Can't evaluate at compile time - NIM - nim-lang

Question

Hi I'm starting to play around with NIM
I get a "can't evaluate at compile time" error on this code:
import strutils
type
Matrix[x, y: static[int], T] = object
data: array[x * y, T]
var n,m: int = 0
proc readFile() =
let f = open("matrix.txt")
defer: f.close()
var graph_size = parseInt(f.readline)
var whole_graph: Matrix[graph_size, graph_size, int]
for line in f.lines:
for field in line.splitWhitespace:
var cell = parseInt(field)
whole_graph[n][m] = cell
m = m + 1
n = n + 1
readFile()
Any help appreciated.

Answer 1

Unless you absolutely positively need array in this scenario while not knowing its size at compile-time, you may want to rather swap to the seq type, whose size does not need to be known at compile-time.
Together with std/enumerate you can even save yourself the hassle of tracking the index with n and m:
import std/[strutils, enumerate]
type Matrix[T] = seq[seq[T]]
proc newZeroIntMatrix(x: int, y: int): Matrix[int] =
result = newSeqOfCap[seq[int]](x)
for i in 0..x-1:
result.add(newSeqOfCap[int](y))
for j in 0..y-1:
result[i].add(0)
proc readFile(): Matrix[int] =
let f = open("matrix.txt")
defer: f.close()
let graph_size = parseInt(f.readline)
var whole_graph = newZeroIntMatrix(graph_size, graph_size)
for rowIndex, line in enumerate(f.lines):
for columnIndex, field in enumerate(line.split):
let cell = parseInt(field)
whole_graph[rowIndex][columnIndex] = cell
result = whole_graph
let myMatrix = readFile()
echo myMatrix.repr
Further things I'd like to point out though are:
array[x * y, T] will not give you a 2D array, but a single array of length x*y. If you want a 2D array, you would most likely want to store this as array[x, array[y, T]]. That is assuming that you know x and y at compile-time, so your variable declaration would look roughly like this: var myMatrix: array[4, array[5, int]]
Your Matrix type has the array in its data field, so trying to access the array with that Matrix type needs to be done accordingly (myMatrix.data[n][m]). That is, unless you define proper []and []= procs for the Matrix type that do exactly that under the hood.