Building a bytestring containing the length/size?

Asked Jan 31 '17 at 19:04

Active Jan 31 '17 at 19:04

Viewed 94 times

This isn't the first time I run into this issue, but I haven't found a really nice solution for it so I thought I'd ask how others approach it.

Many protocols include the size of the message (or a part of it) inside the message itself. For instance, right now I'm looking at ModbusTCP where bytes 5 and 6 should hold the length of the remaining part of the message. Is there some nice and elegant way to deal with this?

The straight-forward builder would look something like this (using Serialize from cereal):

put (ModTcpRequestFrame tid pid uid req)= do
  putWord16be tid
  putWord16be pid
  putWord16be len
  putWord8 uid
  put req

  where
    len = .... -- what to put here?

Can it be done?

asked Jan 31 '17 at 19:04

Magnus

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Isn't it just `length uid + length req` (assuming those values are `Foldable`)? – chepner Jan 31 '17 at 19:23
Unfortunately they are not, in the example above uid is a `Word8` and `req` is a serializable type. – Magnus Jan 31 '17 at 19:35
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Just `let reqBS = runPut put req` first and you can calculate the length then later `putByteString reqBS`. That or find a knot tying solution. – Thomas M. DuBuisson Jan 31 '17 at 21:59
Well, a knot-tying solution would be pretty much what I'm asking for ;) – Magnus Feb 21 '17 at 08:29

Building a bytestring containing the length/size?

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