ppx_integer is a PPX syntax extension to write integer literals with
non standard suffix characters of [g-zG-Z]. Using ppx_integer,
you can easily write non standard integer literals, such as
like 1234u for Uint32, 0xabcdU for Uint64, etc.

Desugaring

ppx_integer does not touch the integer literals with the standard suffix chars, L, l and n.

A literal with a non standard suffix char is expanded to a variable.
For example, 1234u is expanded to:

integer_1234u

ppx_integer also builds a definition declaration of the created variable:

let integer_1234u = Ppx_integer._u "1234"

By default, these bindings are placed at the beginning of the compilation unit.
For example, the following program:

let () = prerr_endline @@ Uint32.to_string 1234u
let () = prerr_endline @@ Uint32.add 1234u 2345u

is converted to

let integer_1234u = Ppx_integer._u "1234"
let integer_2345u = Ppx_integer._u "2345"
let () = prerr_endline @@ Uint32.to_string integer_1234u
let () = prerr_endline @@ Uint32.add integer_1234u integer_2345u

Note that the variable for the same literals is created only once.

Module Ppx_integer

It is the user's responsibility to prepare a module Ppx_integer
which provides functions Ppx_integer._X where X are non standard
integer suffix chars used in the user's code.

Variable declaration placeholder

Using [%%ppx_integer], you can change the place of the generated
integer_... variable declarations. This is useful if the module
Ppx_integer is defined in the same compilation unit:

module Ppx_integer = struct
  let _u s = Uint32.of_string
end

[%%ppx_integer] (* <- variable declarations come here, not to the top *)

let () = prerr_endline @@ Uint32.to_string 1234u
let () = prerr_endline @@ Uint32.add 1234u 2345u

Why not other chars?

In OCaml, integer literal suffix chars are restricted to [g-zG-Z],
since [a-fA-F] are used for hex integers.