It's a runtime check. The following fennel: (fn add-1 [x] (+ x 1)) (lambda add-2 [x] (+ x 2)) transpiles to the following lua: local function ad

It's a runtime check. The following fennel: (fn add-1 [x] (+ x 1)) (lambda add-2 [x] (+ x 2)) transpiles to the following lua: local function add_1(x) return (x + 1) end local function add_2(x) _G.assert((nil ~= x), "Missing argument x on /home/sullyj3/tmp/fn-vs-lambda/fnl/x.fnl:3") return (x + 2) end return add_2