Module Merlin_analysis.Refactor_extract_regionSource

Warning: this feature is experimental.

Region extractor allows extracting arbitrary expressions into a fresh toplevel binding. The extractor detects bound variables inside the extracted expression and performs code generation accordingly.

For instance, let's assume that we want to extract the pi value oustide of the body of circle_area:

  let circle_area radius = 3.14159 *. (radius ** 2.)
                        (* ^^^^^^^ *)

The generated code will look like this:

  let const_name1 = 3.14159
  let circle_area radius = const_name1 *. (radius ** 2.)

Extraction also works on expressions that are functions:

  let all_empty l =
    List.for_all
      (function
        | [] -> true
        | _ -> false)
   (* ^^^^^^^^^^^^^^^ *)
      l

  let is_empty = (function | [] -> true | _ -> false)
  let all_empty l = List.for_all is_empty l

Let's look at a more complicated example where we want to extract the entire body of f:

  let rec f x = 10 + y + x
             (* ^^^^^^^^^^ *)

  and y = 80

Performing the extraction leads to this code:

  let rec f x = fun_name2 x

  and y = 80

  and fun_name2 x = 10 + y + x

We can see that extractor detects this kind of pattern and extracts the expression inside an and binding. It also substitutes the expression by a call to the fresh generated function with the correct parameters.

Finally, if there is no bound variable in the expression, a trailing unit parameter is added to the generated let binding in order to preserve the evaluation order. Let's extract the entire body of my_list:

  let my_list =
    print_endline "Wild side effect!";
    1 :: [ 2; 3; 4 ]

  let my_list () =
    print_endline "Wild side effect!";
    [ 1; 2; 3; 4 ]
  let f = my_list ()

Final remarks:

• Extraction currently works on any typedtree expression that doesn't have a ghost location. This restriction prevents the generation of invalid code.

• The generated code is pretty printed by the compiler libs and may not be formatted according to OCamlformat conventions.