[halostatue]

I generally think about "types" in terms of capabilities more than shapes. When I write Ruby, I don’t generally think "this parameter must be an array". Instead, I think "this parameter must be an Enumerable". Or I think "incoming objects must behave like strings" (that is, they implement #to_str or are Strings)…although most of the time, I would really think "incoming objects must have useful string representations".

In Elixir, I do think about shapes more than capabilities (because Elixir is not OO), but with pattern matching, I can either specify "this must be a MyApp.Account struct" (which is just a fancy map) or I can specify "we will handle any map that has the keys X and Y, and Y must be a map itself".

I replicate this more formally when writing TypeScript, usually by building up type definitions and specifying those.