unikernel/duniverse/ppxlib/test/expansion_context/run.t

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+The three context fields can be accessed in a rewriter, both from within an implementation file
+
+  $ echo "let x = 0" > file.ml
+  $ ./standalone_print_ctxt.exe file.ml | grep -E 'tool_name|input_name|file_path'
+  tool_name: ppx_driver
+  input_name: file.ml
+  file_path: file.ml
+
+and from within an interface file
+
+  $ echo "val x : int" > file.mli
+  $ ./standalone_print_ctxt.exe file.mli | grep -E 'tool_name|input_name|file_path'
+  tool_name: ppx_driver
+  input_name: file.mli
+  file_path: file.mli
+
+In most cases, the input name and the file path coincide. But there are some exceptions, such as
+1. empty files
+
+  $ touch empty_file.ml
+  $ ./standalone_print_ctxt.exe empty_file.ml | grep -E 'input_name|file_path'
+  input_name: empty_file.ml
+  file_path: 
+
+2. files with directives pointing to other files
+
+  $ cat > directive.ml << EOF
+  > # 1 "file.ml"
+  > let y = 0
+  > EOF
+  $ ./standalone_print_ctxt.exe directive.ml | grep -E 'input_name|file_path'
+  input_name: directive.ml
+  file_path: file.ml
+
+3. using `map_structure` (or `map_signature`)
+
+  $ echo "let x = 0" | ./map_structure_print_ctxt.exe | grep -E 'input_name|file_path'
+  input_name: _none_
+  file_path: lexbuf_pos_fname