Should one read previous implementation? That's a difficult question. If you're worried about losing your perspective from seeing a single solution, then perhaps you're right that you shouldn't.

But it's possible that merely implementing it in a different [more functional] language would encourage sufficiently different patterns to allow you to keep perspective.

With all of this said, I am a huge fan of both C++ and Haskell. I had just had an internal realization that these stories might not be extracting the positive effect that a rewrite might have on a code base [even though, as you say, that is likely not relevant here].