package ppx_deriving_madcast

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Library deriving cast functions based on their types


Dune Dependency






Madcast is a library deriving cast functions based on their types.


Base types

For base types, Madcast uses the functions defined in the standard library:

[%madcast: int -> string]

will simply be replaced by string_of_int.

Parsing positions

Say you have to parse a line of coordinates x1, y1, x2, y2, etc. and you want an array of pairs of integers:

let points =
  read_line ()
  |> String.split_on_char ' '
  |> [%madcast: string list -> (int * int) array]


Madcast is primarily meant to be used in conjunction with low level API. Here is an example with MySQL:

let () =
  let result = Mysql.exec conn "SELECT id, name, surname FROM person WHERE username='johndoe'" in
  let row = Mysql.fetch result
            |> [%madcast: string option array option -> (int * string * string option) option]
  in match row with
  | None -> Format.eprintf "Could not find user `johndoe`@."
  | Some (id, name, None) -> Format.eprintf "%s (%d) has no surname.@." name id
  | Some (id, name, Some surname) -> Format.eprintf "%s (%d) has %s for surname.@." name id surname

Try it yourself!

You can see by yourself the code generated for a given type with test/show.exe:

$ dune exec test/show.exe 'string array -> (int * int) array'
fun a  ->
  if ((Array.length a) mod 2) <> 0
  then failwith "madcast: 'a array -> <tuple> array"
    Array.init ((Array.length a) / 2)
      (fun i  ->
         (((fun s  ->
              try int_of_string s
              with | Failure _ -> failwith "madcast: string -> int")
             (a.(0 + (i * 2)))),
           ((fun s  ->
               try int_of_string s
               with | Failure _ -> failwith "madcast: string -> int")
              (a.(1 + (i * 2))))))

Actually, if you feel fancy, we recommend adding ocamlformat and bat to the lot and running:

$ dune exec test/show.exe 'string list -> (int * int) array' \
      | ocamlformat - --impl --enable-outside-detected-project \
      | bat --language ocaml

And if you don't feel like cloning but you love Nix, you can also go for:

$ nix run github:LesBoloss-es/ppx_deriving_madcast#show -- 'string array -> (int * int) array'


Using OPAM

ppx_deriving_madcast is available on OPAM:

$ opam install ppx_deriving_madcast


Madcast also provides an API. This API is in the package ppx_deriving_madcast.api. Its documentation can be built with make doc.


Madcast is distributed under the LGPL License, version 3.