The finale runs a batch of real expressions through the finished library, asserting the exact result of each well-formed one and the exact error of each malformed one. Every layer you built proves itself at once.
Evaluate a batch of expressions to their exact results, and confirm malformed ones report the right errors.
This is the promise the whole project was built to keep: a real expression
evaluator. The well-formed batch exercises every layer at once. 2 + 3 * 4 proves
precedence, (2 + 3) * 4 proves grouping overrides it, 2 ^ 3 ^ 2 proves right
associativity, -2 ^ 2 proves the unary-minus convention, sqrt(16) + abs(-5) proves
functions, and x * 2 + max(a, b) proves variables and precedence together. Each
reduces to one exact number because the tokenizer, the Pratt parser, and the tree
walk all agree.
The malformed batch closes the loop on robustness: a stray operator, an unclosed parenthesis, a division by zero, and an unknown variable each come back as a clear message naming what went wrong and the position where. From a tokenizer that could only recognize a single number, you have built the honest core of a calculator: a Pratt parser whose binding powers capture all of precedence and associativity, an evaluator over float64 with variables and built-in functions, and errors that point exactly at the problem. The same technique scales straight up to real language parsers, and now it is yours.
good := []struct{ in string; env Env; want float64 }{{"2 + 3 * 4", nil, 14}, {"(2 + 3) * 4", nil, 20},{"2 ^ 3 ^ 2", nil, 512}, {"-2 ^ 2", nil, -4},{"sqrt(16) + abs(-5)", nil, 9},{"x * 2 + max(a, b)", Env{"x": 3, "a": 1, "b": 7}, 13},}for _, c := range good {got, err := EvalString(c.in, c.env) // err nil, got == c.want}