> But in a production system, functional purity (no side-effects) is a means to an end, and lazy evaluation comes at a pretty extreme performance cost. In theo

> But in a production system, functional purity (no side-effects) is a means to an end, and lazy evaluation comes at a pretty extreme performance cost. In theory yes, in practice... not in my experience. Or at least, things are almost always fast enough in web backends. > Not everyone disables lazy evaluation in production Haskell, because not everyone thinks they need to for performance, but I've definitely heard about it enough times that it seems like a relevant pattern. Most people don't disable lazy evaluation in the way I think you mean it, but they use either BangPatterns selectively or StrictData. Maybe there's widespread usage of `-XStrict` as well I don't know about. Anecdotally I can say that trying to enable `-XStrict` in production caused performance issues that prevented us from trialing it further. Laziness is frequently talked about as a negative, but it has big advantages in composition and code ordering like another reply talked about with regards to where bindings to enable truly top-down program design.