Code from github.com/anakrish/rego-rs

Authored by anakrish and mingweishih

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

docs/grammar.md

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+
+```
+module: package  imports { rule }
+
+package: "package" path-ref
+
+imports: { "import" path-ref [ "as" var ] }
+
+path-ref: path-ref NO_WS "." NO_WS IDENT
+| path-ref NO_WS "[" STRING "]"
+| IDENT
+
+rule: default-rule
+| spec-rule
+
+default-rule: "default" rule-ref assign-op term
+
+spec-rule: rule-head rule-bodies
+
+rule-head: func-rule
+| contains-rule
+| object-rule
+| set-rule
+| compr-rule
+
+func-rule: rule-ref "(" term { "," term } [","] ")" [ rule-assign ]
+
+contains-rule: rule-ref "contains" or-expr
+
+object-rule: rule-ref NO_WS "[" membership-expr "]" rule-assign
+
+set-rule: rule-ref NO_WS "[" membership-expr "]"
+
+compr-rule: rule-ref [ rule-assign ]
+
+rule-ref: rule-ref NO_WS  "." NO_WS var
+| path-ref NO_WS "[" membership-expr "]"
+| var
+
+rule-assign: assign-op membership-expr
+
+rule-bodies: "if"  "{" query "}" alternatives
+| "if" literal-stmt alternatives
+| "{" query "}" alternatives
+
+alternatives: query-blocks
+| else-blocks
+
+query-blocks: { "{" query "}" }
+
+else-blocks: { else-block }
+
+else-block: "else" [rule-assign] "if" "{" query "}"
+| "else" [rule-assign] "if" literal-stmt
+| "else" [rule-assign] "{" query "}"
+
+assign-op: "=" | ":="
+
+query: literal-stmt { sep  literal-stmt }
+
+sep: ";" | "\n" | "\r\n"
+
+literal-stmt: literal with-modifiers
+
+with-modifiers: { "with" path-ref "as" in-expr }
+
+literal: some
+| every
+| expr
+| not-expr
+
+some: some-vars
+| some-in
+
+some-vars: "some" var { "," var }
+
+some-in: "some" ref [ "," ref ] "in"
+
+every: "every" var [ "," var ] "in" bool-expr "{" query "}"
+
+expr: assign-expr
+
+not-expr: "not" assign-expr
+
+assign-expr: ref assign-op membership-expr
+
+membership-expr: membership-expr "in" bool-expr
+| bool-expr "," bool-expr
+| bool-expr
+
+in-expr: in-expr "in" bool-expr
+| bool-expr
+
+bool-expr: bool-expr bool-op or-expr
+| or-expr
+
+bool-op: "<" | "<=" | "==" | ">=" | ">" | "!="
+
+or-expr: or-expr "|" and-expr
+| and-expr
+
+and-expr: and-expr "&" arith-expr
+| arith-expr
+
+arith-expr: arith-expr ("+" | "-") mul-div-expr
+| mul-div-expr
+
+mul-div-expr: mul-div-expr ("*" | "/") term
+| term
+
+term: ref
+
+ref: scalar-or-var
+| compr-set-or-object
+| compr-or-array
+| unary-expr
+| parens-expr
+| ref-dot
+| ref-brack
+| call-expr
+
+ref-dot: ref NO_WS "." NO_WS var
+
+ref-brack: ref NO_WS "[" in-expr "]"
+
+call-expr: path-ref NO_WS "(" call-args  [","] ")"
+
+call-args: in-expr { "," in-expr }
+
+parens-expr: "(" membership-expr ")"
+
+unary-expr: "-" in-expr
+
+compr-set-or-object: set-compr
+| set
+| object-compr
+| object
+
+set-compr: "{" compr "}"
+
+# Set must have at least one item.
+set: "{" in-expr  { "," in-expr } [","] "}"
+| "set(" ")" # empty set
+
+object: "{" field { "," field } [","] "}"
+| "{" "}" # empty object
+
+field: in-expr ":" in-expr
+
+# Comprehension or array
+compr-or-array: array-compr
+| array
+
+array-compr: "[" compr "]"
+
+array: "[" in-expr { "," in-expr } [","] "]"
+| "[" "]"  # Empty array
+
+# Comprehension
+compr: ref "|" query
+
+scalar-or-var: var
+| NUMBER
+| STRING
+| RAWSTRING
+| "null"
+| "true"
+| "false"
+
+var: IDENT
+| non-imported-future-keyword
+
+non-imported-future-future-keyword: "contains" | "every" | "if" | "in"
+```