[kazinator]

Mostly we have eval-when because of outdated defaults that are worth re-examining.

A Lisp compiler today should by default evaluate every top level form that are compiles, unless the program opts out of it.

I made the decision in TXR Lisp and it's so much nicer that way.

There are fewer surprises and less need for boilerplate for compile time evaluation control. The most you usually have to do is tell the compiler not to run that form which starts your program: for instance (compile-only (main)). In a big program with many files that could well be the one and only piece of evaluation control for the file compiler.

The downside of evaluating everything is that these definitions sit in the compiler's environment. This pollution would have been a big deal when the entire machine is running a single Lisp image. Today I can spin up a process for the compiling. All those definitions that are not relevant to the compile job go away when that exits. My compiler uses a fraction of the memory of something like GCC, so I don't have to worry that these definitions are taking up space during compilation; i.e. that things which could be written to the object file and then discarded from memory are not being discarded.

Note how when eval-when is used, it's the club sandwich 99% of the time: all three toppings, :compile-toplevel, :load-toplevel, :execute are present. The ergonomics are not very good. There are situations in which it would make sense to only use some of these but they rarely come up.