Usage

let foo_bar = [%VAR_NAME]      in
let foo_bar = [%Var_name]      in
let foo_bar = [%var_name]
and foo_bar = [%var_dash_name] in

let%map       foo_bar = [%Var_name] in
let%bind      foo_bar = [%Var_name] in

let%map_open.Command  foo_bar = [%var_dash_name] in
let%bind_open.Command foo_bar = [%var_dash_name] in

let%map_open.Command foo_bar = [%var_dash_name]
and                  baz_qux = [%var_dash_name]
in

let%mapn foo = [%Var_name]
and      bar = [%Var_name]
and      baz = [%Var_name] in

[%map_open.Command
  let foo_bar = [%var_dash_name]
  and baz_qux = [%var_dash_name] in
  ...
  ]

module Foo_Bar = struct
  let name = [%Module_name]
  let name = [%MODULE_NAME]
  let name = [%module_name]
  let name = [%module_dash_name]
end


let name = [%Library_name]
let name = [%library_name]
let name = [%LIBRARY_NAME]
let name = [%library_dash_name]

produces:

let foo_bar = "FOO_BAR" in
let foo_bar = "Foo_bar" in
let foo_bar = "foo_bar"
and foo_bar = "foo-bar" in

let%map       foo_bar = "Foo_bar" in
let%bind      foo_bar = "Foo_bar" in

let%map_open.Command  foo_bar = "foo-bar" in
let%bind_open.Command foo_bar = "foo-bar" in

let%map_open.Command foo_bar = "foo-bar"
and                  baz_qux = "baz-qux"
in

let%mapn foo = "Foo"
and      bar = "Bar"
and      baz = "Baz" in

[%map_open.Command
  let foo_bar = "foo-bar"
  and baz_qux = "baz-qux" in
  ...
  ]

module Foo_bar = struct
  let name = "Foo_Bar"
  let name = "FOO_BAR"
  let name = "foo_bar"
  let name = "foo-bar"
end

let name = "Ppx_var_name"
let name = "ppx_var_name"
let name = "PPX_VAR_NAME"
let name = "ppx-var-name"

The [%Library_name] family depends on the "build unit" specified by the build system. In a library, this is the library name. In non-library contexts (e.g., executables), these extensions may fail to resolve or produce less natural identifiers.

For typical usage you can apply this in bulk to existing code with a command like:

sed -i -E 's/let ([a-zA-Z_][a-zA-Z0-9_]*) = "\1"/let \1 = [%var_name]/g' *.ml