If your control structures are expressions then they can lead to code that the maintainers of the language might not want to promote. Remember that language design is not just about being as flexible as possible. If it were we would have stopped with Lisp. Languages are also designed with readability, portability, and simplicity in mind.

Imagine if you could write this (nonsense) code in Java:

bool a = (while (b) { if (c) { b=d;} else { b=e; }});

While it is efficient, some may balk at how “implicit” it is.

Of course I'd say this as a lisper, but as long as the return values of everything is intuitive and well defined, I don't see an inherent problem with such form.

Indeed, if anything, if the intention is to communicate that the Boolean value is the result of some process that runs through a while loop, then explicitly saying that in the assignment seems to me to almost be the best way to do it :\

ymmv.

Now deeply nested while/assignment/ifs, that's a bad-pattern, but I'm not inherently convinced that's an expression/statement problem.

I've worked extensively with both Java and Clojure, and the latter is far easier to read in my experience.

> Anyone know the language design rationale behind the way Java does it?

Because C did it, because Algol did it, because Fortran did it, because that is how assembly code works. Expressions require code to be compiled to something that uses a temporary value stack (or equivalent, like what some continuation-passing style compilers do by allocating values from garbage-collected memory), which, along with subroutines, was an exotic technology well into the 1960s.

Algol (60) had if-expressions, as did Simula 67 (based on Algol): x := if (y = 0) then 0 else 4 / y

In general the split between expressions and statements has been disastrous IMHO.

I remember the first time I saw 'constexpr' in C++ I was excited, until I realized it's really conststatement.

The elaborate syntactical decorations of most languages drive away from this, unfortunately.

In Kotlin, if is an expression: https://kotlinlang.org/docs/reference/control-flow.html. This is why the language drops Java's ternary operator.