module = {statement} (expression) EOF ;

statement = expression ";" ;

expression = literal 
           | block
           | unary_op expression
           | expression binary_op expression
           | if
           | while
           | let
           | import
           | fn_def
           | fn_extern
           | fn_call
           | "return" expression
           ;

literal = NUMBER
        | STRING
        | BOOLEAN
        ;

block = "{" {statement} (expression) "}" ;

unary_op = "-"
         | "!"
         ;

binary_op = "+"
          | "-"
          | "*"
          | "/"
          | "<"
          | "<="
          | ">"
          | ">="
          | "=="
          | "!="
          | "="
          ;

if = "if" expression expression {"else" "if" expression expression} ["else" expression] ;

while = "while" expression expression ;

type_signature = IDENTIFIER ;

variable_signature = IDENTIFIER ":" [type_signature] ;

let = "let" ["mut"] variable_signature ["=" expression] ;

import = "import" STRING ;

function_signature = "(" [variable_signature {"," variable_signature}] ")" ["->" type_signature] ;

fn_def = function_signature expression ;

fn_extern = "extern" IDENTIFIER function_signature;

fn_call = IDENTIFIER "(" {variable_signature} ")" ;