On Sunday, 28 May 2017 at 23:49:16 UTC, Brad Roberts wrote:
Is there a mechanism for declaring something pure when it's
built from parts which individually aren't?
string foo(string s)
{
// do something arbitrarily complex with s that doesn't
touch globals or change global state except possibly state of
the heap or gc
return s;
}
Ali has answered this two years ago:
http://www.digitalmars.com/d/archives/digitalmars/D/learn/using_memset_withing_a_pure_function_74629.html#N74631
Copying for convenience:
If you want to live dangerously, you can use assumePure, which is
found
in one of the unittest blocks of std.traits:
import std.traits;
auto assumePure(T)(T t)
if (isFunctionPointer!T || isDelegate!T)
{
enum attrs = functionAttributes!T | FunctionAttribute.pure_;
return cast(SetFunctionAttributes!(T, functionLinkage!T,
attrs)) t;
}
int i = 0;
void foo()
{
++i; // foo accesses mutable module-level data
}
void bar() pure
{
auto pureFoo = assumePure(&foo);
pureFoo(); // <-- pure function is calling impure function
}
void main()
{
assert(i == 0);
bar();
assert(i == 1); // mutation through a pure function
}
It also came up in other discussions (the keyword is
`assumePure`), e.g.
- http://forum.dlang.org/post/hpxxghbiomtitrmwe...@forum.dlang.org
- http://forum.dlang.org/post/nfhqvffqtkfsxjewg...@forum.dlang.org
