[SOLVED] How guard nil in math function if it drops with bad argument in arithmetic expression

How guard nil in math function if it drops with bad argument in arithmetic expression

Trying figure out, while generating GQL schema, why if :float data in source var square_plot exists (say 50), there is no ERR but if it is NIL the code generates error while i converting value so sqft with such code:

may be it seems no validation passes

CODE

field(:square_plot, :float)     // my var is `50.0` in DB

field :square_plot_ft, :float do
  if square_plot > 0 do
    resolve(fn _args, %{source: source} -> {:ok, Float.round(source.square_plot*10.7639, 0)} end)
  end
end

if value NOT NIL the GQL response

{
    "data": {
        "unit": {
            "squareTotalFt": 1615.0,
            "squareTotal": 150.0,
            "squarePlotFt": 538.0,
            "squarePlot": 50.0
        }
    }
}

if value is NIL the GQL response

"Internal server error"

and server ERR

** (exit) an exception was raised:
    ** (ArithmeticError) bad argument in arithmetic expression
        (platform) lib/units_web/schema/object.ex:228: anonymous fn/2 in UnitsWeb.Schema.Unit.__abs
inthe_type__/1

Solution

Surprisingly enough, in erlang, and therefore in elixir, comparisons </> involving different types are legal.

Even more surprisingly, any atom is greater than any number (see Atom Ordering in the documentation.)

nil > 0
#⇒ true

and we get into if, attempting square_plot*10.7639 which is nil*10.7639, which obviously raises.

The proper code would be:

field :square_plot_ft, :float do
  if is_number(square_plot) and square_plot > 0 do
    resolve(...)
  end
end