On running code at compile time, what this reads as to me is evaluating constant expressions at compile time. We're thoroughly used to compilers optimising cons

On running code at compile time, what this reads as to me is evaluating constant expressions at compile time. We're thoroughly used to compilers optimising constant expressions away for us: "if 4 < 2 then a else b" will usually result in identical compiler output as just "b": "4" is constant, "2" is constant, "<" is constant, and the application of "<" to "4" and "2" is constant. What about "factorial 8" ? Assuming the compiler knows that "factorial" is a pure function, it also knows that "factorial 8" is a constant, and in theory it could evaluate that function and substitute in the result. In practice this won't generally happen automatically, because compile times would blow out something fierce. But in, say, C++, you can do this: constexpr unsigned factorial(unsigned n) { return n == 1 ? 1 : n * factorial(n - 1); } constexpr unsigned fac8 = factorial(8); And it will compile to the static int value. This looks very much like the author's proposal; reading the various rules around constant-initializer probably will help expose pitfalls. More interesting for me is partial evaluation. Constant folding evaluates fully static parts of a program to produce a static result, shifting that execution cost from run time to compile time. Partial evaluation also allows some of the program's dynamic inputs to be fixed to a static value, slaps a virtual "constexpr" on everything that now only has static inputs (and is a pure function), and reduces everything it can. Partial evaluation gets fun when you start applying it to certain types of program: partial evaluation of an interpreter with a given program's source as an input acts just like a compiler. Partial evaluation of a partial evaluator with an interpreter as fixed input produces a compiler. Partial evaluation of a partial evaluator with a partial evaluator as fixed input produces a compiler generator: feed it an interpreter for any language, get a compiler for that language.