The way TXR's lazy lists are implemented and used, the program ends up reading ahead by one line, which is not nice for processing a stream interactively, such as implementing a command interpreter.

This is because after processing a line of input, there is a cdr(data) operation performed to march down the list to consume it. But cdr has to read from the file in order to determine whether to return nil. This is necessary to maintain a strict mapping in the sense that an N line file corresponds to a list with N conses, and an empty file to an empty list (nil).

There could be an interactive mode which relaxes this requirement. A different kind of lazy list could be used whose cdr operation only a promise to read from the file, such that only the car operation actually reads the line.

A special keyword like @(next :interactive) could set up this mode.