Genprint

Genprint is a small hack of the compiler allowing general printing of values exactly as is seen from the interactive toplevel. The difference is that a value can be printed out from anywhere in the program rather than only for the end result.

open Genprint
(* prs requires both arguments. *)
let _=
    prs "random value" [[[0,1]]];
    prs "random value" (true,"true")

random value =>
[[[(0, 1)]]]
random value =>
(true, "true")
- : unit = ()

It relies on capturing the type of the value at the point of use in the program and as such it is not useful to bury such statements within polymorhic functions:

let printnl x = 
    Genprint.prs "polymorphic and not very useful" x;
    print_endline()
    
let _= printnl 0

polymorphic and not very useful =>
<poly>
- : unit = ()

So use only where the type is partially or completely inferred.

For some control, as Toplevel, there is:

val max_printer_depth : int ref
val max_printer_steps : int ref
val ppf : Format.formatter ref

The printing function is basic and reflects the fact this is only meant as a debug aid. It is available in the toplevel and the compiler. There is an opam switch available:

opam repository add --set-default progman1 https://github.com/progman1/my-opam-repo.git

opam switch create 4.07.1+genprint

See the Makefile above for example targets.

Patches are also available separately: one for the common component here, one specifically for the toplevel and one specifically for the compiler.

From the root of your ocaml distribution:

patch -p1 < <path of patch file>

./configure ... to your requirements then:

make coldstart depend
make world world.opt install

The patches are against ocaml version 4.07.1 but easily adjusted for other versions.

The switch contains both toplevel and compiler patches but this is only for demo'ing - the compiler patch (in addition to the common patch) is sufficient for toplevel operation as well. The reason there is a separate toplevel patch is that it is trivial, making use of existing toplevel infrastructure, and if one is content to develop via the toplevel only, that patch (on top of the common patch) is sufficient.

For example, since it is possible for project sub-component libraries to be loaded into a toplevel either as object files/library or, when undertaking maintenance/development, as source, one can switch between modes. And one benefit of going to source mode is then general value printing. So strictly speaking, the compiler patch is unnecessary. On the other hand, it may be harder to maintain ability to do toplevel work than I have thus far experienced and so the compiler patch would allow the debug printing facility in re-compiled code as well.

Feedback

Feedback welcome, just use the issue tracker.