How does instance_eval work and why does DHH hate it?

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The thing that instance_eval does is that it runs the block in the context of a different instance. In other words, it changes the meaning of self which means it changes the meaning of instance methods and instance variables.

This creates a cognitive disconnect: the context in which the block runs is not the context in which it appears on the screen.

Let me demonstrate that with a slight variation of @Matt Briggs’s example. Let’s say we’re building an email instead of a form:

def mail
  builder = MailBuilder.new
  yield builder
  # executed after the block 
  # do stuff with builder 
end

mail do |f|
  f.subject @subject
  f.name    name
end

In this case, @subject is an instance variable of your object and name is a method of your class. You can use nice object-oriented decomposition and store your subject in a variable.

def mail &block
  builder = MailBuilder.new
  builder.instance_eval &block
  # do stuff with builder 
end

mail do 
  subject @subject
  name    name # Huh?!?
end

In this case, @subject is an instance variable of the mail builder object! It might not even exist! (Or even worse, it might exist and contain some completely stupid value.) There is no way for you to get access to your object’s instance variables. And how do you even call the name method of your object? Everytime you try to call it, you get the mail builder’s method.

Basically, instance_eval makes it hard to use your own code inside the DSL code. So, it should really only be used in cases where there is very little chance that this might be needed.

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