It seems pure ain't so pure after all

[Alex Rønne Petersen]

On 01-10-2012 07:40, Tommi wrote:
> import std.stdio;
>
> int pow2(int val) pure
> {
>      if (__ctfe)
>          return 6;
>      else
>          return val * val;
> }
>
> void main()
> {
>             assert(pow2(3) == 9);
>      static assert(pow2(3) == 6);
>
>      writeln("9 = 6 ... I knew it! '6' was faking it all along");
>      readln();
> }

This is a corner case.

__ctfe is there to allow special-cased CTFE code when absolutely 
necessary. By necessity, this separates a function into two worlds: 
compile time and run time.

As far as purity goes, pow2 *is* pure. It just does something different 
depending on whether you run it at compile time or run time. I don't see 
this as a problem in practice.

-- 
Alex Rønne Petersen
alex at lycus.org
http://lycus.org