Example Codemod

Here's an example of a type-driven codemod!

It finds functions that have a return type of Belt.Result.t(int, string), and wraps the error payloads with Unspecified.

So, for the file:

open Result;

let changeMe = (a, b, c) => Error("hello");

let nonCollapsible = something =>
  if (something > 2) {
    Result.Ok(something);
  } else {
    Error("bad news");
  };

It rewrites as:

open Result;

let changeMe = (a, b, c) => Error("hello");

let nonCollapsible = something =>
  if (something > 2) {
    Result.Ok(something);
  } else {
    Error(Unspecified("bad news"));
  };

Note that the first Error() was not wrapped, because we specified that the 'ok type had to be int. We could have left the ok type argument blank, and it would have picked that one up too.