About updating map value

Questions / Help
1

so i am think about how to update a map value but about the map.update but since the map is use the
def start link
to create it what the map name should be MODULE?
and how do i edit the handle_call set price function is using Map.update right?

defmodule Sequence.Server do
  use GenServer

  #####
  # External API  

  def start_link(catalogue) do
    GenServer.start_link(__MODULE__, catalogue, name: __MODULE__)
  end

  def next_number do
    GenServer.call __MODULE__, :next_number
  end

  def get_price(product) do
    GenServer.call __MODULE__, {:get_price, product}
  end

  def increment_number(delta) do
    GenServer.cast __MODULE__, {:increment_number, delta}
  end
  
  def show_catalogue do
    GenServer.call __MODULE__, :show_catalogue 
  end
  
  def set_price(product, np) do
    GenServer.call __MODULE__, {:set_price, product, np}
  end
  


  #####
  # GenServer implementation

  def handle_call(:next_number, _from, current_number) do
    { :reply, current_number, current_number+1 }
  end

  def handle_call({:get_price, product}, _from(client proessid), catalogue) do
		{:ok, price} = Map.fetch(catalogue,product)
		
    { :reply, price, catalogue }
  end

  def handle_cast({:increment_number, delta}, current_number) do
    { :noreply, current_number + delta}
  end

  def format_status(_reason, [ _pdict, state ]) do
    [data: [{'State', "My current state is '#{inspect state}', and I'm happy"}]] 
  end
  
  def handle_call(:show_catalogue, _from, catalogue) do
    { :reply, catalogue, catalogue}
  end

  def handle_call({:set_price, product, np}, _from, catalogue) do
		{:ok, price} = Map.fetch(catalogue,product)
		Map.update(,:product,np)
	{ :reply, price, catalogue }
  end
  
end
2

Well, they are many errors…

To name a few

  • You don’t have an init function
  • The state of the server is returned by this init function, as {:ok, state}
  • You mix state, once it’s current_number, once it’s catalog… it cannot work like this
  • You define var that are not going to be used, and from is not a function
_from(client proessid)
  • You mix handle call with handle cast, but they should be grouped, and the order is very important
  • This function call is incomplete, and wrong
Map.update(,:product,np)

You can get the signature of the function using h helper in the console…

iex> h Map.update
...
@spec update(map(), key(), value(), (value() -> value())) :: map()
...

It takes 4 parameters, a map, a key, a value if key don’t exists, and a function to update.

Maybe You can ask your question about Map.update without involving a server :slight_smile:

3

yes i just want to edit this part of the code only and ask about the
Map.update(???,:product,np)

the run down is like this

c "Sequence.Server.exs"

Sequence.Server.start_link %{a: 5, b: 10}

so the result should be created a map and then if i want to change map value a: 5 > a: 20

it should be using like this

Sequence.Server.set_Price :a, 20

so the code should be using

def set_price(product, np) do
    GenServer.call __MODULE__, {:set_price, product, np}
  end

def handle_call({:set_price, product, np}, _from, catalogue) do
		{:ok, price} = Map.fetch(catalogue,product)
		Map.update/put(???,:product,np)
	{ :reply, price, catalogue }
  end

but the 1st map value i dont know what should i put on it

4

the handle_call is mean have return and handle_cast doesn’t have return?

5

You are mixing the order… You put handle_call, then handle_cast, then handle_call.

They should be grouped together (call with call, cast with cast), not mixed.

6

Use Map.put if You don’t care what was the previous value… but want to override.

7

Don’t forget Elixir is immutable, and doing something like this…

# product, not :product
Map.put(catalogue, product, np)

… is not going to change the value of catalogue. You should do like this

catalogue = Map.put(catalogue, product, np)