Pull - Cut ties with BindBind, no need for BuildR #2828
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We modify `Bind` class to make the `step` into an abstract method. For the `BindBind` process, we modify the `bindBindAux` method (for applying continuation) to cut ties to the two Bind objects. An intuition for this is that a `BindBind` is just a node in a stack of continuations, applied to the terminal result of a pull. Once applied, the `BindBind` should not be used again. Cutting this ties allows us to remove the use of the `BuildR` intermediate representation, for the Uncons and StepLeg case interpreters, without this causing the memory leaks tests to fail.
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Description of changes
Bindclass to make thestepinto an abstract method.BindBindclass to make the references to the middle and end Bind objects mutable.bindBindAuxmethod (that applies the continuation) to cut the ties with the two Bind objects. That is, set both pointers to null. For an intuition as to why this is correct, it helps to think of aBindBindas a node in a stack of continuations, applied to the terminal result of a pull. Once aBindBindis applied, it should be popped and not used again.Cutting this ties allows us to remove the use of the
BuildRintermediate representation, for theUnconsandStepLegcase interpreters, without this causing the memory leak tests to fail.Reasons
The motivation for these changes are several. On one hand, the
BuildRis a bit of a hack to workaround a problem (the leak) not well understood. On the other hand, avoiding those continuations may shave some memory consumption. A smaller design motivation has always been to make thegofunction of the compile method sort of tail-recursive insideF. That is to say, in each recursive case the call togois the last effect action.Discussion
Removing the use of the intermediate
BuildRand just passing recursively the handlers forUnconsandStepLeghas been tried before, such as in #2445 or #2196. However, this caused reports of memory leaks that were recorded in issues, and were fixed by reverting those changes, as done in #2394. Since each of those memory leak reports was recorded into a minimised IntegrationTest, there is some confidence that these changes would not cause new leaks.