Flutter 2 dimensional List/Array as Int/Object(class)

A list or multi-dimensional lists can only have a single type. Looking at your sample code, it looks like you're trying to associate ingredients and properties of a given recipe to a recipe using a multi-dimensional list.

A better approach would be to use classes. Classes will give you a more structured design and a little more flexibility. For example, the code below overrides the equality operator (==) and hashCode to be able to compare recipes and find a given recipe in a list.

class Recipe {
  final String name;
  final List<Ingredient> ingredients;
  final List<Instruction> instructions;

  Recipe({
    required this.name,
    required this.ingredients,
    required this.instructions,
  });

  @override
  bool operator ==(Object other) =>
      identical(this, other) || other is Recipe && runtimeType == other.runtimeType && name == other.name;

  @override
  int get hashCode => name.hashCode ^ ingredients.hashCode ^ instructions.hashCode;
}

class Ingredient {
  final String name;
  final String description;

  Ingredient({
    required this.name,
    required this.description,
  });
}

class Instruction {
  final String description;
  final String? tip;

  Instruction({
    required this.description,
    this.tip,
  });
}

final recipes = [
  Recipe(
    name: 'Cheese pizza',
    ingredients: [
      Ingredient(name: 'Dough', description: 'Normal yeast based pizza dough'),
      Ingredient(name: 'Tomato Sauce', description: 'Freshly blended organic tomatoes'),
      Ingredient(name: 'Mozzarella Cheese', description: 'Cheesy cheesiness from mount cheese'),
    ],
    instructions: [
      Instruction(description: 'Preheat oven to 400 degrees Fahrenheit'),
      Instruction(description: 'While oven is preheating spread dough evenly in a circle over an oiled pan'),
      Instruction(description: 'Spread sauce evenly over dough leaving enough room for the crust'),
      Instruction(description: 'Cover the sauce with the Mozzarella cheese'),
      Instruction(
        description: 'Roll the edge of the dough to form the crust',
        tip: 'Optionally add cheese within the dough for stuffed crust!',
      ),
      Instruction(description: 'Bake the pizza in the over until golden brown'),
      Instruction(
        description: 'Remove the pizza from the oven and let cool',
        tip: 'Some people like to let the pizza cool before cutting',
      ),
    ],
  ),
];

/// Find the index of a recipe. This uses the equality and hashcode overrides of the recipe class.
int getRecipeIndex(Recipe recipe) => recipes.indexOf(recipe);

/// Find all of the recipes containing the [query] in their name (case-insensitive).
List<Recipe> getRecipeByName(String query) {
  return recipes.where((element) => element.name.toLowerCase().contains(query.toLowerCase())).toList();
}

/// Get the ingredients of a recipe.
final recipeIngredients = recipes[0].ingredients;

/// Get the instructions of a recipe.
final recipeInstructions = recipes[0].instructions;

/// Get a list of all of the tips for a recipe.
final recipeTips = recipes[0].instructions.map((e) => e.tip).whereType<String>();

If for whatever reason you wanted to associate a recipe with a list of recipes (e.g. similar recipes) I would use a map of Recipe to List<Recipe>.

final pizza = recipes[0];

/// How to associate a recipe with similar recipes using a map of `Recipe` to `List<Recipe>`
final similarRecipesMap = { pizza: [recipes[1], recipes[2]]};

final similarRecipesToPizza = similarRecipesMap[pizza];

Hope that helps!