
; It may stop there, or it may continue with an alternative block,
; or possibly with another conditional statement, forming a chain.
; Note that blocks are required in all branches, not merely statements.

branch = %s"if" expression block

conditional-statement = branch
                      / branch %s"else" block
                      / branch %s"else" conditional-statement

; A loop statement implicitly defines a loop variable
; that goes from a starting value (inclusive) to an ending value (exclusive).
; The body is a block.

loop-statement = %s"for" identifier ":" type
                 %s"in" expression ".." [ "=" ] expression
                 block

; An assignment statement is straightforward.
; Based on the operator, the assignment may be simple (i.e. `=`)
; or compound (i.e. combining assignment with an arithmetic operation).

assignment-operator = "=" / "+=" / "-=" / "*=" / "/=" / "**="

assignment-statement = expression assignment-operator expression ";"

; Console statements start with the `console` keyword,
; followed by a console function call.
; The call may be an assertion or a print command.
; The former takes an expression (which must be boolean) as argument.
; The latter takes either no argument,
; or a format string followed by expressions,
; whose number must match the number of containers `{}` in the format string.
; Note that the console function names are identifiers, not keywords.
; There are two kinds of print commands.

console-statement = %s"console" "." console-call ";"

console-call = assert-call
             / print-call

assert-call = %s"assert" "(" expression ")"

print-function = %s"error" / %s"log"

print-arguments = "(" string-literal  *( "," expression ) ")"

print-call = print-function print-arguments

; An annotation consists of an annotation name (which starts with `@`)
; with optional annotation arguments, which are identifiers.

annotation = annotation-name
             [ "(" *( identifier "," ) [ identifier ] ")" ]

; A function declaration defines a function.
; The output type is optional, defaulting to the empty tuple type.
; In general, a function input consists of an identifier and a type,
; with an optional 'const' modifier.
; Additionally, functions inside circuits
; may start with a `&self` or `const self` or `self` parameter.

function-declaration = *annotation [ %s"const" ] %s"function" identifier
                       "(" [ function-parameters ] ")" [ "->" type ]
                       block

function-parameters = self-parameter
                    / self-parameter "," function-inputs
                    / function-inputs

self-parameter = [ %s"&" / %s"const" ] %s"self"

function-inputs = function-input *( "," function-input )

function-input = [ %s"const" ] identifier ":" type

; A circuit member constant declaration consists of
; the `static` and `const` keywords followed by
; an identifier and a type, then an initializer
; with a literal terminated by semicolon.

member-constant-declaration = %s"static" %s"const" identifier ":" type
                              "=" literal ";"

; A circuit member variable declaration consists of
; an identifier and a type, terminated by semicolon.
; For backward compatibility,
; member variable declarations may be alternatively followed by commas,
; and the last one may not be followed by anything:
; these are deprecated, and will be eventually removed,
; leaving only mandatory semicolons.
; Note that there is no rule for a single `member-variable-declaration`,
; but instead one for a sequence of them;
; see the rule `circuit-declaration`.

member-variable-declarations = *( identifier ":" type ( "," / ";" ) )
                               identifier ":" type ( [ "," ] / ";" )

; A circuit member function declaration consists of a function declaration.

member-function-declaration = function-declaration

; A circuit declaration defines a circuit type,
; as consisting of member constants, variables and functions.
; A circuit member constant declaration
; as consisting of member constants, member variables, and member functions.
; To more simply accommodate the backward compatibility
; described for the rule `member-variable-declarations`,
; all the member variables must precede all the member functions;
; this may be relaxed after the backward compatibility is removed,
; allowing member variables and member functions to be intermixed.

circuit-declaration = %s"circuit" identifier
                      "{" *member-constant-declaration
                      [ member-variable-declarations ]
                      *member-function-declaration "}"

; An import declaration consists of the `import` keyword
; followed by a package path, which may be one of the following:
; a single wildcard;
; an identifier, optionally followed by a local renamer;
; an identifier followed by a path, recursively;
; or a parenthesized list of package paths,
; which are "fan out" of the initial path.
; Note that we allow the last element of the parenthesized list
; to be followed by a comma, for convenience.
; The package path in the import declaration must start with an identifier
; (e.g. it cannot be a `*`):
; the rule for import declaration expresses this requirement
; by using an explicit identifier before the package path.

import-declaration = %s"import" identifier "." package-path ";"

package-path = "*"
             / identifier [ %s"as" identifier ]
             / identifier "." package-path
             / "(" package-path *( "," package-path ) [","] ")"

; A type alias declaration defines an identifier to stand for a type.

type-alias-declaration = %s"type" identifier "=" type ";"

; Finally, we define a file as a sequence of zero or more declarations.
; We allow constant declarations at the top level, for global constants.
; Currently variable declarations are disallowed at the top level.

declaration = import-declaration
            / function-declaration
            / circuit-declaration
            / constant-declaration
            / type-alias-declaration

file = *declaration

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

; Format Note
; -----------

; The ABNF standard requires grammars
; to consist of lines terminated by `<CR><LF>`
; (i.e. carriage return followed by line feed, DOS/Windows-style),
; as explained in the background on ABNF earlier in this file.
; This file's lines are therefore terminated by `<CR><LF>`.
; To avoid losing this requirement across systems,
; this file is marked as `text eol=crlf` in `.gitattributes`:
; this means that the file is textual, enabling visual diffs,
; but its lines will always be terminated by `<CR><LF>` on any system.

; Note that this `<CR><LF>` requirement only applies
; to the grammar files themselves.
; It does not apply to the lines of the languages described by the grammar.
; ABNF grammars may describe any kind of languages,
; with any kind of line terminators,
; or even without line terminators at all (e.g. for "binary" languages).