indexing.nim

const
  NumCoord = 3
  maxpts = 8
  
type 
  TPt = object                       # A particle object:
    p, v : array[NumCoord, float64]  # The position and velocity. 
    r, life: float64                 # Radius and remaining lifetime
    bis: bool                        # Living or not

  Tpts = object
    high, low: int              # The maximum index in the pool that currently contains a particle
    pool: array[MaxPts, TPt]    # The pool of particles

proc `[]`(pts: var Tpts, key: int): TPt = pts.pool[key]
proc `[]=`(pts: var Tpts, key: int, val: TPt) = pts.pool[key] = val


var pts = Tpts(high: 0, low: 0)
pts[1].life = 4
echo repr pts[1]

# Compilation result: Error: '[](pts, 1).life' cannot be assigned to
# If I take off the 'var' from the first parameter, then the error is: Error: 'pts.pool[key]' cannot be assigned to

It was just the lack of 'var' on the return variable from []. It seems I don't even need to define []=