[oedo]

Declaring a function with `lambda` prepends runtime nil checks for each argument to the function body (excluding optional args-- specified with a question mark, eg. `?x`-- and variable arguments, eg. `...`[1]).

This Fennel:

  (fn foo [x y ...]
    (print x y ...))

  (lambda bar [x ?y ...]
    (print x ?y ...))

...transpiles to this Lua:

  local function foo(x, y, ...)
    return print(x, y, ...)
  end
  local function bar(x, _3fy, ...)
    _G.assert((nil ~= x), "Missing argument x on test.fnl:3")
    return print(x, _3fy, ...)
  end

[1] Fennel's alternative `& xs` vargs format does create a runtime check in functions declared with lambda, but it always evaluates to true because it simply nil-checks the table (here `xs`) that it dumps Lua `...` vargs into.