[Caml-list] python-style array access

["Yaron M. Minsky" <yminsky@CS.Cornell.EDU>]

One of the things I miss from Python is the small syntactical touches that
simplify common operations.  one of my favorites is the way python does
array indexing.  You can use negative numbers to count from the end of the
array, and you can easily take subranges of arrays or strings.  It's
actually quite simple to get the same functionality in ocaml, and here's
some simple code for doing it.  It adds a bit of overhead (an extra
function call), but where that small overhead isn't an issue, I think this
is a nice solution.  If you include the following code in the beginning of
your module, you'll be able to do the following kind of tricks:

# "foobar".[-1];;
- : char = 'r'
# "foobar".[-2];;
- : char = 'a'
# String.rsub "foobar" 0 (-1);;
- : string = "fooba"
# String.rsub "foobar" 1 0;;
- : string = "oobar"
# String.rsub "foobar" 1 3;;
- : string = "oo"
# String.rsub "foobar" 1 (-1);;
- : string = "ooba"

The same kind of things can be done with arrays.  rsub stans for "range
sub", by the way.  Note that a 0 in the second position of rsub means the
very last as opposed to the very first index position.

------ Snip Here -------

open StdLabels
open MoreLabels

module Array =
struct
  include Array
  let normalize ar i = if i < 0 then length ar + i else i
  let get ar i = get ar (normalize ar i)
  let rsub ar start stop =
    let stop = if stop = 0 then length ar else stop in
    let pos = normalize ar start in
    let len = (normalize ar stop) - pos in
    sub ar ~pos ~len
end

module String =
struct
  include String
  let normalize str i = if i < 0 then length str + i else i
  let get str i = get str (normalize str i)
  let rsub str start stop =
    let stop = if stop = 0 then length str else stop in
    let pos = normalize str start in
    let len = (normalize str stop) - pos in
    sub str ~pos ~len
end

-- 
|--------/            Yaron M. Minsky              \--------|
|--------\ http://www.cs.cornell.edu/home/yminsky/ /--------|

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