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9493 :: Battling with copies & a silly question

• drkameleon (orginal) [2022-09-26T17:16:03+02:00] view original

  OK, so, first of all let me tell you that I have spent countless hours analyzing what the Nim compiler is doing and how Nim code gets converted to C.

  Let's take this naive example:

  • we have a table (string keys, some user ref object values)
  • I'm looking up a key, get the value back
  • Then I want to do a few things with that value (checks, etc)

  A rough idea in code:

  let val = tbl[key] # COPY
  
  if val.something == somethingElse:
     # do something
  
  if val.somethinElse == whatever:
      # do something else
  Run

  Another idea, would be to totally avoid the assignment and make 2 lookups instead of one (yay!).

  Then - right now, as I'm writing this - I'm thinking whether using a var instead of a let would make a difference. But then, I remember more than once being totally baffled by the output, after minor changes (that, at least according to what I would expect appeared either auto-magical, or not very predictable).

  I've found my own ways of doing things like that, but mostly they look like weird hacks (ok, plus I have developed some weird phobia towards assigning things; you never know you may end up with 3 trillion copies lol)

  So... how do you usually handle this type of thing?

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• Araq (orginal) [2022-09-26T18:13:45+02:00] view original

  Just use more ref in your code. For tables mgetOrPut also works wonders but others already told you about it.

  There is also this idiom:

  let x = addr table[key]
  use x[]
  useAgain x[]
  Run

  It's not really all that hard...

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• drkameleon (orginal) [2022-09-26T21:03:57+02:00] view original

  Thanks for this addr table[key]. I guess I needed some confirmation to make 100% sure I won't mess with the GC.

  Awesome :)

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