`dhall text` and `dhall to-directory-tree` require the expression (or file leaves) to be Text. But what if it was able to also render expressions of type `(Text -> Text) -> Text` (or `(Text -> Text) -> directory tree`), and be given a shell argument as the `Text -> Text` ? This is essentially a very minimal FFI for dhall, since it doesn't require extending anything in the language. It just requires you to parameterize your program on that ffi function.