It’s also an odd feature of a programming language. Imagine the classical equivalent:

    square : constant integer -> constant integer

square(2) is 4. square(4) is 8. square(readint()) doesn’t compile because readint() may return a mixture (if, say the user rolled a die and entered the result on the keyboard), so the input isn’t pure and you can’t square it. This is, of course, almost useless.

(Amusingly, gcc can do this. asm’s "i" has precisely this restriction. Use with caution.)