The Single Responsibility Principle
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The Single Responsibility Principle
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Re: The Single Responsibility Principle
#2Prior to reading this article, I always took the "single responsibility principle" literally, but largely do this in practice whenever possible.
Thank you.
Re: The Single Responsibility Principle
#3This is nice and drives home something particularly important to me. In Parnas' quote he suggests modularizing such that "things that change together stay together". I think this is highly sensible. Another way of saying it is that your APIs should be fixed.
But often this gets conflated in OO because objects are the hearth of modularization (via encapsulation) along with state and interaction and any number of other things.
---
As a comparison point, you might examine ML modules. They look a bit like this
module counter(X)
count : X -> Int
incr : X -> X
incCounter : counter
incCounter = structure(Int)
count n = n
incr n = n + 1
---
So why does this fall down in OO? Because objects lend themselves to being thought of as entities which move through time and space in a stateful fashion. This means you're also likely to encapsulate differences of entity without regard for how they might change together or apart.
Returning to Parnas' quote: it's a bad idea to decompose into modules based on a flowchart. Flowcharts allow you to emphasize the entities of your system, but they are not demonstrating the boundaries of change.
So you can probably get better OO design by being clear when you're using objects as entities (and thus perhaps you do not need encapsulation at all!) and when you're using them as modules. Once this distinction is made it can be clear when objects will derive from flowcharts and when objects will derive from selections of choices made by people.
Re: The Single Responsibility Principle
#4The most important principle in software design. Follow SRP and so many other things just fall into place.
Re: The Single Responsibility Principle
#5While he might understand what he is talking about, based on a corpus of experience, people who learn about this "principle" as they learn to program do not typically understand it and tend to draw only the wrong conclusions from it. What makes for coupling, and how to avoid it, you simply learn with a lot of practice, and there are not many clearcut guidelines you can formulate, it all varies depending on the problem. At best you can read SICP and see how they decouple everything by doing lots of data-driving and dynamic dispatch and things of this sort.
This is too vague to be a "principle", and only causes confusion. It is about as precise, and as useful, as the "write good code" principle.
Re: The Single Responsibility Principle
#6I personally find it easier to think of it as Conway's Law:
"organizations which design systems ... are constrained to produce designs which are copies of the communication structures of these organizations" - M. Conway
More on http://c2.com/cgi/wiki?ConwaysLaw and http://en.wikipedia.org/wiki/Conway's_law
However, I also hate this mental model of software engineering because I have often found it easier to refactor the organization. Maybe because I'm a prima donna and only like working at startups.
I think it's better to require each portion of code has a narrow interface so you can reason in your mind easily about what that code segment does and should do into the future. A function or a class must promise what it should deliver given some inputs and not violate that expectation. If you ever had to reason about code using invariants, you'll grok this.
Re: The Single Responsibility Principle
#7While he might understand what he is talking about, based on a corpus of experience, people who learn about this "principle" as they learn to program do not typically understand it and tend to draw only the wrong conclusions from it. What makes for coupling, and how to avoid it, you simply learn with a lot of practice, and there are not many clearcut guidelines you can formulate, it all varies depending on the proble…
A lot of people seem to chafe at these principles because they convey truths about a intangible reality they may not even be able to perceive. They only see more work for little payoff. They don't have the body of knowledge and experience (nor the intuition) to know when when to abstract more. We're also in a post-enterprise era, where any sort of deliberate design is often mocked by way of strawman AbstractComposerFactorySingleton references. Rhetoric masquerades as honest intellectual debate for self-promotional purposes.
I consider myself fortunate to have written enough C++ to learn coupling and cohesion. When you screw these up in C++, you pay for it, over and over, through increased compile times and link times.
Re: The Single Responsibility Principle
#8(Warning: bikeshedding/off-topic angry rant ahead. Please ignore.)
"The Single Responsibility Principle (SRP) states that each software module should have one and only one reason to change. This sounds good, and seems to align with Parnas' formulation. However it begs the question: What defines a reason to change?" (emphasis added)
Ok I realize that languages evolve and all; and I see how the nearly universal appeal of using the phrase "begging the question" in this way will ensure it will soon make its way into the dictionary; but I think people should at least know the original meaning of the phrase [1] and that, in some pedantic or predominantly academic circles today, it is considered as incorrect usage. That is all.
Re: The Single Responsibility Principle
#9While he might understand what he is talking about, based on a corpus of experience, people who learn about this "principle" as they learn to program do not typically understand it and tend to draw only the wrong conclusions from it. What makes for coupling, and how to avoid it, you simply learn with a lot of practice, and there are not many clearcut guidelines you can formulate, it all varies depending on the proble…
Yup. Which is why I believe that if you really want to teach someone software design principles, you have to be prepared show them an actual, real software system; the real problem you encountered; the reason why you picked a particular solution; and the results of that choice (both pros and cons). All the details.
Given that data, its easy for a developer to extrapolate (more general) principles that may apply for their own concrete situations and problems, if any.
If you do the extrapolation of the principle yourself but hide the data that caused you to arrive to that principle, nobody will learn anything.
I have no idea why software architects do this. I guess because they're so used to abstracting all the details away in software, they start thinking it also applies to teaching/writing. It does not.
Re: The Single Responsibility Principle
#10While he might understand what he is talking about, based on a corpus of experience, people who learn about this "principle" as they learn to program do not typically understand it and tend to draw only the wrong conclusions from it. What makes for coupling, and how to avoid it, you simply learn with a lot of practice, and there are not many clearcut guidelines you can formulate, it all varies depending on the proble…
A lot of people seem to chafe at these principles because they convey truths about a intangible reality they may not even be able to perceive. They only see more work for little payoff. They don't have the body of knowledge and experience (nor the intuition) to know when when to abstract more. We're also in a post-enterprise era, where any sort of deliberate design is often mocked by way of strawman AbstractComposerF…
They don't chafe because they convey truths about a intangible reality they may not even be able to perceive. They chafe because nothing is actually being conveyed. Its a bunch of words removed from any actual meaning - from the concrete problems that caused the person to arrive to the truth being "conveyed". They're like the conclusion part of an essay with all the rest left out. They tell nothing to people that don't already know, and nothing new to people that do.
Its no wonder people either tend to discard them or to apply them incorrectly.