I just got it! (invariant/const)

[Janice Caron]

On 10/04/2008, Lionello Lunesu <lionello at lunesu.remove.com> wrote:
>  Why? If pure-ness (uh, purity?) can be checked, it can also be deducted

"purity" is the right word. (But "deducted" should be "deduced" :-))

> (*), so the compiler could optimize/complain in either case.

It would slow down compilation no end though. For example, a pure
function is not allowed to call non-pure functions. Without the "pure"
keyword, you'd have to follow through every function call recursively,
and that could take a long time.

Moreover, just because the compiler decides the function is impure,
that doesn't necessarily mean that the programmer /intended/ for it to
be impure. Perhaps there was a bug? Perhaps there was a silly, simple,
one line bug, that the programmer could fix in a jiffy if the compiler
would only say "Error: Line 53: Pure function cannot do XYZ"? It is
far better for the programmer to document their intentions, so that
the compiler can check that those intentions are actually met.