List Sugar
The list operations work, but building lists is still verbose:
call(call("cons", 1), call(call("cons", 2), call(call("cons", 3), "nil")))With sugar:
cons(1, cons(2, cons(3, "nil")))Write two helpers following the same pattern as the pair sugar. cons(x, rest) builds the curried call, isEmpty(list) builds a single call.
What You’ve Just Built
You started this module with an evaluator that could add numbers. Now you have a language with variables, functions, conditionals, recursion, closures, syntactic sugar, pairs, and lists. And along the way you added a new type, nil, that exists for one reason: to mark the end of a list so recursive functions can stop.
Every type in the language serves a different purpose. Numbers represent quantity. Booleans represent choices. Functions represent computation. And nil represents absence. A type with exactly one value, carrying no information except “there is nothing here.”