Sum of list of maps

list 
|> Enum.map(fn %{"price" => price} -> price 
|> String.to_integer() end) 
|> Enum.sum()

Result: 500

If the end result you want is an integer, then it’s best to start with an integer as the accumulator, like this:

Enum.reduce(list, 0, fn item, acc ->
  acc + String.to_integer(item[”price”])
end)

Could do pure recursion:

def reduce(list), do: reduce(list, 0)

def reduce([], acc), do: acc
def reduce([%{"price" => value} | rest], acc), do: reduce(rest, acc + String.to_integer(value))
def reduce([_hd | rest], acc), do: reduce(rest, acc)

I just want to point out why you received that error.

reduce(list, fn item, acc -> do_something(...) end)

The result of the do_something(...) will be passed as the new acc. It is a map (%{val: 200}) after the first round of run.

String.to_integer(%{val: 200}["price"]}) leads to the exception. So you just need to change :val into "price" as the map key to bypass the exception. But the above approaches provided by @hudsonbay @APB9785 and @mpope are better IMO.