Grammar for the interpreter language in EBNF syntax:
function ::= fn-keyword fn-name { identifier } fn-operator expression
fn-name ::= identifier
fn-operator ::= '=>'
fn-keyword ::= 'fn'
expression ::= factor | expression operator expression
factor ::= number | identifier | assignment | '(' expression ')' | function-call
assignment ::= identifier '=' expression
function-call ::= fn-name { expression }
operator ::= '+' | '-' | '*' | '/' | '%'
identifier ::= letter | '_' { identifier-char }
identifier-char ::= '_' | letter | digit
number ::= { digit } [ '.' digit { digit } ]
letter ::= 'a' | 'b' | ... | 'y' | 'z' | 'A' | 'B' | ... | 'Y' | 'Z'
digit ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'