Re: Nested list comprehension

[dawkot]

This should work: 
    
    
    import tables, strutils
    
    proc cross(xs, ys: string): seq[string] =
      for x in xs: (for y in ys: result.add x & y)
    
    let
      rows = "ABCDEFGHI"
      cols = "123456789"
      squares = cross(rows, cols)
    
    var unitSeq: seq[string]
    for c in cols: unitSeq.add cross(rows, $c)
    for r in rows: unitSeq.add cross(cols, $r)
    for rs in ["ABC", "DEF", "GHI"]: (for cs in ["123", "456", "789"]: 
unitSeq.add cross(rs, cs))
    
    var units, peers: Table[string, seq[string]]
    for s in squares:
      var tmp: seq[string]
      for u in unitSeq: (if s in u: tmp.add u)
      units[s] = tmp
      if (let i = tmp.find s; i >= 0): tmp.del i
      peers[s] = tmp
    
    
    Run