immutable bug?

[Peter Alexander]

On Sunday, 12 January 2014 at 00:50:30 UTC, Manu wrote:
> On 12 January 2014 04:52, Adam D. Ruppe 
> <destructionator at gmail.com> wrote:
>
>> On Saturday, 11 January 2014 at 18:48:15 UTC, Maxim Fomin 
>> wrote:
>>
>>> It is legal exactly because function is marked as pure. 
>>> Result of pure
>>> function is implicitly convertible to immutable.
>>>
>>
>> It shouldn't be here though... the reason it is implicitly 
>> convertable is
>> that pure means the result is unique.
>
>
> Can you explain how this is true? I can't see anything about 
> the concept of
> purity that suggests the result should be unique...
> Pure just means given the same inputs, it will produce the same 
> outputs;
> external state can't affect the calculation.

How could the result not be unique, or at least immutable? Pure 
functions cannot read mutable global state, so any global state 
returned must be immutable. Strong pure functions can also only 
have immutable arguments, so anything returned from those will be 
immutable. The only other thing that can be returned must be 
created within the function, which will be unique, and safely 
converted to immutable.