purity question

[ag0aep6g via Digitalmars-d-learn]

On 05/30/2017 11:12 AM, Rene Zwanenburg wrote:
> If malloc were marked as pure, wouldn't that mean it must return the 
> same pointer every time you call it with the same size?

D's `pure` mostly means: "does not access mutable state, and does not do 
input/output".

There is never a requirement that a function must return the same value 
for the same input. But a compiler is allowed to memoize the result of a 
`pure` function when it has no mutable indirections in its parameter and 
return types. Such a function is "strongly pure".

When there are mutable indirections, the function is "weakly pure". 
Weakly pure functions are not assumed to be memoizable, but "weakly 
pure" still has meaning:

* Can call weakly pure functions from strongly pure ones.

* When a weakly pure function has mutable indirections in the return 
type but not in the parameters (like malloc), then it must be returning 
freshly allocated memory. That means, the result cannot be referenced 
from anywhere else. So it can be converted to const/immutable/shared 
implicitly. The spec calls that a "pure factory function".

Repeating Biotronic's links, the spec and David Nadlinger's article are 
the go-to resources for D's take on purity:

https://dlang.org/spec/function.html#pure-functions
http://klickverbot.at/blog/2012/05/purity-in-d/