kanon

https://github.com/MaksimenkoPG/kanon

A YAML file becomes a frozen tree of nodes with dotted access, and the values that belong to the environment are declared in that same file. A read that misses a mandatory value fails naming every absent variable at once, and a typo in a key raises instead of returning nil. ERB tags keep working, so a file that already reads the environment that way needs no rewriting. The standard library only, with the config directory and the environment name injected from outside.