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 fixed at the beginning. 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.
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.
Scientists (and, in my experience, especially bioinformaticians) tend to make horrible, awful messes no matter how maintainable you think a language is. (You can hand them Inform 7 and it'll still end up looking like Fortran ate the csh manual and vomited all over an APL keyboard.)
I don't listen to the radio hardly at all anymore. Recently, I was with my wife while she was just idly flipping through, and I was astounded.
The electro/techno stuff was autotuned, but that's less of a surprise.
Autotune, autotune, AUTOTUNE!
Not even subtly, either, but cranked up as far as it will go before the high end simply explodes with noise.
Is there anyone left in the music industry that can carry a tune?
|Past Posts ->|