Re: [Caml-list] Problems with printing MetaOCaml generated code

["Ömer Sinan Ağacan" <omeragacan@gmail.com>]

> Personally, I think it is best to avoid this auto-magic and just do your
> lifting by hand. The type-based auto-magic doesn't work with polymorphism,
> whilst the value-based auto-magic is not parametric.

Hm, I didn't know this was possible. I just tried and it seems to work really
well:

  type stx =
    | A
    | B of stx
    | C of (stx * stx)

  let rec lift_stx (stx : stx) : stx code =
    match stx with
    | A              -> .< A >.
    | B stx'         -> .< B .~ (lift_stx stx') >.
    | C (stx1, stx2) -> .< C ( .~ (lift_stx stx1), .~ (lift_stx stx2) ) >.

  let stx = C (C (C (A, A), C (A, A)), C (C (A, A), C (A, A))) in
  print_code std_formatter .< .~ (lift_stx stx) >.;

So it seems like I finally have a workaround now, thanks!

The story of MetaOCaml code printing is still not completely clear to me, and
this workaround means lots of boilerplace, but at least I have a workaround now
and I can make progress.

2015-05-03 11:24 GMT-04:00 Leo White <leo@lpw25.net>:
>> I don't think this is true, as far as I can see MetaOCaml doesn't have open code
>> values that are only runnable in environments that bind free variables in the
>> code.
>>
>>   # .<a>.;;
>>   Error: Unbound value a
>>   # let a = 1 in .<a>.;;
>>   - : int code = .<1>.
>>
>> In this example `.<a>.` didn't mean a code that is just the variable `a`,
>> instead, `a` is lifted in code and code is now `.<1>.`.
>>
>> I think this automatic lifting is why MetaOCaml doesn't have explicit "lift"
>> operation like MetaML does: variables bound in the environment automatically
>> lifted, so no open code values are possible.
>
> It is true that MetaOCaml tries to auto-magically lift *some* things for you
> (e.g. ints), as in this example. If type information is available, then for known
> types it can produce a call to a function such as `Trx.lift_constant_int` to do the
> lifting. For other types it will examine the value at run-time and attempt to lift it.
> For example, if the value has a "string tag" then it can be represented by a string
> literal. If this fails then it will fall back to using true CSP -- which can be run but
> cannot be printed.
>
> Personally, I think it is best to avoid this auto-magic and just do your lifting by
> hand. The type-based auto-magic doesn't work with polymorphism, whilst the
> value-based auto-magic is not parametric.
>
> Regards,
>
> Leo
>
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