nimforum mirror - Hopefully simple Nim syntax question

DMisener (orginal) [2022-04-14T17:35:47+02:00] view original

Probably very simple....but stumps me :-(

Any explanation would be appreciated...

Why is line 25:

lastStoneWeight(weights.mapIt(Stone(weight: it)).toHeapQueue)

not equivalent to commented out lines at 23-24:

var stones = weights.mapIt(Stone(weight: it)).toHeapQueue
  result = lastStoneWeight(stones)

It is just a simple substitution. No?

The first variant works and second genrates as big scary compilation error message (see below)

# See: http://leetcode.com/problems/last-stone-weight/

import sequtils, std/heapqueue # See: https://nim-lang.org/docs/heapqueue.html

type
  Stone = object
    weight: int
  Stones = Heapqueue[Stone]

proc `<`(a, b: Stone): bool = b.weight < a.weight # maxHeap ordering for score

proc lastStoneWeight(stones: var Stones): int =
  case stones.len:
  of 0: 0
  of 1: stones.pop.weight
  else:
    let (weight1, weight2) = (stones.pop.weight, stones.pop.weight)
    let weightDiff = weight1 - weight2
    if weightDiff > 0: stones.push Stone(weight: weightDiff)
    lastStoneWeight stones

proc lastStoneWeight(weights: openArray[int]): int =
# var stones = weights.mapIt(Stone(weight: it)).toHeapQueue
# result = lastStoneWeight(stones)
  lastStoneWeight(weights.mapIt(Stone(weight: it)).toHeapQueue)

proc test() =
  assert lastStoneWeight([1])           == 1
  assert lastStoneWeight([1,1])         == 0
  assert lastStoneWeight([1,3])         == 2
  assert lastStoneWeight([3,8,12])      == 1
  assert lastStoneWeight([1,3,8,12])    == 0
  assert lastStoneWeight([2,7,4,1,8,1]) == 1

if isMainModule: test()

Error text for the second variant:


/Users/DM/work/trunk/coding_dojo/lastStoneWeight.nim(23, 18) Error: type mismatch: got <HeapQueue[lastStoneWeight.Stone]>
but expected one of:
proc lastStoneWeight(stones: var Stones): int
  first type mismatch at position: 1
  required type for stones: var Stones
  but expression 'toHeapQueue(
  type
    OutType`gensym1 = typeof(
      block:
        var it: typeof(items(weights), typeOfIter)
        Stone(weight: it), typeOfProc)
  block:
    template s2_452985056(): untyped =
      weights
    
    var i`gensym1 = 0
    var result`gensym1 = newSeq(len(weights))
    for it in items(weights):
      result`gensym1[i`gensym1] = Stone(weight: it)
      i`gensym1 += 1
    result`gensym1)' is immutable, not 'var'
proc lastStoneWeight(weights: openArray[int]): int
  first type mismatch at position: 1
  required type for weights: openArray[int]
  but expression 'toHeapQueue(
  type
    OutType`gensym1 = typeof(
      block:
        var it: typeof(items(weights), typeOfIter)
        Stone(weight: it), typeOfProc)
  block:
    template s2_452985056(): untyped =
      weights
    
    var i`gensym1 = 0
    var result`gensym1 = newSeq(len(weights))
    for it in items(weights):
      result`gensym1[i`gensym1] = Stone(weight: it)
      i`gensym1 += 1
    result`gensym1)' is of type: HeapQueue[lastStoneWeight.Stone]

expression: lastStoneWeight(toHeapQueue(
  type
    OutType`gensym1 = typeof(
      block:
        var it: typeof(items(weights), typeOfIter)
        Stone(weight: it), typeOfProc)
  block:
    template s2_452985056(): untyped =
      weights
    
    var i`gensym1 = 0
    var result`gensym1 = newSeq(len(weights))
    for it in items(weights):
      result`gensym1[i`gensym1] = Stone(weight: it)
      i`gensym1 += 1
    result`gensym1))

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