Interpolating Ruby Strings

Let’s talk about interpolation. Sure, you probably know the Ruby string interpolation syntax:

arg = "world"
puts "Hello #{arg}!" #=> "Hello world!"

This syntax is pretty useful. However, it has it’s limitations. Let’s say that we are generating strings that are almost the same each time, but with each, one word changes. How are you going to do this now?

My first inclination was something like this:

template = "Today the weather is \#{ weather }"
weather_strings = {}

["warm", "cloudy", "windy"].each do |weather|
  weather_strings[weather] = template
end

It’s not hard to see why weather wasn’t interpolated into the strings. Well, it occurred to me shortly thereafter that ERB might be the right way to go with this.

First try with ERB:

template = "Today the weather is <%= weather %>"
weather_strings = {}

["warm", "cloudy", "windy"].each do |weather|
  weather_strings[weather] = ERB.new(template).result
end

This, however, resulted in the following error:

NameError - undefined local variable or method `weather' for main:Object:

After some searching about on the internet, it appears that there is an ever-present binding variable that is an instance of the Binding class. Strange as that was, it seems to be needed by ERB#result to properly interpolate variables. I also discovered that we can also get by with only building the ERB instance once:

template = ERB.new("Today the weather is <%= weather %>")
weather_strings = {}

["warm", "cloudy", "windy"].each do |weather|
  weather_strings[weather] = template.result(binding)
end

Another approach, if you don’t want to pass ERB all variables in your binding is to use result_with_hash (using a Hash instead of a Binding):

template = ERB.new("Today the weather is <%= weather %>")
weather_strings = {}

["warm", "cloudy", "windy"].each do |weather|
  weather_strings[weather] = template.result_with_hash({weather: weather})
end

Not very intuitive, but it works!