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2314 :: Problem sorting seq[Tup]

• bobg (orginal) [2016-06-14T16:18:52+02:00] view original

  I'm following examples on the Internet, but getting compiler Errors. (The Error message seems to be written in a different language?)

  Here is the code:

  
  import strutils, sequtils
  import algorithm
  
  type
    Tup = tuple[amt: float, time: string]
    TupSeq = seq[Tup]
  
  const data: TupSeq = @[(amt: 24.95, time: "04:35"),
                         (amt: 10.45, time: "03:30")
                        ]
  # Sort using proc syntax (From goran.krampe.se)
  # s.sort(proc(x, y: Fruit) :int =
  #   cmp(x.origin, y.origin))
  
  data.sort(proc(x, y: Tup): int =
    cmp(x.time, y.time) )
  
  for da in data:
    echo da.time & " " & $da.amt
  Run

  And here is the Error message:

  
  sortSeq.nim(15, 5) Error: type mismatch: got (TupSeq, proc (x: Tup, y: Tup): int{.gcsafe, locks: 0.})
  but expected one of:
  proc sort[T](a: var openArray[T]; cmp: proc (x, y: T): int; order = SortOrder.Ascending)
  Run

  If I duplicate the syntax in the Error message, I also get errors.. If I comment out the sort statement, it compiles and runs fine. Here is the compiler version.

  
  Nim Compiler Version 0.14.3 (2016-06-13) [MacOSX: amd64]
  Copyright (c) 2006-2016 by Andreas Rumpf
  Run

  Have fun

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• Parashurama (orginal) [2016-06-14T16:29:45+02:00] view original

  I believe you are trying to sort in-place a const sequence.

  change

  const data: TupSeq
  Run

  to

  var data: TupSeq
  Run

  and it should work.

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• bobg (orginal) [2016-06-14T16:38:15+02:00] view original

  Super - that did it.

  The compiler error messages were not so helpful, but the Forum is the way to go.

  Thanks much!

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• bobg (orginal) [2016-06-14T17:02:40+02:00] view original

  @Parashurama - How did you figure out the problem? From the compiler error message, or experience (you made/solved the same error in the past)?

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• OderWat (orginal) [2016-06-14T17:32:39+02:00] view original

  I think that error message is just plain useless in that case. In the case of a "type" mismatch because of constant data there should be a better hint. The error itself is IMHO very obvious though. You can't change a const at runtime http://nim-lang.org/docs/manual.html#constants_1

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• andrea (orginal) [2016-06-14T19:00:45+02:00] view original

  Well, it is not the friendliest error message, but you can compare the two signatures:

  provided

  
  (TupSeq, proc (x: Tup, y: Tup): int{.gcsafe, locks: 0.})
  Run

  vs expected

  
  (a: var openArray[T]; cmp: proc (x, y: T): int; order = SortOrder.Ascending)
  Run

  Let us remove the annotations from the provided one, getting

  
  (TupSeq, proc (x: Tup, y: Tup): int)
  Run

  You are not sending the optional parameter order, so let us omit that from the expected one. Also, let us forget parameter names, which are not relevant anyway. You get

  
  (var openArray[T], proc (x, y: T): int)
  Run

  Now thw two start looking similar. In your case, you know that T is Tup, so specializing the expected signature you get

  
  (var openArray[Tup], proc (x, y: Tup): int)
  Run

  Expanding TupSeq shows that what you provided has signature

  
  (seq[Tup], proc (x, y: Tup): int)
  Run

  Now a seq is an openarray, so the only difference is that var at the beginning.

  This is because Nim distinguishes mutable and immutable objects by type. Hence what the compiler is telling you is that the first parameter ought to be mutable, but it isn't.

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• OderWat (orginal) [2016-06-14T19:38:01+02:00] view original

  Yes. Now I wonder that I did not spot that "var" immediately. I wonder if that error output could get some kind of "highlighting" of the differences with each of the candidates.

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• bobg (orginal) [2016-06-14T21:37:45+02:00] view original

  @andrea - Yes, excellent tutorial. I completely missed the 'var' in the Error message. Maybe I will begin to believe the compiler error messages...

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