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.

unikernel/duniverse/ppxlib/examples/simple-extension-rewriter/dune

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+(library
+ (name ppx_get_env)
+ (kind ppx_rewriter)
+ (libraries ppxlib))

unikernel/duniverse/ppxlib/examples/simple-extension-rewriter/ppx_get_env.ml

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+open Ppxlib
+
+let expand ~ctxt env_var =
+  let loc = Expansion_context.Extension.extension_point_loc ctxt in
+  match Sys.getenv env_var with
+  | value -> Ast_builder.Default.estring ~loc value
+  | exception Not_found ->
+      let ext =
+        Location.error_extensionf ~loc "The environement variable %s is unbound"
+          env_var
+      in
+      Ast_builder.Default.pexp_extension ~loc ext
+
+let my_extension =
+  Extension.V3.declare "get_env" Extension.Context.expression
+    Ast_pattern.(single_expr_payload (estring __))
+    expand
+
+let rule = Ppxlib.Context_free.Rule.extension my_extension
+let () = Driver.register_transformation ~rules:[ rule ] "get_env"