Map, filter and pure functions

[realhet]

Hi,

I have an input range.
I use the map! on it to transform using a function.
Finally I use filter! on the transformed data.

When I do a foreach on this, i noticed, that the transform 
function is called twice for each element.
If I remove the filter! it does only one transform function call 
per element.

Is this because the function is NOT PURE, so Phobos thinks that 
the function can give different results for the same input?

If I modify the function to be pure, will the transform function 
be called only once?