Using ruby's 'case'/'when' method with || or && operators, etc

Question

I just tried to run some code that looked like this

def get_proj4(srid, type=nil)
  type.downcase! if type
  case type
  when nil || "epsg"
   open("http://spatialreference.org/ref/epsg/#{srid}/proj4/").read
  when "esri"
   open("http://spatialreference.org/ref/esri/#{srid}/proj4/").read
  end
end

and it didn't run properly, returning nil every time. wrapping the nil || "epsg" in parentheses didn't work either

It turns out ruby wouldn't allow me to use the || operator in this

Now I assume ruby takes the case/when method and ultimately breaks it down into a group of conditionals looking something like

x = type
  if x == (nil || "epsg")
    y = ...runs code...
  elsif x == "esri"
    y = ...
  end
x = nil
y

but obviously it does not. What's going on here?

Thanks

Answer 1

The expression is evaluated first so when nil || "espg" is equivalent to when "espg"¹ - it will never match nil.

To match either-or, separate the options with a comma:

case type
when nil, "espg" ..
when "esri" ..

Or, alternatively, perhaps normalize the value:

case (type || "espg")
when "espg" ..
when "esri" ..

Or use the other form that resembles an if-else:

case
when type.nil? || type == "espg" ..
when type == "esri" ..

Or some combination of everything :)

---

¹ This is also the same reason why the example if is suspect. It should probably be written like:

if type.nil? || type == "espg"