On Fri, 27 Apr 2012 13:25:30 -0400, Joseph Rushton Wakeling
<joseph.wakel...@webdrake.net> wrote:
On 27/04/12 19:08, Steven Schveighoffer wrote:
weak purity, I think, is one of the most revolutionary ideas to come
from D.
Essentially, we get compiler-checked pure functions that can be written
imperatively instead of functionally.
I do agree with that; it's a very attractive aspect of D that a function
or object can be internally imperative but pure as far as the outside
world is concerned. Seeing that documented in Andrei's writings was
another "Wow!" moment for me about D. :-)
What concerned me here was that the way my reputation() function was set
up actually did change the public state of the object, which to my mind
isn't even weakly pure. But I just wrote a fix which, contrary to my
fears, didn't affect the speed of the program.
Weakly pure simply means it can be called by a strong-pure function. It
has no other benefits except for that, and is basically a normal non-pure
function otherwise. It's a weird thing, and it's hard to wrap your mind
around. I don't think anyone has ever done it before, so it's hard to
explain.
But the benefits are *enormous*. Now, instead of pure versions of so many
normal functions, much of the standard library can be marked as pure, and
callable from either pure or unpure functions. It opens up the whole
library to pure functions which would otherwise be a painstaking porting
effort.
If the function is weakly pure, then it cannot be optimized based on
purity, so
I don't think there is any way marking it pure *hurts*.
I was more concerned that the compiler wasn't identifying what to me was
a violation of purity. I'm fairly sure I can also find a way to make
some of those "nothrow" functions throw an error ...
It depends on what your definition of "purity" is. For D's definition,
it's pure, otherwise the compiler would reject it.
The definition of pure for D is, it cannot accept any shared data as
parameters, and it cannot access any global non-immutable data. That's
it. There's no requirement for immutability of parameters or results.
In hindsight, D's 'pure' keyword really should be named something else,
since it varies from the traditional definition of 'pure'. But it is what
it is now, and no changing it.
nothrow means it cannot throw an Exception. It can still throw an error,
or technically even something else derived from Throwable. This is
necessary, because otherwise, nothing could be nothrow (anything might
throw an out of memory error).
-Steve
