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122 :: Unexpected `==` required for empty seq initialization

• gradha (orginal) [2013-02-01T13:05:00+01:00] view original

  I was implementing a proc which accepted a nil value for a sequence as default parameter. But the idiom of assigning the variable locally and checking for nil surprised me with a compilation error:

  
  
  type Tannoying = object
    tempt: bool
    tease: string
  
  proc test(a: seq[Tannoying] = nil) =
    var a = a
    if a == nil: a = @[]
    echo repr(a)
  
  when isMainModule:
    test()
  Run

  The compiler expects the comparison line to invoke the == proc, so adding this will calm the compiler:

  
  
  proc `==`(a, b: Tannoying): bool =
    if a.tempt == b.tempt and a.tease == b.tease:
      result = true
  Run

  It is however strange, since another option to calm the compiler would be to remove the if and change the default parameter for the list from nil to @[] directly. I wonder why of the difference between the nil/empty list case, why do I need a comparison proc for object/sequence creation, and if it can be avoided, or at least display a less menacing compiler error.

  For the moment I'll go with the default @[] parameter since that requires less code and moves the if check to the compiler generating the call.

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• Araq (orginal) [2013-02-01T21:18:37+01:00] view original

  == does a deep comparison for sequences as sequences are mostly treated as value types. Use system.isNil for the nil check instead. (And yes, it's planned to get rid of nil for sequences and strings...)

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