; ABNF syntax based on RFC 5234 ; ; The character encoding for Dhall is UTF-8 ; ; Some notes on implementing this grammar: ; ; First, do not use a lexer to tokenize the file before parsing. Instead, treat ; the individual characters of the file as the tokens to feed into the parser. ; You should not use a lexer because Dhall's grammar supports two features which ; cannot be correctly supported by a lexer: ; ; * String interpolation (i.e. "foo ${Natural/toInteger bar} baz") ; * Nested block comments (i.e. "{- foo {- bar -} baz -}") ; ; Second, this grammar assumes that your parser can backtrack and/or try ; multiple parses simultaneously. For example, consider this expression: ; ; List ./MyType ; ; A parser might first try to parse the period as the beginning of a field ; selector, only to realize immediately afterwards that `/MyType` is not a valid ; name for a field. A conforming parser must backtrack so that the expression ; `./MyType` can instead be correctly interpreted as a relative path ; ; Third, if there are multiple valid parses then prefer the first parse ; according to the ordering of alternatives. That is, the order of evaluation ; of the alternatives is left-to-right. ; ; For example, the grammar for single quoted string literals is: ; ; single-quote-continue = ; "'''" single-quote-continue ; / "${" complete-expression "}" single-quote-continue ; / "''${" single-quote-continue ; / "''" ; / %x20-10FFFF single-quote-continue ; / tab single-quote-continue ; / end-of-line single-quote-continue ; ; single-quote-literal = "''" single-quote-continue ; ; ... which permits valid parses for the following code: ; ; "''''''''''''''''" ; ; If you tried to parse all alternatives then there are at least two valid ; interpretations for the above code: ; ; * A single quoted literal with four escape sequences of the form "'''" ; * i.e. "''" followed by "'''" four times in a row followed by "''" ; * Four empty single quoted literals ; * i.e. "''''" four times in a row ; ; The correct interpretation is the first one because parsing the escape ; sequence "'''" takes precedence over parsing the termination sequence "''", ; according to the order of the alternatives in the `single-quote-continue` ; rule. ; ; Some parsing libraries do not backtrack by default but allow the user to ; selectively backtrack in certain parts of the grammar. Usually parsing ; libraries do this to improve efficiency and error messages. Dhall's grammar ; takes that into account by minimizing the number of rules that require the ; parser to backtrack and comments below will highlight where you need to ; explicitly backtrack ; ; Specifically, if you see an uninterrupted literal in a grammar rule such as: ; ; "->" ; ; ... or: ; ; %x66.6f.72.61.6c.6c ; ; ... then that string literal is parsed as a single unit, meaning that you ; should backtrack if you parse only part of the literal ; ; In all other cases you can assume that you do not need to backtrack unless ; there is a comment explicitly asking you to backtrack ; ; When parsing a repeated construct, prefer alternatives that parse as many ; repetitions as possible. On in other words: ; ; [a] = a / "" ; ; a* = a* a / "" ; ; Note that the latter rule also specifies that repetition produces ; left-associated expressions. For example, function application is ; left-associative and all operators are left-associative when they are not ; parenthesized. ; ; Additionally, try alternatives in an order that minimizes backtracking ; according to the following rule: ; ; (a / b) (c / d) = a c / a d / b c / b d ; NOTE: There are many line endings in the wild ; ; See: https://en.wikipedia.org/wiki/Newline ; ; For simplicity this supports Unix and Windows line-endings, which are the most ; common end-of-line = %x0A ; "\n" / %x0D.0A ; "\r\n" tab = %x09 ; "\t" block-comment = "{-" block-comment-continue block-comment-chunk = block-comment / %x20-10FFFF / tab / end-of-line block-comment-continue = "-}" / block-comment-chunk block-comment-continue not-end-of-line = %x20-10FFFF / tab ; NOTE: Slightly different from Haskell-style single-line comments because this ; does not require a space after the dashes line-comment = "--" *not-end-of-line end-of-line whitespace-chunk = " " / tab / end-of-line / line-comment / block-comment whsp = *whitespace-chunk ; nonempty whitespace whsp1 = 1*whitespace-chunk ; Uppercase or lowercase ASCII letter ALPHA = %x41-5A / %x61-7A ; ASCII digit DIGIT = %x30-39 ; 0-9 HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F" ; A simple label cannot be one of the reserved keywords ; listed in the `keyword` rule. ; A PEG parser could use negative lookahead to ; enforce this, e.g. as follows: ; simple-label = ; keyword 1*simple-label-next-char ; / !keyword (simple-label-first-char *simple-label-next-char) simple-label-first-char = ALPHA / "_" simple-label-next-char = ALPHA / DIGIT / "-" / "/" / "_" simple-label = simple-label-first-char *simple-label-next-char quoted-label-char = %x20-5F ; %x60 = '`' / %x61-7E quoted-label = 1*quoted-label-char ; NOTE: Dhall does not support Unicode labels, mainly to minimize the potential ; for code obfuscation label = ("`" quoted-label "`" / simple-label) ; A nonreserved-label cannot not be any of the reserved identifiers for builtins (unless quoted). ; Their list can be found in semantics.md. This is not enforced by the grammar but ; should be checked by implementations. The only place where this restriction applies ; is bound variables. ; A PEG parser could use negative lookahead to avoid parsing those identifiers. nonreserved-label = label ; An any-label is allowed to be one of the reserved identifiers. any-label = label ; Dhall's double-quoted strings are equivalent to JSON strings except with ; support for string interpolation (and escaping string interpolation) ; ; Dhall uses almost the same escaping rules as JSON (RFC7159) with one ; exception: Dhall adds a new `\$` escape sequence for dollar signs. This ; additional escape sequences lets you escape string interpolation by writing ; `\${` ; ; > The representation of strings is similar to conventions used in the C ; > family of programming languages. A string begins and ends with ; > quotation marks. All Unicode characters may be placed within the ; > quotation marks, except for the characters that must be escaped: ; > quotation mark, reverse solidus, and the control characters (U+0000 ; > through U+001F). ; > ; > Any character may be escaped. If the character is in the Basic ; > Multilingual Plane (U+0000 through U+FFFF), then it may be ; > represented as a six-character sequence: a reverse solidus, followed ; > by the lowercase letter u, followed by four hexadecimal digits that ; > encode the character's code point. The hexadecimal letters A though ; > F can be upper or lower case. So, for example, a string containing ; > only a single reverse solidus character may be represented as ; > "\u005C". ; > ; > Alternatively, there are two-character sequence escape ; > representations of some popular characters. So, for example, a ; > string containing only a single reverse solidus character may be ; > represented more compactly as "\\". ; > ; > To escape an extended character that is not in the Basic Multilingual ; > Plane, the character is represented as a 12-character sequence, ; > encoding the UTF-16 surrogate pair. So, for example, a string ; > containing only the G clef character (U+1D11E) may be represented as ; > "\uD834\uDD1E". double-quote-chunk = interpolation ; '\' Beginning of escape sequence / %x5C double-quote-escaped / double-quote-char double-quote-escaped = %x22 ; '"' quotation mark U+0022 / %x24 ; '$' dollar sign U+0024 / %x5C ; '\' reverse solidus U+005C / %x2F ; '/' solidus U+002F / %x62 ; 'b' backspace U+0008 / %x66 ; 'f' form feed U+000C / %x6E ; 'n' line feed U+000A / %x72 ; 'r' carriage return U+000D / %x74 ; 't' tab U+0009 / %x75 4HEXDIG ; 'uXXXX' U+XXXX ; Printable characters except double quote and backslash double-quote-char = %x20-21 ; %x22 = '"' / %x23-5B ; %x5C = "\" / %x5D-10FFFF double-quote-literal = %x22 *double-quote-chunk %x22 ; NOTE: The only way to end a single-quote string literal with a single quote is ; to either interpolate the single quote, like this: ; ; ''ABC${"'"}'' ; ; ... or concatenate another string, like this: ; ; ''ABC'' ++ "'" ; ; If you try to end the string literal with a single quote then you get "'''", ; which is interpreted as an escaped pair of single quotes single-quote-continue = interpolation single-quote-continue / escaped-quote-pair single-quote-continue / escaped-interpolation single-quote-continue / single-quote-char single-quote-continue / "''" ; End of text literal ; Escape two single quotes (i.e. replace this sequence with "''") escaped-quote-pair = "'''" ; Escape interpolation (i.e. replace this sequence with "${") escaped-interpolation = "''${" single-quote-char = %x20-10FFFF / tab / end-of-line single-quote-literal = "''" end-of-line single-quote-continue interpolation = "${" complete-expression "}" text-literal = (double-quote-literal / single-quote-literal) ; RFC 5234 interprets string literals as case-insensitive and recommends using ; hex instead for case-sensitive strings ; ; If you don't feel like reading hex, these are all the same as the rule name. ; Keywords that should never be parsed as identifiers if = %x69.66 then = %x74.68.65.6e else = %x65.6c.73.65 let = %x6c.65.74 in = %x69.6e as = %x61.73 using = %x75.73.69.6e.67 merge = %x6d.65.72.67.65 missing = %x6d.69.73.73.69.6e.67 Infinity = %x49.6e.66.69.6e.69.74.79 NaN = %x4e.61.4e Some = %x53.6f.6d.65 ; Unused rule that could be used as negative lookahead in the ; `simple-label` rule for parsers that support this. keyword = if / then / else / let / in / using / missing / as / Infinity / NaN / merge / Some ; Reserved identifiers, only needed for some special cases of parsing Optional = %x4f.70.74.69.6f.6e.61.6c Text = %x54.65.78.74 List = %x4c.69.73.74 combine = %x2227 / "/\" combine-types = %x2A53 / "//\\" prefer = %x2AFD / "//" lambda = %x3BB / "\" forall = %x2200 / %x66.6f.72.61.6c.6c arrow = %x2192 / "->" exponent = "e" [ "+" / "-" ] 1*DIGIT numeric-double-literal = [ "+" / "-" ] 1*DIGIT ( "." 1*DIGIT [ exponent ] / exponent) minus-infinity-literal = "-" Infinity plus-infinity-literal = Infinity double-literal = ; "2.0" numeric-double-literal ; "-Infinity" / minus-infinity-literal ; "Infinity" / plus-infinity-literal ; "NaN" / NaN natural-literal = 1*DIGIT integer-literal = ( "+" / "-" ) natural-literal ; The implementation should recognize reserved names for builtins and treat them as special ; values instead of variables. identifier = any-label [ whsp "@" whsp natural-literal ] ; Printable characters other than " ()[]{}<>/\," ; ; Excluding those characters ensures that paths don't have to end with trailing ; whitespace most of the time path-character = ; %x20 = " " %x21 ; %x22 = "\"" ; %x23 = "#" / %x24-27 ; %x28 = "(" ; %x29 = ")" / %x2A-2B ; %x2C = "," / %x2D-2E ; %x2F = "/" / %x30-3B ; %x3C = "<" / %x3D ; %x3E = ">" ; %x3F = "?" / %x40-5A ; %x5B = "[" ; %x5C = "\" ; %x5D = "]" / %x5E-7A ; %x7B = "{" / %x7C ; %x7D = "}" / %x7E quoted-path-character = %x20-21 ; %x22 = "\"" / %x23-2E ; %x2F = "/" / %x30-10FFFF unquoted-path-component = 1*path-character quoted-path-component = 1*quoted-path-character path-component = "/" ( unquoted-path-component / %x22 quoted-path-component %x22 ) ; The last path-component matched by this rule is referred to as "file" in the semantics, ; and the other path-components as "directory". path = 1*path-component local = parent-path / here-path / home-path ; NOTE: Backtrack if parsing this alternative fails ; ; This is because the first character of this alternative will be "/", but ; if the second character is "/" or "\" then this should have been parsed ; as an operator instead of a path / absolute-path parent-path = ".." path ; Relative path here-path = "." path ; Relative path home-path = "~" path ; Home-anchored path absolute-path = path ; Absolute path ; `http[s]` URI grammar based on RFC7230 and RFC 3986 with some differences ; noted below scheme = %x68.74.74.70 [ %x73 ] ; "http" [ "s" ] ; NOTE: This does not match the official grammar for a URI. Specifically: ; ; * this replaces `path-abempty` with `path`, so an empty path is ; not valid ; * this does not support fragment identifiers, which have no meaning within ; Dhall expressions and do not affect import resolution http-raw = scheme "://" authority path [ "?" query ] ; NOTE: Backtrack if parsing the optional user info prefix fails authority = [ userinfo "@" ] host [ ":" port ] userinfo = *( unreserved / pct-encoded / sub-delims / ":" ) host = IP-literal / IPv4address / reg-name port = *DIGIT IP-literal = "[" ( IPv6address / IPvFuture ) "]" IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" ) ; NOTE: Backtrack when parsing each alternative IPv6address = 6( h16 ":" ) ls32 / "::" 5( h16 ":" ) ls32 / [ h16 ] "::" 4( h16 ":" ) ls32 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 / [ *4( h16 ":" ) h16 ] "::" ls32 / [ *5( h16 ":" ) h16 ] "::" h16 / [ *6( h16 ":" ) h16 ] "::" h16 = 1*4HEXDIG ls32 = ( h16 ":" h16 ) / IPv4address IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet ; NOTE: Backtrack when parsing these alternatives and try them in reverse order dec-octet = DIGIT ; 0-9 / %x31-39 DIGIT ; 10-99 / "1" 2DIGIT ; 100-199 / "2" %x30-34 DIGIT ; 200-249 / "25" %x30-35 ; 250-255 reg-name = *( unreserved / pct-encoded / sub-delims ) pchar = unreserved / pct-encoded / sub-delims / ":" / "@" query = *( pchar / "/" / "?" ) pct-encoded = "%" HEXDIG HEXDIG unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" sub-delims = "!" / "$" / "&" / "'" / "(" / ")" / "*" / "+" / "," / ";" / "=" http = http-raw [ whsp using whsp1 (import-hashed / "(" whsp import-hashed whsp ")") ] ; Dhall supports unquoted environment variables that are Bash-compliant or ; quoted environment variables that are POSIX-compliant env = "env:" ( bash-environment-variable / %x22 posix-environment-variable %x22 ) ; Bash supports a restricted subset of POSIX environment variables. From the ; Bash `man` page, an environment variable name is: ; ; > A word consisting only of alphanumeric characters and under-scores, and ; > beginning with an alphabetic character or an under-score bash-environment-variable = (ALPHA / "_") *(ALPHA / DIGIT / "_") ; The POSIX standard is significantly more flexible about legal environment ; variable names, which can contain alerts (i.e. '\a'), whitespace, or ; punctuation, for example. The POSIX standard says about environment variable ; names: ; ; > The value of an environment variable is a string of characters. For a ; > C-language program, an array of strings called the environment shall be made ; > available when a process begins. The array is pointed to by the external ; > variable environ, which is defined as: ; > ; > extern char **environ; ; > ; > These strings have the form name=value; names shall not contain the ; > character '='. For values to be portable across systems conforming to IEEE ; > Std 1003.1-2001, the value shall be composed of characters from the portable ; > character set (except NUL and as indicated below). ; ; Note that the standard does not explicitly state that the name must have at ; least one character, but `env` does not appear to support this and `env` ; claims to be POSIX-compliant. To be safe, Dhall requires at least one ; character like `env` posix-environment-variable = 1*posix-environment-variable-character ; These are all the characters from the POSIX Portable Character Set except for ; '\0' (NUL) and '='. Note that the POSIX standard does not explicitly state ; that environment variable names cannot have NUL. However, this is implicit ; in the fact that environment variables are passed to the program as ; NUL-terminated `name=value` strings, which implies that the `name` portion of ; the string cannot have NUL characters posix-environment-variable-character = %x5C ; '\' Beginning of escape sequence ( %x22 ; '"' quotation mark U+0022 / %x5C ; '\' reverse solidus U+005C / %x61 ; 'a' alert U+0007 / %x62 ; 'b' backspace U+0008 / %x66 ; 'f' form feed U+000C / %x6E ; 'n' line feed U+000A / %x72 ; 'r' carriage return U+000D / %x74 ; 't' tab U+0009 / %x76 ; 'v' vertical tab U+000B ) ; Printable characters except double quote, backslash and equals / %x20-21 ; %x22 = '"' / %x23-3C ; %x3D = '=' / %x3E-5B ; %x5C = "\" / %x5D-7E import-type = missing / local / http / env hash = %x73.68.61.32.35.36.3a 64HEXDIG ; "sha256:XXX...XXX" import-hashed = import-type [ whsp hash ] ; "http://example.com" ; "./foo/bar" ; "env:FOO" import = import-hashed [ whsp as whsp1 Text ] expression = ; "\(x : a) -> b" lambda whsp "(" whsp nonreserved-label whsp ":" whsp1 expression whsp ")" whsp arrow whsp expression ; "if a then b else c" / if whsp1 expression whsp then whsp1 expression whsp else whsp1 expression ; "let x : t = e1 in e2" ; "let x = e1 in e2" ; "let x = e1 let y = e2 in e3" / 1*let-binding in whsp1 expression ; "forall (x : a) -> b" / forall whsp "(" whsp nonreserved-label whsp ":" whsp1 expression whsp ")" whsp arrow whsp expression ; "a -> b" ; ; NOTE: Backtrack if parsing this alternative fails / operator-expression whsp arrow whsp expression ; "merge e1 e2 : t" ; "merge e1 e2" / merge whsp1 import-expression whsp import-expression [ whsp ":" whsp1 application-expression ] ; "[] : List t" ; "[] : Optional t" ; "[x] : Optional t" ; ; NOTE: Backtrack if parsing this alternative fails since we can't tell ; from the opening bracket whether or not this will be an empty list or ; a non-empty list / "[" whsp (empty-collection / non-empty-optional) ; "x : t" / annotated-expression ; Nonempty-whitespace to disambiguate `env:VARIABLE` from type annotations annotated-expression = operator-expression [ whsp ":" whsp1 expression ] ; "let x = e1" let-binding = let whsp1 nonreserved-label whsp [ ":" whsp1 expression whsp ] "=" whsp expression whsp ; "] : List t" ; "] : Optional t" empty-collection = "]" whsp ":" whsp1 (List / Optional) whsp import-expression ; "x] : Optional t" non-empty-optional = expression whsp "]" whsp ":" whsp1 Optional whsp import-expression operator-expression = import-alt-expression ; Nonempty-whitespace to disambiguate `http://a/a?a` import-alt-expression = or-expression *(whsp "?" whsp1 or-expression) or-expression = plus-expression *(whsp "||" whsp plus-expression) ; Nonempty-whitespace to disambiguate `f +2` plus-expression = text-append-expression *(whsp "+" whsp1 text-append-expression) text-append-expression = list-append-expression *(whsp "++" whsp list-append-expression) list-append-expression = and-expression *(whsp "#" whsp and-expression) and-expression = combine-expression *(whsp "&&" whsp combine-expression) combine-expression = prefer-expression *(whsp combine whsp prefer-expression) prefer-expression = combine-types-expression *(whsp prefer whsp combine-types-expression) combine-types-expression = times-expression *(whsp combine-types whsp times-expression) times-expression = equal-expression *(whsp "*" whsp equal-expression) equal-expression = not-equal-expression *(whsp "==" whsp not-equal-expression) not-equal-expression = application-expression *(whsp "!=" whsp application-expression) ; Import expressions need to be separated by some whitespace, otherwise there ; would be ambiguity: `./ab` could be interpreted as "import the file `./ab`", ; or "apply the import `./a` to label `b`" application-expression = [ Some whsp1 ] import-expression *(whsp1 import-expression) import-expression = import / selector-expression ; `record.field` extracts one field of a record ; ; `record.{ field0, field1, field2 }` projects out several fields of a record ; ; NOTE: Backtrack when parsing the `*("." ...)`. The reason why is that you ; can't tell from parsing just the period whether "foo." will become "foo.bar" ; (i.e. accessing field `bar` of the record `foo`) or `foo./bar` (i.e. applying ; the function `foo` to the relative path `./bar`) selector-expression = primitive-expression *(whsp "." whsp selector) selector = any-label / labels labels = "{" whsp [ any-label whsp *("," whsp any-label whsp) ] "}" ; NOTE: Backtrack when parsing the first three alternatives (i.e. the numeric ; literals). This is because they share leading characters in common primitive-expression = ; "2.0" double-literal ; "2" / natural-literal ; "+2" / integer-literal ; '"ABC"' / text-literal ; "{ foo = 1 , bar = True }" ; "{ foo : Integer, bar : Bool }" / "{" whsp record-type-or-literal whsp "}" ; "< Foo : Integer | Bar : Bool >" ; "< Foo : Integer | Bar = True | Baz : Bool >" ; "< Foo | Bar : Bool >" / "<" whsp union-type-or-literal whsp ">" ; "[1, 2, 3]" ; `empty-collection` handles empty lists / non-empty-list-literal ; "x" ; "x@2" / identifier ; "( e )" / "(" complete-expression ")" record-type-or-literal = empty-record-literal / non-empty-record-type-or-literal / empty-record-type empty-record-literal = "=" empty-record-type = "" non-empty-record-type-or-literal = any-label whsp (non-empty-record-literal / non-empty-record-type) non-empty-record-type = ":" whsp1 expression *(whsp "," whsp record-type-entry) record-type-entry = any-label whsp ":" whsp1 expression non-empty-record-literal = "=" whsp expression *(whsp "," whsp record-literal-entry) record-literal-entry = any-label whsp "=" whsp expression union-type-or-literal = non-empty-union-type-or-literal / empty-union-type empty-union-type = "" non-empty-union-type-or-literal = any-label [ whsp ( union-literal-variant-value / union-type-or-literal-variant-type) ] ; = True | ... union-literal-variant-value = "=" whsp expression *(whsp "|" whsp union-type-entry) union-type-entry = any-label whsp ":" whsp1 expression ; : Integer | ... ; | ... union-type-or-literal-variant-type = [ ":" whsp1 expression ] [ whsp "|" whsp non-empty-union-type-or-literal ] non-empty-list-literal = "[" whsp expression whsp *("," whsp expression whsp) "]" ; This just adds surrounding whitespace for the top-level of the program complete-expression = whsp expression whsp