This blogpost was posted on Hacker News with several upvotes. Coming from C++, m

Question

This blogpost was posted on Hacker News with several upvotes. Coming from C++, most of these examples seem to go against what I've been taught.

Such as example #2:

Bad:

def check_for_overheating(system_monitor)
if system_monitor.temperature > 100
    system_monitor.sound_alarms
end
end
versus good:

system_monitor.check_for_overheating

class SystemMonitor
def check_for_overheating
    if temperature > 100
      sound_alarms
    end
end
end
The advice in C++ is that you should prefer free functions instead of member functions as they increase encapsulation. Both of these are identical semantically, so why prefer the choice that has access to more state?

Example 4:

Bad:

def street_name(user)
if user.address
    user.address.street_name
else
    'No street name on file'
end
end
versus good:

def street_name(user)
user.address.street_name
end

class User
def address
    @address || NullAddress.new
end
end

class NullAddress
def street_name
    'No street name on file'
end
end
Why is it the responsibility of User to format an unrelated error string? What if I want to do something besides print 'No street name on file' if it has no street? What if the street is named the same thing?

Could someone enlighten me on the "Tell, Don't Ask" advantages and rationale? I am not looking for which is better, but instead trying to understand the author's viewpoint.

Explanation / Answer

Asking the object about its state, and then calling methods on that object based on decisions made outside of the object, means that the object is now a leaky abstraction; some of its behavior is located outside of the object, and internal state is exposed (perhaps unnecessarily) to the outside world.

You should endeavor to tell objects what you want them to do; do not ask them questions about their state, make a decision, and then tell them what to do.

The problem is that, as the caller, you should not be making decisions based on the state of the called object that result in you then changing the state of the object. The logic you are implementing is probably the called object

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