Conversation

I’ve come to the point where I’m removing stuff from my Lisp interpreter instead of adding it, as a sort of challenge.

Some Lisps have the conditional forms

(when <predicate> <expression>)

(unless ...)

which are complementary.

It turns out that you can define “when” and a standard “if-then-else” form in terms of “unless” alone.

I should hardly be surprised. All logic gates can be implemented with NAND gates. “unless” is NOT and conditional evaluation is AND, so...