Sum type not always the best choice

gistfile1.txt

When you came from the Ocaml world, tree and AST are easly done with a sum type. The sum type have the very nice feature of 
not compiling if a case is missing, wich is super nice for code refactoring.

But a new kind of technique that only use module and function exist, it's called tagless final. Module are kind of golang package
with parameters. It's look like mother class but with types as parameters.

So you use simple function call to describe an ast, then you could interpret it the way you want. Typical examples are 
type checking, pretty print and execution.

I don't think it's possible to do it in Golang with interface and struct because of the lack of type as parameter. 
Maybe if a package could be created at compile time with some parameter, the technique became possible. 

[ENG] http://okmij.org/ftp/tagless-final/index.html
[fr] https://linuxfr.org/users/kantien/journaux/tagless-final-ou-l-art-de-l-interpretation-modulaire