Pure does not imply const in D. If you want stronger
guarantees, just turn 'test' into a const (or immutable) member
function. In D, a function can change anything that is mutable
and reachable through its parameters, this includes the
implicit 'this' pointer. The reason for this design is simple:
There is no need to be overly restrictive, because the
additional restrictions are already trivially expressed in the
type system. Furthermore, any mutation of a parameter can be
turned into a less efficient protocol that only requires a
const pure function, so there would be no gain if pure implied
const, it would only make pure less useful.
void foo(int* x)pure{*x+=2;}
int bar(const(int)*x)pure{return *x+2;}
void main() pure{
int x, y;
foo(&x); // those two lines have
y = bar(&y); // equivalent functionality!
}
Only external data I was implying, that was not based off the
input arguments. Examples in the book refer that calculating Pi,
or the square root of 2 is a constant and will always result in
the same output, regardless the situation.
Quote TDPL pg 165:
"In D, a function is considered pure if returning a result is
it's only effect and the result depends only on the function's
arguments.
Also, pure functions can run literally in parallel because they
don't interact with the rest of the program except through their
result."
So... If we take a struct.
struct X {
int i;
pure int squaredPlus(int x) {
return x*x + i
}
alias squaredPlus sqp;
}
X st(15);
writeln(st.sqp(0)); //15
int i1 = st.sqp(10); st.i++;
int i2 = st.sqp(10); st.i++;
int i3 = st.sqp(10); st.i++;
int i4 = st.sqp(10); st.i++;
assert(i1 == 100); //pass/fail?
assert(i2 == 101); //pass/fail?
assert(i3 == 102); //pass/fail?
assert(i4 == 103); //pass/fail?
assert(s1.i == 104); //probably pass.
If the compiler can reorder or run these in parallel (for
optimization) or caches the result the first time (since it's
suppose to always return the same value), what's correct in this
case? This afterall isn't synchronized, even if it was, what's
correct behavior? Am I wrong in understanding this?
