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Blueprint pure nodes were initially intended to be similar to "functional pure". In other words, they're deterministic and produce no side effects (eg: doesn't mutate state). However, pure nodes have violated both conditions for a while now.
Instead, pure nodes are simply function nodes with output values and no visible exec pins. While this can be convenient, they come with a downside: the pure node is evaluated for each connected output. This can lead to unexpected performance issues if the node is expensive to evaluate. In the case of non-deterministic nodes, this can lead to unexpected behavior. In both cases, the user often needs to cull multiple outputs of a pure node and cache the result manually.
The solution here is to add support for toggling purity at the call site. When a function node is placed, right-clicking on it and selecting either "Hide Exec pins" or "Show Exec Pins" will toggle purity. Additionally, the meaning of BlueprintPure and the "Pure" check box changes slightly: it now means that the function node _defaults_ to a pure state when placed in a graph. However, it can be toggled to show its exec pins.
In future changes, we'll also reevaluate which common library functions should continue to default as pure.
#jira UE-196156
#rb dan.oconnor, jodon.karlik, ben.zeigler
[CL 35309072 by dave jones2 in ue5-main branch]