unikernel/duniverse/ppxlib/examples/simple-extension-rewriter/README.md

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+# ppx_get_env
+
+This folder contains an example of a very simple ppx rewriter that will expand
+`[%get_env "SOME_ENV_VAR"]` into the value of the env variable `SOME_ENV_VAR` at compile time,
+as a string.
+
+E.g., assuming we set `MY_VAR="foo"`, it will turn:
+
+```ocaml
+let () = print_string [%get_env "MY_VAR"]
+```
+
+into:
+
+```ocaml
+let () = print_string "foo"
+```
+
+Note that this is just a toy example and we'd actually advise you against this type of ppx
+that have side effects or rely heavily on the file system or env variables unless you absolutely know
+what you are doing.
+
+In particular in this case it won't work well with dune since dune won't know about the dependency
+on the env variables specified in the extension's payload.