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6647 :: How to instantiate custom `TableRef` without repeating individual types?

• sschwarzer (orginal) [2020-08-06T17:12:02+02:00] view original

  I have the following type declarations:

  import tables
  
  type
    A = object
      i: int
    
    ATableRef = TableRef[string, A]
  Run

  where ATableRef is kind of a slightly higher-level shortcut for a specific``TableRef``. I'm using ATableRef as parameter type in procs to avoid repeating the individual types.

  Now I want to create a new ATableRef. This works:

  var newTable = newTable[string, A]()
  Run

  but has the disadvantage that it repeats the types I used in the definition of ATableRef.

  I tried the "seemingly obvious"

  var newTable: ATableRef = newTable()
  Run

  hoping that the compiler would be able to infer the type, but the compiler tells me:

  
  /usercode/in.nim(16, 35) template/generic instantiation of `newTable` from here
  /playground/nim/lib/pure/collections/tables.nim(784, 16) Error: cannot instantiate: 'A'
  Run

  How can I create an object of ATableRef without repeating the types string and A?

  The above code is combined here for experimenting: https://play.nim-lang.org/#ix=2tqq .

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• zevv (orginal) [2020-08-06T17:14:57+02:00] view original

  var myTable: ATableRef new myTable

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• Yardanico (orginal) [2020-08-06T17:18:08+02:00] view original

  I don't think that this will work since https://github.com/nim-lang/Nim/blob/version-1-2/lib/pure/collections/tables.nim#L784 calls https://github.com/nim-lang/Nim/blob/version-1-2/lib/pure/collections/tableimpl.nim#L162

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• zevv (orginal) [2020-08-06T17:20:02+02:00] view original

  https://play.nim-lang.org/#ix=2tqy

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• Yardanico (orginal) [2020-08-06T17:24:40+02:00] view original

  Relying on internal details is bad :P It might break without you knowing

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• sschwarzer (orginal) [2020-08-06T17:25:53+02:00] view original

  Interesting idea. For the example in the playground it works, but what if newTableRef got changed so that it needed some set-up for the data structure, that new alone wouldn't do? I guess usually there's a reason why you should use newSomething() instead of new Something. :-) Or do I misunderstand how new works?

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• sschwarzer (orginal) [2020-08-06T17:46:12+02:00] view original

  I guess if the original goal isn't achievable with some use of TableRef or newTableRef, probably the cleanest approach is defining a new "constructor":

  func newATable(): ATableRef =
    newTable[string, A]()
  Run

  :-) . That would also help if newATable needs more set-up in the future. If you're very strict about it, you could consider this approach a "YAGNI violation", but I think it's fine. ;-)

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• jibal (orginal) [2020-08-06T21:00:47+02:00] view original

  Couldn't render post #41228.

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• sschwarzer (orginal) [2020-08-06T21:20:02+02:00] view original

  What's wrong with

  var myTable = ATableRef()

  It wasn't clear whether this would be enough.

  It's in the spec that tables don't need explicit initialization: "Starting from Nim v0.20, tables are initialized by default and it is not necessary to call this function explicitly." That means that plain new works and will always work.

  Awesome, now that is useful information.

  Unnecessary and poor practice.

  The only difference I see between your and my code is the line to create the actual table. And that's different because it wasn't clear whether the second form would be reliable (see the discussion so far).

  Thanks anyway! ;-)

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