And the cat's in the cradle and the silver spoon,
Little boy blue and the man in the moon.
"When you coming home, dad?" "I'm home right now,
having some fun and how,
You know we're having fun and how!"
Go comes in part from Rob Pike and Ken Thompson, both influential in early UNIX. Both Rob Pike and Ken Thompson also were influential in working on Plan 9, a followup to UNIX.
UNIX's ideal is that "everything is a file". In Go terminology, this is a declaration that everything should be accessible via a uniform interface, which the OS specially privileges. One of Plan 9's core reasons for existing is that UNIX didn't take this anywhere near as far as it could be taken, and it goes much further in making everything accessible as a file in a directory structure.
I'm skeptical of both of these approaches. Everything isn't a "file".
There's numerous "files" that require ioctls to correctly manipulate, which are arbitrary extensions outside of the file interface. On the flip side, there are all kinds of "files" that can't be seeked, such as sockets, or files that can't be closed, like UDP streams. Pretty much every element of the file interface is one that doesn't apply to some "file", somewhere.
The Procrustean approach to software engineering tends to have the same results as Procrustes himself did, gravely or even fatally wounding the code in question.
Supervisor trees are one of the core ingredients in Erlang's reliability and let it crash philosophy. A well-structured Erlang program is broken into multiple independent pieces that communicate via messages, and when a piece crashes, the supervisor of that piece automatically restarts it.
This may not sound very impressive if you've never used it. But I have witnessed systems that I have written experience dozens of crashes per minute, but function correctly for 99% of the users. Even as I have been writing suture, I have on occasion been astonished to flip my screen over to the console of Go program I've written with suture, and been surprised to discover that it's actually been merrily crashing away during my manual testing, but soldiering on so well I didn't even know.
(This is, of course, immediately followed by improving my logging so I do know when it happens in the future. Being crash-resistant is good, but one should not "spend" this valuable resource frivolously!)
I've been porting a system out of Erlang into Go for various other reasons, and I've missed having supervisor trees around. I decided to create them in Go. But this is one of those cases where we do not need a transliteration of the Erlang code into Go. For one thing, that's simply impossible as the two are mutually incompatible in some fundamental ways. We want an idiomatic translation of the functionality, which retains as much as possible of the original while perhaps introducing whatever new local capabilities into it make sense.
To correctly do that, step one is to deeply examine not only the what of Erlang supervision trees, but the why, and then figure out how to translate.
One of the things I've been really enjoying about Go is how easy testing is. The pervasive use of interfaces and composition-instead-of-inheritance synergize nicely for testing. But as I've expressed this online on reddit and Hacker News a couple of times, I've found that this does not seem to be a universally-shared opinion. Some have even commented on how hard it is to test in Go.
Since we are all obviously using the same language, the difference must lie in coding behavior. I've internalized a lot of testing methodology over the years, and I find some of the things work even better in Go that most other imperative languages. Let me share one of my core tricks today, which I will call the Environment Object pattern, and why Go makes it incrementally easier to use than other similar (imperative) environments.
There are a number of errors made in putative Monad tutorials in languages other than Haskell. Any implementation of monadic computations should be able to implement the equivalent of the following in Haskell:
minimal :: Bool -> [(Int, String)] minimal b = do x <- if b then [1, 2] else [3, 4] if x `mod` 2 == 0 then do y <- ["a", "b"] return (x, y) else do y <- ["y", "z"] return (x, y)
This should yield the local equivalent of:
Prelude> minimal True [(1,"y"),(1,"z"),(2,"a"),(2,"b")] Prelude> minimal False [(3,"y"),(3,"z"),(4,"a"),(4,"b")]
At the risk of being offensive, you, ahhh... really ought to understand why that's the result too, without too much effort... or you really shouldn't be writing a Monad tutorial. Ahem.
- Many putative monadic computation solutions only work with a "container" that contains zero or one elements, and therefore do not work on lists. >>= is allowed to call its second argument (a -> m b) an arbitrary number of times. It may be once, it may be dozens, it may be none. If you can't do that, you don't have a monadic computation.
- A monadic computation has the ability to examine the intermediate results of the computation, and make decisions, as shown by the if statement. If you can't do that, you don't have a monadic computation.
- In statically-typed languages, the type of the inner value is not determined by the incoming argument. It's a -> m b, not a -> m a, which is quite different. Note how x and y are of different types.
- The monadic computation builds up a namespace as it goes along; note we determine x, then somewhat later use it in the return, regardless of which branch we go down, and in both cases, we do not use it right away. Many putative implementations end up with a pipeline, where each stage can use the previous stage's values, but can not refer back to values before that.
- Monads are not "about effects". The monadic computation I show above is in fact perfectly pure, in every sense of the term. And yes, in practice monad notation is used this way in real Haskell all the time, it isn't just an incidental side-effect.
If you can translate the above code correctly, and obtain the correct result, I don't guarantee that you have a proper monadic computation, but if you've got a bind or a join function with the right type signatures, and you can do the above, you're probably at least on the right track. This is the approximately minimal example that a putative implementation of a monadic computation ought to be able to do.
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