The Read-Eval-Print-Loop can yield only integers. That is not very useful. I resist introducing more primitive data types for now. The following is a kludge but the least kludge I have thought of.
The language is purely functional and without side effects. I propose to add a print character operator P. Code with access to P, can print on the terminal. Such code must rely on the fact that (fa) consistently evaluates f before a. The other order, if consistent, would print the output backwards. Limiting access to a primordial function can be done with shadowing.
With such a primordial function S-expressions may be printed. A primordial S-expression printer would be possible, but less powerful. Also I want to do the logic in the λ calculus.
The primordial function P was added to the code here with two lines of new C code. They each have the identifier Putchar. That was too easy, but I have not yet thought of a real use for it—perhaps monads.