Discussion Thread: DIP 1033--Implicit Conversion of Expressions to Delegates--Final Review

Q. Schroll qs.il.paperinik at gmail.com
Sun Nov 22 20:49:27 UTC 2020

In the Feedback thread, Walter wrote:

> All this [the proposed changes] does is replace the 'lazy' with 
> the delegate syntax. D already uses this for variadic lazy 
> arguments, and nothing has come up bad about it.

Have you ever tried to use attributes on them? It's a mess. The 
expressions converted to delegates are not at all equivalent to 
literal delegates when attributes are at play.

int fun(int delegate() pure [] xs...) pure
     foreach (x; xs)
     if (auto value = x())
         return value;
     return 0;

void impureContext()
     import std.stdio : write;
     int impureFactory() { write(""); return 0; }

     fun(impureFactory());        // passes, but why??
     fun(&impureFactory);         // fails, no surprise
     fun(() => impureFactory());  // fails, no surprise

void pureContext() pure
     int pureFactory() pure { return 0; }

     fun(pureFactory());       // passes, no surprise
     fun(&pureFactory);        // passes, no surprise
     fun(() => pureFactory()); // passes, no surprise

The DIP should make clear, that "simply" lowering expressions to 
delegates isn't what's going on.

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