> And certainly when you end up copy-pasting code because you're using the same logic but the language doesn't let you express how it's the same, that's a problem with your language.

That is not the case. Sometimes language designers and maintainers _deliberately_ avoid language constructs that allow for more generic code. More generic is not always better, definitely not when you have to read and understand the code, and find a bug in it, for example. The more layers of abstractions you have, the harder it is to construct a model of it in your mind.

(An example of this approach is Erlang, where parameterized modules were introduced as an unsupported, "beta" feature at some point, then a few major versions later they were removed. Why? Take this opinion, for example:

"I would gladly reconsider my somewhat harshly critical position on parameterized modules if shown a clear example of how their use improves the readability or expressiveness of code. Until then I would urge that they didn't become an official part of the Erlang language."

http://erlang.org/pipermail/erlang-questions/2010-February/0...)