Explanation on how "Tell, Don't Ask" is considered good OO
- by Pubby
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.
