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Date: -- (:)
From: Richard Jones <rich@a...>
Subject: Re: [Caml-list] xpath or alternatives
On Wed, Sep 30, 2009 at 09:33:07AM -0400, Till Varoquaux wrote:
> OCamlduce (Alain correct me if I am wrong) basically maintains two
> separate type systems side by side (the Xduce one and the Ocaml one).
> This is done in order to make Ocamlduce maintainable by keeping a
> clear separation. As a result you have to explicitly convert values
> between type systems using {:...:}. These casts are type safe but do
> lead to some work at runtime.
> 
> Also note that ocaml's string are Latin1 and not String in the XML world. So:
> 
>  let devs = match xml with
>    | {{ <domain>[<devices>[<source dev=(Latin1 & dev) ..>[]]] }} -> {:dev:}
>    | {{ <domain>[<devices>[<source file=(Latin1 & file) ..>[]]] }} ->
> {:file:} in
> 
> Should work (I'm rusty and have nothing to check handy).

I tried variations on the above, but couldn't get it to work.
ocamlduce is very fond of a mysterious error called "Error: Subtyping
failed", which is very difficult for me to understand, and therefore
must be absolutely impossible for someone not used to strong typing.

This is where I'm heading at the moment (sorry, my previous
example missed a <disk> level inside <devices>), so:

  let xml = from_string xml in
  prerr_endline (Ocamlduce.to_string xml);

  let devs = {{ map [xml] with
    | <domain..>[<devices..>[<disk..>[<source dev=(Latin1 & s) ..>_]]]
    | <domain..>[<devices..>[<disk..>[<source file=(Latin1 & s) ..>_]]] -> [s]
    | _ -> [] }} in
  prerr_endline (Ocamlduce.to_string devs);

+1 : this compiles
-1 : it doesn't work, devs is empty

This is what the first prerr_endline prints:

<domain
  type="kvm"
  id="2">[
  <name>[ 'CentOS5x32' ]
  <uuid>[ '2ce397d9-1931-feb1-8ad8-15f22c4f18af' ]
  <memory>[ '524288' ]
  <currentMemory>[ '524288' ]
  <vcpu>[ '1' ]
  <os>[ <type arch="x86_64" machine="pc-0.11">[ 'hvm' ] <boot dev="hd">[ ] ]
  <features>[ <acpi>[ ] <apic>[ ] <pae>[ ] ]
  <clock offset="utc">[ ]
  <on_poweroff>[ 'destroy' ]
  <on_reboot>[ 'restart' ]
  <on_crash>[ 'restart' ]
  <devices>[
    <emulator>[ '/usr/bin/qemu-kvm' ]
    <disk
      type="block"
      device="disk">[
      <source dev="/dev/vg_trick/CentOS5x32">[ ]
      <target bus="ide" dev="hda">[ ]
      ]
    <interface
      type="network">[
      <mac address="54:52:00:3c:76:11">[ ]
      <source network="default">[ ]
      <target dev="vnet0">[ ]
      ]
    <serial type="pty">[ <source path="/dev/pts/7">[ ] <target port="0">[ ] ]
    <console
      type="pty"
      tty="/dev/pts/7">[
      <source path="/dev/pts/7">[ ]
      <target port="0">[ ]
      ]
    <input type="mouse" bus="ps2">[ ]
    <graphics autoport="yes" port="5900" type="vnc">[ ]
    <video>[ <model type="cirrus" vram="9216" heads="1">[ ] ]
    ]
  ]

and what the second prerr_endline prints:

""

Rich.

-- 
Richard Jones
Red Hat