[sqrt17]

"Mostly functional", in my eyes, works as well as contract-based specification of interfaces works. The property of being free of (surprising/undesirable) side effects is part of a contract that can, but should not be violated by an implementation. To me Meijer's argument sounds like a strawman, since the alternatives to encapsulating side-effects in the terms of a (informal, software) contract are all not very appealing.

Declarative sublanguages such as LINQ work precisely because of this "contract" of being mostly-functional, even if the work behind the scenes (database accesses etc.) certainly changes the state of the world.