bnf

program -> form* EOF

form -> (expression | assignment)

assignment -> "let" IDENT "=" expression

expression -> "if" logic_or "then" expression (else expression)?
            | "fn" "(" params? ")" "->" expression 
            | block
            | logic_or

params -> IDENT ("," IDENT)*

block -> "do" form* "done" ";"

logic_or -> logic_and ("or" logic_and)*

logic_and -> equality ("and" equality)*

equality -> comparison (("==" | "!=") comparison)*

comparison -> addition (("<" | ">" | "<=" | ">=") addition)*

addition -> multiplication (("+" | "-") multiplication)*

multiplication -> unary (("*" | "/") unary)*

unary -> ("!" | "-") unary | call

call -> primary ("(" args? ")")*

args -> expression ("," expression)*

primary -> NUMBER | STRING | IDENT
         | "true" | "false" | "nil"
         | "(" expression ")"