According to Uncle Bob in his Clean Code book: Functions should either do something or answer something.
So commands (do something) must not to return some information. But, it's useful to know if the command was executed without error.
For that, we can throw an exception that we catch in try/catch.
But, the reactive way (rxjs) seems to be little bit different.
If we take this code sample:
function test(var: number): Observable<void> {
if(var == 2)
return Rx.Observable.throw("error")
return Rx.Observable.of(var)
}
let foo = test(1).catch(err => Rx.Observable.of(err))
foo.subscribe(val => console.log(val))
let foo2 = test(2).catch(err => Rx.Observable.of(err))
foo2.subscribe(val => console.log(val))
Console log of foo displays 1 and it's quite logic. Console log of foo2 displays "error" and it's logic too.
Maybe, I don't use correctly throw and .catch?
To use the catch operator, I have to return an Observable otherwise I have a javascript error (Cannot read property 'catch' of undefined).
That's mean we always must to return something?
Related
In my controller, I extend the Controller class instead of ActiveController
I have a simple actionIndex() method:
public function actionIndex(){
return json_encode(["text" => "some text"]);
}
When I access the certain route in browser, in the debugger, I see that this function is executing (the breakpoint stops inside the function), but I get 500 status code (Internal server error). How can I find the cause of the error? I have implemented the actionError() method, but it is not executing.
public function actionError() {
$exception = Yii::$app->errorHandler->exception;
if ($exception !== null) {
return $this->render('error', ['exception' => $exception]);
}
}
If I put the logic of the actionError() method in the actionIndex(), the $exception variable is null
The only output I get is:
{
name: "PHP Warning",
message: "Expected array for frame 0",
code: 2,
type: "yii\base\ErrorException",
file: "Unknown",
line: 0,
stack-trace: []
}
but it's warning, not error. May this cause the status code 500?
How can I get the error? Thanks!
According to this thread https://github.com/yiisoft/yii2/issues/11120 this is related to Xdebug for Yii 2 < 2.0.9.
When handling an exception, XDebug modifies the exception and Yii2 was not able to handle it correctly.
So several possible solutions here
The best idea is to update Yii2 to a version >= 2.0.9
Also you sould correct the source of the exception (the warning). It is never a good idea to have warnings. It can hide some more issues.
And as a workaround, you can disable XDebug. It is very useful during development but must be disabled in production in all cases.
Don't know about your error, but there is generally no need to return a response as json encoded. Yii checks the Accept-header of the request and adjust output accordingly. In your case I would just use:
public function actionIndex()
{
return ["text" => "some text"];
}
Possibly it might also solve your error...
I have the following service in Angular2:
#Injectable()
export class MyService{
private myServiceUrl= ....
constructor(private http: Http) { }
getService(): Promise<MyObject> {
return this.http.get(this.myServiceUrl).map(response => response.json())
.toPromise();
}
}
And I have this function in one of my components:
myFunction(): any{
let toReturn: any;
this.myService.getService().then(result => toReturn = result);
console.log(toReturn);
return toReturn;
}
Basically, as you can see, I want to store in toReturn, the objects that it is returned by the promise of myService. I am looking to the rest call I have, and I am getting the proper json, but when I am trying to store it in the internal variable, I get undefinied in toReturn.
However, if I try this:
this.myService.getService().then(result=> console.log(result));
I am able to see the json I want.
I can do a function like this:
getService(address: string) {
this.myService.getService().then(result=> this.result= result);
}
But I prefer to make my function to return an object. What am I doing wrong?
Remember that getService is asynchronous, so when your code reaches the line
return toReturn;
The toReturn variable has not yet received the data coming from the server.
The cleanest way to deal with this is to return the promise itself, so that the calling code will extract the data when it arrives from the server:
myFunction(): Promise<MyObject>
{
//return the promise that will output data in the future
return this.myService.getService();
}
The calling code can access the data this way:
myFunction().then( result =>
this.result= result
)
You can even remove a step by getting rid of myFunction because it's just a thin wrapper around getService(). Instead the calling code can simply do:
this.myService.getService().then(result => this.result = result)
And that's it.
Addendum to address your comments:
You must understand two concepts about asynchronous ops and promises:
A promise is executed in a different thread, so the lines after the call to getService() are executed before the data arrives. That's why console.log() doesn't show you data at that point. That's also why trying to return the result there doesn't work
the only place in code where you can capture the result of a promise is .then(). There you can do whatever you want with the result, including storing it in any variable. Just remember point 1 above, and don't expect to have access to the result on the very next line because then() occurs later in time on a different thread.
assign the json that comes from that service (which returns a
promise), to an internal variable that I defined in the line above, it
is not working. How can store it to an internal variable and return it
You cannot.
I was struggling to describe this succintly in the title so I'll paste in my typescript code that achieves what I'm talking about -
aggregate<T, A>(args: A[], invokable: (arg: A) => promise<T>): promise<T[]> {
let allPromises = new Array<promise<T>>();
for (let arg of args) {
allPromises.push(invokable(arg));
}
return promise.all(allPromises);
}
This takes a list of arguments of type A and for each of them invokes some function (which returns a promise which returns type T). Each of these promises are collected into a list which is then all-ified and returned.
My question is, does this function already exist in Bluebird as I'd rather do things properly and use that existing, tested functionality! I had problems getting my head around some of the documentation so I might not have grokked something I should have!
Your problem is perfectly solvable with Array.prototype.map.
Your code can be turned into:
aggregate<T, A>(args: A[], invokable: (arg: A) => promise<T>): promise<T[]> {
return promise.all(args.map(invocable));
}
I would like to create a smart recursion prevention mechanism. I would like to be able to annotate a piece of code somehow, to mark that it should not be executed in recursion, and if it is indeed executed in recursion, then I want to throw a custom error (which can be caught to allow executing custom code when this happens)
Here is my attempt until here:
import scala.collection.mutable.{Set => MutableSet, HashSet => MutableHashSet }
case class RecursionException(uniqueID:Any) extends Exception("Double recursion on " + uniqueID)
object Locking {
var locks:MutableSet[Any] = new MutableHashSet[Any]
def acquireLock (uniqueID:Any) : Unit = {
if (! (locks add uniqueID))
throw new RecursionException(uniqueID)
}
def releaseLock (uniqueID:Any) : Unit = {
locks remove uniqueID
}
def lock1 (uniqueID:Any, f:() => Unit) : Unit = {
acquireLock (uniqueID)
try {
f()
} finally {
releaseLock (uniqueID)
}
}
def lock2[T] (uniqueID:Any, f:() => T) : T = {
acquireLock (uniqueID)
try {
return f()
} finally {
releaseLock (uniqueID)
}
}
}
and now to lock a code segment I do:
import Locking._
lock1 ("someID", () => {
// Custom code here
})
My questions are:
Is there any obvious way to get rid of the need for hard coding a unique identifier? I need a unique identifier which will actually be shared between all invocations of the function containing the locked section (so I can't have something like a counter for generating unique values, unless somehow scala has static function variables). I thought on somehow
Is there any way to prettify the syntax of the anonymouse function? Specifically, something that will make my code look like lock1 ("id") { /* code goes here */ } or any other prettier look.
A bit silly to ask in this stage, but I'll ask anyway - Am I re-inventing the wheel? (i.e. does something like this exist?)
Wild final thought: I know that abusing the synchronized keyword (at least in java) can gaurantee that there would be only one execution of the code (in the sense that no multiple threads can enter that part of the code at the same time). I don't think it prevents from the same thread to execute the code twice (although I may be wrong here). Anyway, if it does prevent it, I still don't want it (even thoug my program is single threaded) since I'm pretty sure it will lead to a deadlock and won't report an exception.
Edit: Just to make it clearer, this project is for error debugging purposes and for learning scala. It has no real useage other than easily finding code errors at runtime (for detecting recursion where it shouldn't happen). See the comments to this post.
Not quite sure what you're aiming at, but a few remarks:
First, you do not need to do lock1 and lock2 to distinguish Unit and the other type. Unit is a proper value type, the generic method will work for it too. Also, you should probably use a call by name argument => T, rather than a function () => T, and use two argument lists:
def lock[T] (uniqueID:Any)(f: => T) : T = {
acquireLock (uniqueID)
try {
f
} finally {
releaseLock (uniqueID)
}
}
Then you can call with lock(id){block} and it looks like common instructions such as if or synchronized.
Second, why do you need a uniqueId, why make Lock a singleton? Instead, make Lock a class, an have as many instances as you would have had ids.
class Lock {
def lock[T](f: => T): T = {acquireLock() ...}
}
(You may even name your lock method apply, so you can just do myLock{....} rather than myLock.lock{...})
Multithreading aside, you now just need a Boolean var for acquire/releaseLock
Finally, if you need to support multithreading, you have to decide whether several thread can enter the lock (that would not be recursion). If they can, the boolean should be replaced with a DynamicVariable[Boolean] (or maybe a java ThreadLocal, as DynamicVariable is an InheritableThreadLocal, which you may or may not want). If they cannot, you just need to synchronize access in acquire/releaseLock.
Is there any obvious way to get rid of the need for hard coding a unique identifier?
Since for what you said on the comments this is not prod code, I guess you could use the functions hashCode property like this:
def lock1 (f:() => Unit) : Unit = {
acquireLock (f.hashCode)
try {
f()
} finally {
releaseLock (f.hashCode)
}
Is there any way to prettify the syntax of the anonymouse function?
With the before-mentioned change the syntax should be prettier:
lock1 {
If you're planning on keeping the identifier (if hashcode doesn't cut it for you) you can define your method like this:
def lock1 (uniqueID:Any)(f:() => Unit) : Unit = {
That will let you call the lock1 method with:
lock("foo") {
}
Cheers!
I am trying to understand how you return non-primitives (i.e. types that do not implement Copy). If you return something like a i32, then the function creates a new value in memory with a copy of the return value, so it can be used outside the scope of the function. But if you return a type that doesn't implement Copy, it does not do this, and you get ownership errors.
I have tried using Box to create values on the heap so that the caller can take ownership of the return value, but this doesn't seem to work either.
Perhaps I am approaching this in the wrong manner by using the same coding style that I use in C# or other languages, where functions return values, rather than passing in an object reference as a parameter and mutating it, so that you can easily indicate ownership in Rust.
The following code examples fails compilation. I believe the issue is only within the iterator closure, but I have included the entire function just in case I am not seeing something.
pub fn get_files(path: &Path) -> Vec<&Path> {
let contents = fs::walk_dir(path);
match contents {
Ok(c) => c.filter_map(|i| { match i {
Ok(d) => {
let val = d.path();
let p = val.as_path();
Some(p)
},
Err(_) => None } })
.collect(),
Err(e) => panic!("An error occurred getting files from {:?}: {}", pa
th, e)
}
}
The compiler gives the following error (I have removed all the line numbers and extraneous text):
error: `val` does not live long enough
let p = val.as_path();
^~~
in expansion of closure expansion
expansion site
reference must be valid for the anonymous lifetime #1 defined on the block...
...but borrowed value is only valid for the block suffix following statement
let val = d.path();
let p = val.as_path();
Some(p)
},
You return a value by... well returning it. However, your signature shows that you are trying to return a reference to a value. You can't do that when the object will be dropped at the end of the block because the reference would become invalid.
In your case, I'd probably write something like
#![feature(fs_walk)]
use std::fs;
use std::path::{Path, PathBuf};
fn get_files(path: &Path) -> Vec<PathBuf> {
let contents = fs::walk_dir(path).unwrap();
contents.filter_map(|i| {
i.ok().map(|p| p.path())
}).collect()
}
fn main() {
for f in get_files(Path::new("/etc")) {
println!("{:?}", f);
}
}
The main thing is that the function returns a Vec<PathBuf> — a collection of a type that owns the path, and are more than just references into someone else's memory.
In your code, you do let p = val.as_path(). Here, val is a PathBuf. Then you call as_path, which is defined as: fn as_path(&self) -> &Path. This means that given a reference to a PathBuf, you can get a reference to a Path that will live as long as the PathBuf will. However, you are trying to keep that reference around longer than vec will exist, as it will be dropped at the end of the iteration.
How do you return non-copyable types?
By value.
fn make() -> String { "Hello, World!".into() }
There is a disconnect between:
the language semantics
the implementation details
Semantically, returning by value is moving the object, not copying it. In Rust, any object is movable and, optionally, may also be Clonable (implement Clone) and Copyable (implement Clone and Copy).
That the implementation of copying or moving uses a memcpy under the hood is a detail that does not affect the semantics, only performance. Furthermore, this being an implementation detail means that it can be optimized away without affecting the semantics, which the optimizer will try very hard to do.
As for your particular code, you have a lifetime issue. You cannot return a reference to a value if said reference may outlive the value (for then, what would it reference?).
The simple fix is to return the value itself: Vec<PathBuf>. As mentioned, it will move the paths, not copy them.