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Genprint

A one function library and PPX extension to provide general value printing from anywhere within a program, as opposed to the ocaml toplevel/REPL behaviour of printing only evaluated expression results.

Used like so:

[%pr v];

or:

[%pr arbitrary text with_ Uppercase but no keywords followed by the expression (123,456)];

It is a unit value expression therefore terminated by ';'

Another form is available when the printing instruction stands aside from the value:

[%prr] v

or:

[%prr some descriptive text] (123,456)

The result is the value given.

Printing is as per the OCaml REPL and printing depth/maximum steps, can be set along with the formatter.

Genprint.max_printer_depth := 100;
Genprint.max_printer_steps := 300;
Genprint.formatter := Format.std_formatter;

Two examples:

let h = Hashtbl.create 1 in
Hashtbl.add h 'X' "the data";
[%pr hash table (888,h,888)];

results in:

hash table => (888,
               Stdlib.Hashtbl.{size = 1;
                               data =
                                [|Empty; Empty; Empty; Empty; Empty;
                                  Cons
                                   {Stdlib__hashtbl.key = 'X';
                                    data = "the data"; next = Empty};
                                   Empty; Empty; Empty; Empty; Empty; Empty;
                                   Empty; Empty; Empty; Empty|];
                                seed = 0; initial_size = 16},
                               888)

This printer will look behind abstracting interfaces created by functor application:

module SM = Map.Make(struct type t = string let compare = compare end)
let s=SM.add "singleton" 'Z' SM.empty in
[%pr stringmap s];

for output:

stringmap => SM.(Node
                  {SM.l = Empty; v = "singleton"; d = 'Z';
                   r = Empty; h = 1})

The type of the value is retrieved from the containing source file's .cmt file which can be had via the compiler option [-bin-annot] or the recommended way of setting an environment variable permanently in one's .profile etc:

export OCAMLPARAM="_,-bin-annot=1"

This will ensure that all compilations generate annotation files, which are needed anyway for Merlin to function fully, and Dune has this flag set by default.

Genprint assumes the standard library defines no sub-modules (excepting Ephemerons). If there were, while being constrained by a signature (in the ml source rather than by mli) then Genprint would not remove this abstraction and any value described by such a module would be printed as

<abstr>

By default other installed libraries (if OCaml is at [lib/ocaml] then other libraries are assumed under [lib]) are treated as if they may have such sub-modules. This may not always be appropriate as it imposes a performance cost. If this is annoying the environment variable [GENPRINT_ALL_LIBS=1] can be used to prevent the search, which is appropriate if the values one is interested in are not displayed as <abstr> as a result. Local project code is always scanned for sub-modules.

Customised Printing?

There is the syntax form [%install_printer <name>] equivalent to the toplevel/REPL [#install_printer] that allows to handle those data types that remain abstract, such as [Bigarray.Genarray.t] which as a C level value has no OCaml representation.

  let open Bigarray in
  let open Array1 in
  let a1=create Float32 C_layout 1 in
  set a1 0 99.9;
  [%pr bigarray is abstract a1];
  let a1print ppf (a: _ Array1.t) = Format.fprintf ppf "got it...%f" (get a 0) in
  (* %printer only accepts an identifier, not a closure *)
  [%install_printer a1print];
  [%pr bigarray unabstracted a1];

Creating a custom printer is what this library seeks to avoid generally. But where it is necessary, as the foregone example demonstrates, or indeed where there is an existing printing function, it is rather convenient to deploy it through the [%printer] mechanism, especially when values it is concerned with are embedded within other structures, as otherwise one would have to extend one's printing function or break apart the structure to get to the embedded part of interest.

Installation's counterpart:

  [%remove_printer a1print]

Compiling

For building, the library is [genprint] and the PPX extension is [genprint.ppx], for both byte and optimising compilation:

ocamlc -ppx '~/.opam/default/lib/genprint/ppx/ppx.exe --as-ppx' -I +../genprint genprint.cma <src>

and with ocamlfind:

ocamlfind ocamlc -package genprint -package genprint.ppx -linkpkg  <name>.ml -o <name>

and with Dune:

 ...
 (libraries genprint ... )
 (preprocess (staged_pps genprint.ppx))
 ...

where staged_pps is used rather than pps in order for the .cmt file locating to work.

To compile with print statements left in-place while removing them from the compiled code set the environment variable [GENPRINT_REMOVE=1]. The resulting parsetree still has no-op artefacts but these are removed by ocamlopt or the flambda variant. This would be a nice convenience if it were not for the fact that the resultant binary is still linked with the compiler-libs library, on which there should be no dependency. I do not know why that is at present.

To disable printing at runtime with print statements compiled in, set the variable [GENPRINT_NOPRINT=1].

Ocamldebug

A personal obstacle to using ocamldebug has been the frustrating appearance of <abstr> for many a value I wanted to look at, especially at the thrashing-around-without-a-clue stage of debugging when I'd rather not be coding printing functions and installing them under the debugger only to abandon them when it becomes apparent a particular value is of no interest after all. This repo contains two dunified versions of the debugger extracted from compiler sources, modified to use Genprint as the default printer. The modifications are minor and easily transferred to other versions. An example invocation:

dune exec debugger/4.10.0/main.bc -- <path to sample cmo> -I <includes>

or drop into your Dune project as required. By default Genprint will issue a warning for modules unfound. Values involving types from such will display as <abstr>. Adjust the include-arguments to correct - unlike with the emdedded printing such paths are not automatically taken care of. To disable the warning set the environment variable [GENPRINT_IGNORE_MISSING=1]

Limitations

This Genprint library cannot be used in the ocaml toplevel except in as much as loading objects already compiled with embedded printing statements and for which a .cmt file exists.

Genprint uses the compiler internals to do the actual printing and so will display <poly> for values that do not have an instantiated type (or for parts thereof). So avoid embedding this printer into a polymorphic context ie. do not create a printing function around this without ensuring a concrete type is being inferred by the typechecker or otherwise providing an explicit constraint.

Printing can sometimes be slow due to the amount of processing needed to reveal concrete types built-up from layers of modules. This is minimised after the first compilation due to caching of results. The file [.genprint.cache] is used for this. It can be deleted at any point. It should be deleted whenever setting/unsetting the [GENPRINT_ALL_LIBS] environment variable or changing the value via [Genprint.all_libs_opaque]. [GENPRINT_ALL_LIBS=1] or [Genprint.all_libs_opaque true] has the effect of preventing the recursive analysis of library modules outside of the OCaml library directory. This only pertains to disovering the presence of functor generated modules and by not searching for such, printing is faster. On the other hand the introduction of caching renders this less important.

The abstraction of data types removed by this printer relates only to that introduced at the module implementation level (.mli interface files) and explicit signatures written into .ml sources as used in module expressions. It does not deal at all with class types, another means of abstraction. Indeed the REPL printer upon which Genprint is based outputs only <obj> for any object.

I think, in principle, something better could be achieved.

Feedback

This is experimental software! If you encounter a problem or can suggest an improvement/addition please raise an issue.