purity question

[Seb via Digitalmars-d-learn]

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/hpxxghbiomtitrmwendu@forum.dlang.org
- http://forum.dlang.org/post/nfhqvffqtkfsxjewgeix@forum.dlang.org