A let expression isn't parameterised by x and y but binds their meaning, and the bound variable isn't free in the expression, and you can't substitute it at all. A simple example is an expression that prints the syntactic name of its argument. You can do that in C, C++, Lisp and Rust, but not in Zig (this is OK, because Zig obviates the need for that with excellent backtraces). But this means that you can understand all Zig code as simple Zig code, and that there is no complex meta-language with super-powers as there is in all those other languages I mentioned.