The assert statement method only works in development. Is there a standard Dart function which works like assert but which will be executed when running production code?
I'm looking for something like this:
void myFunc(List myList) {
validate(myList.isNotEmpty);
...
}
There is nothing in the Dart SDK, but Google's quiver package provides a checkState function that does what you want.
Related
Debugging with extension methods seems to be a bit problematic currently.
Consider the following code:
import 'dart:developer';
void main() {
final a = A();
workaround = a.extMethod;
debugger();
a.normalMethod();
a.extMethod();
}
class A {
String normalMethod() => 'hello from class';
}
extension AX on A {
String extMethod() => 'hello from extension';
}
When debugging this, stepping into both methods (normalMethod() and extMethod()) will work fine, the only difference is that this isn't available for extMethod() for some reason (there is #this instead, but it can't be used anyhow).
The extension method (extMethod()) is impossible to use for evaluations in the debug console, though.
a.normalMethod()
"hello from class"
a.extMethod()
Unhandled exception:
NoSuchMethodError: Class 'A' has no instance method 'extMethod'.
Receiver: Instance of 'A'
Tried calling: extMethod()
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:38:5)
#1 Eval ()
workaround()
"hello from extension"
I have found kind of a workaround as you can see, but it has to be in place before running the program so I'd definitely prefer a better method that can be used on fly without any preparation.
I have found two very old issues describing this exact problem but both are closed and from what I understand, unresolved.
https://github.com/dart-lang/sdk/issues/44801
https://github.com/dart-lang/sdk/issues/44472
I'm using Dart 2.18.6 (latest stable).
In the following code snippet, an exception is thrown, as is to be expected:
#Test
public void testError()
{
Mono.error(new NullPointerException())
.subscribe();
}
However, in the following code snippet no exception is thrown:
#Test
public void testErrorWithDoOnSuccess()
{
Mono.error(new NullPointerException())
.doOnSuccess(aValue -> {})
.subscribe();
}
When I use the block operator instead of the subscribe operator, the exception works again:
#Test
public void testErrorWithDoOnSuccessAndBlock()
{
Mono.error(new NullPointerException())
.doOnSuccess(aBoolean -> {})
.block();
}
The documentation of the Mono.doOnSuccess operator does not say anything specifically about its behavior w.r.t. errors, but I wouldn't expect this behavior at all given the name of the operator. Why does it suppress exceptions (and only when not using the block operator)? Is this intended behavior, or is this a bug?
Reactor version used is the most recent one at time of writing, 3.1.8.RELEASE.
This is a bug, the operator should suppress the "No error callback implemented" exception only in the case where you use doOnTerminate/doAfterTerminate, not doOnSuccess...
That said, operator still propagates the NPE and this bug is easily worked around by implementing best practices: always define at least value handler and error handler in subscribe(...).
I am having difficulty referring to parameterless functions in Fable.
With this example:
let f1 () =
1
let someRefTof1 = f1
I'd expect the generated js to look something like this:
function f1() {
return 1;
}
var someRefTof1 = f1;
but what I actually get is:
function f1() {
return 1;
}
var someRefTof1 = exports.someRefTof1 = function someRefTof1(arg00_) {
return f1(arg00_);
};
I'm unclear on the purpose of arg00_ or how to avoid it being generated?
(As a bit of background, I am struggling to call a function in an external js library which expects a function to be passed as a parameter)
Edit:
Background
The above is what i believe to be a minimal, verifiable, reproduction of my question but, after comments, I thought it may be useful to provide a touch more context on why this is causing issues. What I am actually trying to do is use angularjs from Fable.
So my example looks more like this:
let app = AngularFable.NgFable.angular.``module``("app",[||])
type TestCtrl() =
member this.Val1() = "boom";
app?controller("test", TestCtrl)
which gets compiled to:
var app = exports.app = angular.module("app", []);
var TestCtrl = exports.TestCtrl = function () {
function TestCtrl() {
_classCallCheck(this, TestCtrl);
}
TestCtrl.prototype.Val1 = function Val1() {
return "boom";
};
return TestCtrl;
}();
_fableCore.Util.setInterfaces(TestCtrl.prototype, [], "App.TestCtrl");
app.controller("test", function (unitVar) {
return new TestCtrl();
});
with unitVar being the problematic parameter introduced in this example. When I use this in my html with something like:
<div ng-app="app">
<div ng-controller="test as vm">
{{vm.Val1()}}
</div>
</div>
I run into an unknown provider error whereas if I simply change the compiled javascript to remove the unitVar parameter from the last line like this:
app.controller("test", function () {
return new TestCtrl();
});
then my example works as expected.
I'd really like to know if there is a way to avoid having the Fable compiler generate this parameter. I'm 99% sure this reduces to the same problem as in my original question but I've included this additional context to better explain why this is an issue
Thank you very much for your question and detailed explanations. There're two things here that are a bit tricky and are caused by optimisations both of the F# compiler and Fable.
In the AST provided by the F# compiler, methods (functions that are members of a type or module) are compiled as usual methods as in C#. This is for optimization.
However, when you create an anonymous lambda or make a reference to a method, the F# compiler will keep F# semantics, that is, all functions have a single argument (as John Palmer says, unit is an argument too) and can be curried.
Ok, this info is just to make clear why the F# compiler/Fable represent methods and lambdas differently. Let's go with the issue of argumentless functions: the obvious solution would be of course to remove the F# compiler generated argument for functions accepting unit (as it's already done for methods). In fact, I also had problems with libraries like Mocha because of this.
I did try to remove the unit argument at the beginning but I got fails in some scenarios because of this. TBH, I don't remember now exactly which tests were failing but because of the expectation that there'll be always an argument, in some cases function composition or inlining was failing when the unit argument was removed.
Other attempts to modify the semantics of F# functions in the JS runtime have always failed because they don't cover all scenarios. However, we can be more lenient with delegates (System.Func<>) as it's usually safe to assume these ones should behave more like functions in languages like C# or F#. I can try to remove the unit argument just for delegates and see what happens :)
For more info about sending F# functions to JS code you can check the documentation.
UPDATE: Scratch all that, please try fable-compiler#0.6.12 and fable-core#0.6.8. This version eliminates unit arguments, the solution was actually simpler than I thought and (hopefully) shouldn't create issues with existing projects. (The explanation about methods and lambdas compiled differently still applies.)
I have a Java 7 project which makes a lot of use of Javascript for scripting various features. Until now I was using Rhino as script engine. I would now like to move to Java 8, which also means that I will replace Rhino by Nashorn.
How compatible is Nashorn to Rhino? Can I use it as a drop-in replacement, or can I expect that some of my scripts will not work anymore and will need to be ported to the new engine? Are there any commonly-used features of Rhino which are not supported by Nashorn?
One problem is that Nashorn can no longer by default import whole Java packages into the global scope by using importPackage(com.organization.project.package);
There is, however, a simple workaround: By adding this line to your script, you can enable the old behavior of Rhino:
load("nashorn:mozilla_compat.js");
Another problem I ran into is that certain type-conversions when passing data between java and javascript work differently. For example, the object which arrives when you pass a Javascript array to Java can no longer be cast to List, but it can be cast to a Map<String, Object>. As a workaround you can convert the Javascript array to a Java List in the Javascript code using Java.to(array, Java.type("java.util.List"))
To use the importClass method on JDK 8, we need to add the following command:
load("nashorn:mozilla_compat.js");
However, this change affect the execution on JDK 7 (JDK does not gives support to load method).
To maintain the compatibility for both SDKs, I solved this problem adding try/catch clause:
try{
load("nashorn:mozilla_compat.js");
}catch(e){
}
Nashorn can not access an inner class when that inner class is declared private, which Rhino was able to do:
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
public class Test {
public static void main(String[] args) {
Test test = new Test();
test.run();
}
public void run() {
ScriptEngineManager factory = new ScriptEngineManager();
ScriptEngine engine = factory.getEngineByName("JavaScript");
Inner inner = new Inner();
engine.put("inner", inner);
try {
engine.eval("function run(inner){inner.foo(\"test\");} run(inner);");
} catch (ScriptException e) {
e.printStackTrace();
}
}
private class Inner {
public void foo(String msg) {
System.out.println(msg);
}
}
}
Under Java8 this code throws following exception:
javax.script.ScriptException: TypeError: kz.test.Test$Inner#117cd4b has no such function "foo" in <eval> at line number 1
at jdk.nashorn.api.scripting.NashornScriptEngine.throwAsScriptException(NashornScriptEngine.java:564)
at jdk.nashorn.api.scripting.NashornScriptEngine.evalImpl(NashornScriptEngine.java:548)
I noticed that Rhino didn't have a problem with a function called 'in()' (although 'in' is a reserved JavaScript keyword).
Nashorn however raise an error.
Nashorn cannot call static methods on instances! Rhino did this, therefore we had to backport Rhino to Java 8 (Here's a short summary: http://andreas.haufler.info/2015/04/using-rhino-with-java-8.html)
Nashorn on Java8 does not support AST. So if you have Java code that inspects the JS source tree using Rhino's AST mechanism , you may have to rewrite it (using regex maybe) once you port your code to use Nashorn.
I am talking about this API https://mozilla.github.io/rhino/javadoc/org/mozilla/javascript/ast/AstNode.html
Nashorn on Java9 supports AST though.
One feature that is in Rhino and not Nashorn: exposing static members through instances.
From http://nashorn-dev.openjdk.java.narkive.com/n0jtdHc9/bug-report-can-t-call-static-methods-on-a-java-class-instance : "
My conviction is that exposing static members through instances is a
sloppy mashing together of otherwise separate namespaces, hence I
chose not to enable it.
I think this is deeply wrong. As long as we have to use two different constructs to access the same java object and use package declarations unnecessarily in javascript, code becomes harder to read and write because cognitive load increases. I will rather stick to Rhino then.
I have not found a workaround for this obvious "design bug" yet.
I'm using the HotTowel SPA template which makes use of Durandal. In my Durandal ViewModels I am using Breeze to get some data from the database.
I have a datacontext class that I put all my breeze queries in and the queries all follow the pattern like the following:
getAthletes: function (queryCompleted) {
var query = breeze.EntityQuery.from("Athletes");
return manager
.executeQuery(query)
.then(queryCompleted)
.fail(queryFailed)
}
Since I'm doing an asynchronous call in the activate method of the view model, I have to return the promise that comes back from these calls in the activate method.
Using a single query works great like this:
function activate() {
datacontext.getAthlete(loadAthlete);
}
However, if I need to perform two queries I run into problems, but only in the release version of my application. I have tried doing this with the following syntax:
function activate() {
datacontext.getAthlete(loadAthlete).then(datacontext.getOtherData(loadOtherData));
}
This will work fine in debug mode, but when I deploy it out to the server and my scripts get bundled, I get an exception which isn't very clear.
t is not a function
I've also tried chaining them together in my datacontext class like below, but I still get the same error.
getAthleteAndEfforts: function (athleteId, athleteQueryCompleted, effortsQueryCompleted) {
var athleteQuery = breeze.EntityQuery.from("Athletes").where("id", "==", athleteId);
var effortsQuery = breeze.EntityQuery.from("BestEfforts").where("athleteId", "==", athleteId);
return manager.executeQuery(athleteQuery).then(athleteQueryCompleted)
.then(manager.executeQuery(effortsQuery).then(effortsQueryCompleted))
.fail(queryFailed);
}
So I'm assuming I just don't understand the Q.defer() enough to use it properly or there is something else going on.
What is the correct syntax to accomplish this?
Ok, thanks to RainerAtSpirit for pointing me in the right direction to find this. I looked at John Papa's jumpstarter examples and he has a datacontext that does this under the primeData function.
So using the syntax he used there I was able to get it to work correctly like this:
getAthleteAndEfforts: function (athleteId, athleteQueryCompleted, effortsQueryCompleted) {
return Q.all([
datacontext.getAthlete(athleteId, athleteQueryCompleted),
datacontext.getAthleteEfforts(athleteId, effortsQueryCompleted)]);
}
I had seen the Q.all in the Q documentation but wasn't sure how to use it, but this example helped. I tested this and it works both in debug and release modes.
Not sure why the first version is working at all, but you'd return a promise when datacontext is making async calls.
function activate() {
return datacontext.getAthlete(loadAthlete);
}
or
function activate() {
return datacontext.getAthlete(loadAthlete).then( return datacontext.getOtherData(loadOtherData));
}
Check #John Papa's jumpstarter for more examples: https://github.com/johnpapa/PluralsightSpaJumpStartFinal/search?q=activate