Purity of delegate-style toString

[ag0aep6g]

On 05/01/2018 01:44 PM, Per Nordlöw wrote:
> In which cases (if any) is it possible to make a delegate-style 
> implementation of toString such as
> 
>      void toString(scope void delegate(const(char)[]) sink) const 
> @trusted pure
>      {
>          // sink("...");
>          // sink.formattedWrite!"..."(...);
>      }
> 
> pure?

You have to mark `sink` as pure, too:

     void toString(scope void delegate (const(char)[]) pure sink)
         const @trusted pure

Then the toString method itself works, but it may not be callable by 
other code that wants to use an impure sink.

For example, `format` works, but `writeln` doesn't:

----
struct S
{
     void toString(scope void delegate(const(char)[]) pure sink)
         const @trusted pure
     {
         import std.format: formattedWrite;
         sink("...");
         sink.formattedWrite!"%s"(" ...");
     }
}
void main()
{
     import std.format: format;
     import std.stdio: writeln;
     writeln(format("%s", S())); /* Ok. Prints "... ...". */
     writeln(S()); /* Nope. writeln would like to use an impure sink. */
}
----

By the way, you shouldn't mark toString as @trusted when `sink` is @system.