It is true you need to pass the environment around, but if the language you are implementing also has objects or structs then you will need to implement that an

It is true you need to pass the environment around, but if the language you are implementing also has objects or structs then you will need to implement that anyway. My main objection to the grand parent is the idea that functional languages are 10x more difficult to implement than mainstream languages. Maybe that is true for Haskell, but most functional features can be compiled efficiently without much additional complexity. The complexity added by closures for instance would be closer to 1% than 10x.