Very cool!

Does it replace identical expressions in the same scope? Like:

    let tau = 3.14 +. 3.14

becomes

    let pi = 3.14
    let tau = pi +. pi

?

EDIT: Or even crazier with function:

    let _ = (x + 1) + (y + 1)

becomes

    let plus_one a = a + 1
    let _ = (plus_one x) + (plus_one y)

(I ask this just out of curiosity. Even the "simpler" version is very impressive!)

    let tau =
      let pi = 3.14 in 
      pi + pi

    let fun_name1 pi = 
       pi + pi

    let tau = 
       let pi = 3.14 in 
       fun_name1 pi

I’ve been working on similar refactoring and grepping utility functions for vim, though mine are meant to work in any language.

The most recent one I’ve made runs 'git grep' on the word under cursor or on the visual selection and puts everything into a quick fix list.

Since it is generic, it works on any phrases as well, and helps me find prose snippets and phrases in docs or other writings.

Extract expression is such a common refactoring, happy to see support for it. I'm even more curious to see now if there are going to be more advanced refactoring possible. A List.map, to for loop, and back refactoring would be such a great thing to have in how I program.

Only thing that would go on my OCD nerve, is the lack of an empty newline when the show_markup function is extracted. Kind of "common sense" when writing top level bindings to leave some breathing room between them.