Codegen is more and more rare these days, because languages have so many tools to help you write less code - like generics. LLMs could, theoretically, help you crank out similar repetitive implementations of things.
Gleam doesn’t support interfaces. Not generics. You are completely right.
Haskell allows both sorts of generics. In Haskell parlance they call this higher-kinded polymorphism and the generic version of map they call fmap (as a method of the class Functor).
Gauche has a generic sequence interface which is great, and it's one of the reasons as a Python user I like Gauche as my "daily driver" Scheme.
It might be the same with gleam, with first version in 2019 and 1.0 in 2024. The language authors might think they are either uneeded and lead to anti patterns, or are waiting to see the best way to implement them.
Which feels super strange, but doesn't seem to really be a problem, e.g. imagine a language where you'd write
fun sum_all_numbers(Iterable<T> it) { it.fold(...) } # an interface
sum_all_numbers(a_list) # a list implements iterable
fun sum_all_numbers(Iterator<T> it) { it.fold(...) } # a concrete type
sum_all_numbers(a_list.iterator) # get the caller to build the right object
By the developers own action of adding generics ultimately the golang team admits they were wrong or that generics are better. If gleam gets popular I think much of the same will occur.
There’s simply too much repeated code without generics. I tried writing a parser combinator in gleam and it wasn’t pretty.