It's a great question. The answer is `pure` came along after D was already established with a lot of existing code. Making it the default would break pretty much all of it.

Part of the problem is `pure` is transitive, i.e. a pure function calling an impure one is not allowed. This makes it hard to incrementally make the code pure.

But when one succeeds at it, it is very satisfying.

There is an escape hatch, however. Prefixing a statement with `debug` allows the dependent statement to be impure. This is very handy when one needs to insert some print statements to debug a program. (Calls to printf are impure!)