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Newbie question: OCaml equivalent of Haskell's show?
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Date: -- (:)
From: Jon Harrop <jon@f...>
Subject: Re: [Caml-list] Newbie question: OCaml equivalent of Haskell's show?
On Sunday 06 July 2008 16:33:35 Antony Courtney wrote:
> I'm an experienced Haskell hacker trying OCaml for the first time.
>
> One thing I am desperately searching for but have been unable to find
> is some direct runtime access to the string representation of
> arbitrary OCaml values.

OCaml has no run-time type information, no structural pretty printers in 
compiled code and no type classes to help you implement your own in source 
code.

> I have written a little option pricer that constructs a
>    float option array array
> in a function.  I've got a little buglet in my function so I'd like to
> print the intermediate states of this value from inside the function.
> How do I do that, short of writing my own recursive pretty printer /
> formatter?

You basically have two choices:

. Run in the top-level and get the value you require as the result of 
evaluating an expression (e.g. by raising an exception, or by setting a 
mutable).

. Write your own print function(s) and call them.

The benefit of the latter is, of course, that it works from compiled code.

> An OCaml form of the Haskell Show type class would be great,

IMHO, type classes are great for some things but pretty printing is not one of 
them.

Start with a function to print a list with an arbitrary separator:

# open Format;;
# let rec fprintfs sep f ff = function
    | [] -> ()
    | [h] -> fprintf ff "%a" f h
    | h::t -> fprintf ff "%a%a%a" f h sep () (fprintfs sep f) t;;
val fprintfs :
  (Format.formatter -> unit -> unit) ->
  (Format.formatter -> 'a -> unit) -> Format.formatter -> 'a list -> unit =
  <fun>

For example, we can now write a function to pretty print a list in Mathematica 
notation:

# let fprintf_mma ff list =
    let sep ff () = fprintf ff ",@ " in
    let int ff = fprintf ff "%d" in
    fprintf ff "{@[%a@]}" (fprintfs sep int) list;;
val fprintf_mma : Format.formatter -> int list -> unit = <fun>

# fprintf std_formatter "%a\n" fprintf_mma (Array.to_list(Array.init 100 (fun 
n -> n)));;
{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39,
 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58,
 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77,
 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96,
 97, 98, 99}
- : unit = ()

Note the automatically-aligned wrapping thanks to OCaml's pretty 
printing "Format" module and the use of "@[", "@ " and "@]" in my code. This 
also works from compiled code and is an excellent way to print indented 
source code (e.g. when generating other languages).

To pretty print a generic array:

# let fprintf_array f ff array =
    let sep ff () = fprintf ff ";@ " in
    fprintf ff "[|@[%a@]|]" (fprintfs sep f) (Array.to_list array);;
val fprintf_array :
  (Format.formatter -> 'a -> unit) -> Format.formatter -> 'a array -> unit =
  <fun>

To print an option:

# let rec fprintf_opt f ff = function
    | None -> fprintf ff "None"
    | Some x -> fprintf ff "Some %a" f x;;
val fprintf_opt :
  (Format.formatter -> 'a -> unit) -> Format.formatter -> 'a option -> unit =
  <fun>

Now you can print your array of arrays of float options:

# let print ff xss =
    let float ff = fprintf ff "%g" in
    fprintf_array (fprintf_array (fprintf_opt float)) ff xss;;
val print : Format.formatter -> float option array array -> unit = <fun>

# fprintf std_formatter "%a\n" print
    [|[|Some 3.; None; None|]; [|None; Some 5.; None|]; [|Some 1.|]|];;
[|[|Some 3; None; None|]; [|None; Some 5; None|]; [|Some 1|]|]
- : unit = ()

> but a hack to provide programmatic access to the polymorphic 
> pretty printer that obviously already exists in the OCaml toplevel
> would be fine.

In addition to Richard's suggestions, you might look at generating OCaml 
expressions using camlp4 and then printing those instead. Just an idea (I've 
never tried it).

> I've scoured the standard library docs, manual, tutorials and a few
> books, looked at the FAQ and am amazed that I haven't found the answer
> to this.  I am hopefully just missing something obvious; would be
> grateful if someone could point me towards the answer!

Surprisingly, structural printing is something that OCamlers rarely do. As 
part of my evolution as a programmer I had migrated everything from C++ and 
Mathematica to OCaml a few years ago but I am actually about to move a little 
parsing-related project from OCaml to F# simply because its built-in pretty 
printers make life so much easier.

Anyway, I shall write an OCaml Journal article about OCaml's 
wonderful "Format" module. Incidentally, the OCaml Journal has recovered from 
a lull in Q1 and has really started to pick up now. So I still think there is 
plenty of opportunity for budding authors to write OCaml books!

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/products/?e