package cinaps

README.org

* CINAPS - Cinaps Is Not A Preprocessing System

Cinaps is a trivial Metaprogramming tool for OCaml using the OCaml
toplevel.

It is intended for two purposes:
- when you want to include a bit of generated code in a file, but
  writing a proper generator/ppx rewriter is not worth it
- when you have many repeated blocks of similar code in your program,
  to help writing and maintaining them

It is not intended as a general preprocessor, and in particular cannot
only be used to generate static code that is independent of the
system.

** How does it work?

Cinaps is a purely textual tool. It recognizes special syntax of the
form =(*$ <ocaml-code> *)= in the input. =<ocaml-code>= is evaluated
and whatever it prints on the standard output is compared against what
follows in the file until the next =($ ... *)= form, in the same way
that expectation tests works.

A form ending with =$*)= stops the matching and switch back to plain
text mode. In particular the empty form =(*$*)= can be used to mark
the end of a generated block.

If the actual output doesn't match the expected one, cinaps creates a
=.corrected= file containing the actual output, diff the original file
against the actual output and exits with an error code. Other it
simply exits with error code 0.

For instance:

#+begin_src sh
$ cat file.ml
let x = 1
(*$ print_newline ();
    List.iter (fun s -> Printf.printf "let ( %s ) = Pervasives.( %s )\n" s s)
      ["+"; "-"; "*"; "/"] *)
(*$*)
let y = 2

$ cinaps file.ml
---file.ml
+++file.ml.corrected
File "file.ml", line 5, characters 0-1:
  let x = 1
  (*$ print_newline ();
      List.iter (fun s -> Printf.printf "let ( %s ) = Pervasives.( %s )\n" s s)
        ["+"; "-"; "*"; "/"] *)
+|let ( + ) = Pervasives.( + )
+|let ( - ) = Pervasives.( - )
+|let ( * ) = Pervasives.( * )
+|let ( / ) = Pervasives.( / )
  (*$*)
  let y = 2

$ echo $?
1
$ cp file.ml.corrected file.ml
$ cinaps file.ml
$ echo $?
0
#+end_src

You can also pass =-i= to override the file in place in case of
mismatch. For instance you can have a =cinaps= target in your build
system to refresh the files in your project.

** Capturing text from the input

In any form =(*$ ... *)= form, the variable =_last_text_block=
contains the contents of the text between the previous =(*$ ... *)=
form or beginning of file and the current form.

For instance you can use it to write a block of code and copy it to a
second block of code that is similar except for some simple
substitution:

#+begin_src ocaml
(*$*)
let rec power_int32 n p =
  if Int32.equal p 0 then
    Int32.one
  else
    Int32.mul n (power n (Int32.pred p))

(*$ print_string (Str.global_replace (Str.regexp "32") "64" _last_text_block) *)
let rec power_int64 n p =
  if Int64.equal p 0 then
    Int64.one
  else
    Int64.mul n (power n (Int64.pred p))

(*$*)
#+end_src

Now, whenever you modify =power_int32=, you can just run cinaps to update
the =power_int64= version.

** Sharing values across multiple files

The toplevel directive ~#use~ works in CINAPS, and can be used to read in values
from other files. For example,

1. In ~import.cinaps~,

   #+BEGIN_SRC ocaml
     (* -*- mode: tuareg -*- *)
     include StdLabels
     include Printf

     let all_fields = [ "name", "string"; "age", "int" ]
   #+END_SRC

2. In ~foo.ml~,

   #+BEGIN_SRC ocaml
     (*$ #use "import.cinaps";;
              List.iter all_fields ~f:(fun (name, type_) ->
                printf "\n\
                  external get_%s : unit -> %s = \"get_%s\"" name type_ name) *)
     external get_name : unit -> string = "get_name"
     external get_age : unit -> int = "get_age"(*$*)
   #+END_SRC

3. In ~stubs.h~,

   #+BEGIN_SRC C
     /*$ #use "import.cinaps";;
              List.iter all_fields ~f:(fun (name, _) ->
                printf "\n\
                  extern value get_%s(void);" name) */
     extern value get_name(void);
     extern value get_age(void);/*$*/
   #+END_SRC

Etc.

Note that the ~#use~ directive will read in OCaml from files of any extension.
~*.cinaps~ is a safe choice in the presence of jenga and dune, which by default
try to use all ~*.ml~ files in the directory for the executables or library.

** Automatic reformatting of CINAPS output

In files managed by automatic formatting tools such as ocp-indent or
ocamlformat, the code need not come out of CINAPs already formatted correctly.

~cinaps.exe -styler FOO~ uses ~FOO~ to reformat its output, before diffing
against the source file.