[CakeML-dev] New string/bytearray operations

Ramana Kumar Ramana.Kumar at cl.cam.ac.uk
Mon Mar 20 09:12:28 UTC 2017

I think you're right. Because I can't copy into an existing string,
the result would need to be copied at the end... hmm, so maybe concat
should be primitive again? Or maybe there's another better set of
primitives? The SML basis doesn't have concat, but it does have copy
and copyVec and the same on slices. (We don't have

To think about this it helped me to try writing concat, and I want to
keep it here for reference later.

fun sumlengths [] acc = acc
  | sumlengths (a::as) acc = sumlengths as (length a + acc)
fun copy [] dst _ = dst
  | copy (a::as) dst n =
   let val l = length a
        val () = CopyStrAw8 a 0 l dst n
  in copy as dst (n+l) end
fun concat ls = let
  val l = sumlengths ls
  val dst = alloc l 0w
in CopyAw8Str (copy ls dst 0) 0 l end

On 20 March 2017 at 19:56, Scott Owens <S.A.Owens at kent.ac.uk> wrote:
> I don’t see how you implement string concat without extra copying.
> Scott
>> On 2017/03/20, at 05:29, Ramana Kumar <Ramana.Kumar at cl.cam.ac.uk> wrote:
>> I've been rethinking these primitives after the discussion at the last hangout, and have come up with a different set altogether. Can you see a simpler or more elegant approach to the one described below?
>> Here is the new approach I am considering:
>> 4 copying primitives in the source language going from string/bytearray to string/bytearray.
>> The source comes with an offset and a length to copy.
>> If the destination is a string, a new string is created. If the destination is a bytearray, it must be provided.
>> Concatenation (of lists of strings/arrays) and conversions between (whole) strings and (whole) bytearrays can be implemented in the basis library in terms of these primitives. And the primitives should be efficiently implementable in terms of a byte-based memcpy primitive further down. (There will need to be bounds checking in the source-level semantics (i.e., Subscript exception can be raised), and this will sometimes be unfortunate (i.e., when obviously in bounds), but I don't think this is too costly.)
>> On 14 March 2017 at 16:52, Ramana Kumar <Ramana.Kumar at cl.cam.ac.uk> wrote:
>> Hi all,
>> I've started adding string/bytearray conversion and concatenation
>> primitives (issues 244 and 245). Before getting too deep into updating
>> the compiler etc., may I request a review of the semantics? Here they
>> are:
>> https://github.com/CakeML/cakeml/commit/67dd15bbd03f516be618ba72f1d56a2764209263
>> I noticed that v_to_char_list might be better as vs_to_char_list, to
>> be run after v_to_list (rather than duplicating its
>> list-deconstruction functionality). But I leave such refactoring for
>> another time.
>> Cheers,
>> Ramana
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