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[caml] Closures and efficiency
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Date: 1999-10-28 (17:09)
From: Christopher Jeris <cjeris@m...>
Subject: [caml] Closures and efficiency
I have a sort of naive question regarding the efficiency of higher-order
functions in data structures, and how well the Ocaml compiler does common
subexpression elimination on these.

The problem is the following.  I wish to write a universal voice editor
for music synthesizers.  Now a voice of a synth is characterized by a
large set of parameters, and this set (the names, ranges, meanings of each
parameter) is different for each synth.  The first thing it occurred to me
to do was to write a type:

type yamaha_AN1x_patch = { name: string; (* lots of parameters *) }
type yamaha_FS1R_patch = (* ... continue ad nauseam *)

But types are not first class; there is no way to write a general piece of
code which will look at a record type, see what its fields are and do
something (like display an edit window) which depends on the types of the
fields.  So instead I write types for meta-data:

type parameter = { name: string; doc: string; values: param_value_set; ... }
and param_value_set =
  Enumeration of enum_value list
| Int_subrange of int * int
| Strings
| ...
and enum_value = { name: string; encoded_value: int }

Now the description of a synth's voice architecture looks like

val yamaha_AN1x_patch =
  [ {name = "Patch name"; values = Strings};
    {name = "Filter cutoff frequency"; values = Int_subrange (0, 127)};
    ... ]

Okay.  Now synths can dump information about their voices via the MIDI
port, and the protocol for decoding this "sysex dump" into an intelligible
voice description is different for each synth.  So I need to have a way,
for each parameter appearing in each synth architecture, to specify its
translation to and from the raw byte-strings which appear on the MIDI
port.  If this were a C program, there would be one big translation
function that would do something like

- to encode an enumerated value, use its index in the list of enumerated
values for this parameter
- to encode a string value, use its ASCII representation
- to encode a member of an integer subrange, use its value minus the
bottom value of this subrange

But this is too monolithic, and in Ocaml we can put functional values in
data structures ... so I would rather have, along with each parameter, a
little function "encoder: param_value -> bytestring" and
"decoder: bytestring -> param_value" which know how to do the translation
for _this_ parameter.

Here's the nub of my question.  In a typical synth architecture there are
very many parameters which take values 0..127.  If I had a function

  int_subrange_encoder: int -> int -> param_value -> bytestring

which took the bounds of a subrange and generated an encoder function for
values of that subrange, and then in a voice-architecture description I
had a list of very many records each of which contained an entry

  int_subrange_encoder 0 127

would I suddenly have six million little closures, or would the compiler
do common-subexpression elimination on them ?  Or am I just taking a
totally wrongheaded approach ?

Chris Jeris