I think you're right - this might not be a good fit for Streamy to have the actual functionality. On the other hand, the function is pretty simple, so it's not that big of a deal whether memoization is actually implemented by us or uses a function from Quiver.

I do think 'memoize' as an optional argument is a feature that Streamy should provide, though. One common thing clients do is preprocess entities, computing fields that will never change. Memoization does this lazily.