I really like this. I've been following gcanti's work in tcomb for a while now, and I feel that it's way too unpopular for how well designed it is. I think I should just replace all my Immutable.js records with tcomb structs, and time is the only reason I haven't.

Does anyone know whether there's any immutable collections that work well with tcomb? I'm a bit weary of adding yet another data structure thing to my code (after Immutable and Ramda), so I'd prefer to replace Immutable by tcomb but I'm afraid that'll make add-to-immutable-sequence kind of operations significantly slower. Any tips?

Type checked state is known as "typestate". A Google search will bring up the typestate paper.