Example:

var v = Immutable.Vector(1, 2, 3)
v.toString() // "Vector [ 1, 2, 3 ]"
var v2 = v.map(function (x) { return x + 1; })
v2.toString() // "Seq [ 2, 3, 4 ]"
v2.delete(0) // TypeError: undefined is not a function

So, one must issue toVector() after the transformation to get original structure type back. This is very inconvenient for a library user.

I understand this is due to laziness so possible options may be:

• provide strict versions of map/filter/etc;
• somehow preserve set of methods and observable behavior of the original data structure;
• provide single method replacing toVector/toMap/etc and allowing to get original data structure after transformation back, e.g. force();
• ...