Exactly. We'd only call the interface 'Mappable' if the method was called 'map', and that's something that's come from mathematical terminology.

(I'd probably have called the 'Map' class 'Dictionary', if that hadn't already been taken!)

I'd avoid 'replaceEach', because it sounds like it mutates the receiver. Maybe 'withEachReplaced'? Way clunky. In Smalltalk, this method is called 'collect', which is better than 'map', to my eyes, but still not all that obvious [1].

[1] Smalltalk has the names collect, inject, select, and reject for its map, foreach, filter, and complementary filter methods, because of a song: http://smalltalkzen.wordpress.com/2011/02/02/arlo-guthrie-an...