Pure functions as initializers for immutable structures?

[Don]

Michel Fortin wrote:
> On 2010-10-18 14:31:26 -0400, Tomek Sowiński <just at ask.me> said:
> 
>> Call me crazy, but I think it is safe to implicitly convert a pure  
>> function's return value to immutable. What you think?
> 
> Well, it depends on the arguments of the pure function. Here's two cases 
> where it won't work.
[snip]
> So I'd precise your assertion by saying it is safe to implicitly convert 
> a pure function's return value to immutable, but only when all the 
> arguments you feed to it are immutable.

Tomek stated that at the end of his original post. I think he is correct.
And no, I don't think it would be difficult to implement.