// PEG grammar for a minimal programming language

Program
  = (_ Function / Statement _)*

Function
  = "fun" _ id:Identifier _ "(" Parameters ")" _ (":" _ Identifier _)? _ (Statement)* _ "end"

Parameter = Identifier _ ":" _ Identifier

Parameters = (Parameter (_ "," _ Parameter)*)?

Statement
  = Variable
  / Assignment
  / Repeat
  / While
  / If
  / Expression

Variable
  = "var" _ id:Identifier _ (":" _ Identifier _)? "=" Expression

Assignment
  = id:Identifier _ "=" _ Expression

Repeat
  = "repeat" _ Expression _ "times" _ (Statement)* _ "end"

While
  = "while" _ Expression _ "do" _ (Statement)* _ "end"

If
  = "if" _ Expression _ "then" _ (Statement)* _ ("elseif" _ Expression _ "then" _ (Statement)*)* ("else" _ (Statement)*)? _ "end"

Expression
  = head:Term tail:(_ ("+" / "-") _ Term)*

Term
  = head:Factor tail:(_ ("*" / "/") _ Factor)*

Factor
  = "(" _ expr:Expression _ ")"
  / Integer
  / String
  / CallOrIdentifier

CallOrIdentifier "function call or variable name"
  = Identifier (Arguments)?

Arguments "arguments"
  = "(" _ (Expression ( _ "," Expression)*) ")"

Integer "integer"
  = [0-9]+

String "string"
  = '"' chars:DoubleStringCharacter* '"' {
      return { type: "DoubleString", value: JSON.stringify(chars.join("")) };
    }
  / "'" chars:SingleStringCharacter* "'" {
      return { type: "SingleString", value: JSON.stringify(chars.join("")) };
    }

DoubleStringCharacter
  = '\\' '"' { return '"'; }
  / !'"' SourceCharacter { return text(); }

SingleStringCharacter
  = '\\' "'" { return "'"; }
  / !"'" SourceCharacter { return text(); }

SourceCharacter
  = .

Identifier "identifier"
  = !Reserved [a-zA-Z_]+

Reserved
  = "var"
  / "fun"
  / "end"
  / "repeat"
  / "while"
  / "if"
  / "then"
  / "else"
  / "elseif"
  / "times"

_ "whitespace"
  = [ \t\n\r]*