Issue
Attempting to identify which is the best practice for executing sequential methods. Either, nesting conditionals one after another, or nesting conditionals one inside another, within a main function. In addition, if you could supply "why" one method would be better than the other besides what's most acceptable, I'd sincerely appreciate it. Here are my examples:
Nesting one after another
int main()
{
// conditional 1
if (!method_one())
{
... do something
}
else
{
... prompt error for method 1!
}
// conditional 2
if (!method_two())
{
... do something
}
else
{
... prompt error for method 2!
}
// conditional 3
if (!method_three())
{
... do something
}
else
{
... prompt error for method 3!
}
return 0;
}
Nesting one inside another
int main()
{
// conditional 1
if (!method_one())
{
if (!method_two())
{
if (!method_three())
{
... next steps in sequence
}
else
{
... prompt error for method 3!
}
... prompt error for method 2!
}
... prompt error for method 1!
}
return 0;
}
Observations
I've seen both used, however, not sure which is better practice and/or more commonly acceptable.
The two options aren't actually entirely logically identical - in the "Nesting one after another", for example, method_two() will run even if method_one() fails; if method_two() has any side effects this may be undesirable. Furthermore, if both method_one() and method_two() are destined to fail, "Nesting one after another" will print two error prompts, whereas 'Nesting one inside another" will only error prompt on method_one().
You could close the difference by appending a goto End at the end of each else in "Nesting one after another", so it skips over the remaining checks, but the use of goto would probably get you slapped. Alternatively, you could return at the end of each else, perhaps with an error code, and let whoever is calling your main function deal with understanding what went wrong.
With that in mind, "Nesting one after another" is probably easier to read and understand, since there's less indentation/the code is kept flat, and what happens on failure is immediately next to the check. (That 2nd point can be addressed by reordering the error prompt for method_one() to before the check for method_two() for "Nesting one inside another")
I'm new to Kotlin and I'm trying to do a simple task - create and write to a file. For some reason, using use() with a block function on printWriter() doesn't actually write.
File("test2.txt").printWriter().use { out ->
{
println("hmmm")
out.println("what's up")
log.info { "finished writing" }
}
}
In fact, the block function doesn't seem to be called at all - both "hmmm" and "finished writing" never show up, although the file itself is created (but is totally empty).
The much simpler writeText() works just fine - the file is created and the given text is written to the file.
File("test3.txt").writeText("testing")
What am I doing wrong in my use() version?
Edit: it seems to be a syntax issue with my block function. Looks like I have an extra pair of brackets? Would love a brief explanation as to why it makes it not work.
Edit 2: I think I understand now. The way I wrote it, I was essentially returning the block function itself rather than running through it.
So the problem was that the way I was writing the block function caused it to just return the inner block function, and not actually call it.
Here are two ways that work:
File("test2.txt").printWriter().use {
println("hmmm")
it.println("what's up")
log.info { "finished writing!" }
}
File("test2.txt").printWriter().use(fun(out) {
println("hmmm")
out.println("what's up")
log.info { "finished writing!" }
})
Although, for my purposes writeText() actually works just fine and is much shorter haha.
I see the following code in Perl 6:
await do for #files -> $file {
start {
#do something ... }
}
which runs in async mode.
Why does the above code need do? What is the purpose of do in Perl 6? Could someone please explain the above code in detail?
Also is there are an option to write something like this:
for #files -> $file {
start {
#do something ... }
}
and await after the code for the promises to be fulfilled?
The purpose of do
The for keyword can be used in two different ways:
1) As a stand-alone block statement:
for 1..5 { say $_ }
2) As a statement modifier appended to the end of a statement:
say $_ for 1..5;
When the bare for keyword is encountered in the middle of a larger statement, it is interpreted as that second form.
If you want to use the block form inside a larger statement (e.g. as the argument to the await function), you have to prefix it with do to tell the parser that you're starting a block statement here, and want its return value.
More generally, do makes sure that what follows it is parsed using the same rules it would be parsed as if it were its own statement, and causes it to provide a return value. It thus allows us to use any statement as an expression inside a larger statement. do if, do while, etc. all work the same way.
Explanation of your code
The code you showed...
await do for #files -> $file {
start {
#do somthing ... }
}
...does the following:
It loops of over the array #files.
For each iteration, it uses the start keyword to schedule an asynchronous task, which presumably does something with the current element $file. (The $*SCHEDULER variable decides how the task is actually started; by default it uses a simple thread pool scheduler.)
Each invocation of start immediately returns a Promise that will be updated when the asynchronous task has completed.
The do for collects a sequence of all the return values of the loop body (i.e. the promises).
The await function accepts this sequence as its argument, and waits until all the promises have completed.
How to "await after the code"
Not entirely sure what you mean here.
If you want to remember the promises but not await them just jet, simply store them in an array:
my #promises = do for #files -> $file {
start {
#do something ... }
}
#other code ...
await #promises;
There is no convenience functionality for awaiting all scheduled/running tasks. You always have to keep track of the promises.
Unit testing and xdebug usage aside, I wish to have a way to throw some browser message is a value is not expected to be present.
Let's say: $className = 45;
If we have:
public function setMainClass($className) {
if (is_string($className)) {
$this->_mainClass = $className;
} else {
echo Yii::trace(CVarDumper::dumpAsString($className),'vardump');
}
}
We will get this output to the browser on development stage.
It's great.
I'm not sure however, if this is a proper way of use Yii::trace of if I'm miss using it.
Please advice.
It is not necessary to echo the call Yii::trace() (it returns void so the echo does nothing). The other recommendation is that you might consider changing category to resemble a path alias as discussed in the documentation. For example-
} else {
Yii::trace(CVarDumper::dumpAsString($className), 'application.models.MyGreatModel');
}
Since getting started in Dart I've been watching for a way to execute Dart (Text) Source (that the same program may well be generating dynamically) as Code. Like the infamous "eval()" function.
Recently I have caught a few hints that the communication port between Isolates support some sort of "Spawn" that seems like it could allow this "trick". In Ruby there is also the possibility to load a module dynamically as a language feature, perhaps there is some way to do this in Dart?
Any clues or a simple example will be greatly appreciated.
Thanks in advance!
Ladislav Thon provided this answer on the Dart forum:
I believe it's very safe to say that Dart will never have eval. But it will have other, more structured ways of dynamically generating code (code name mirror builders). There is nothing like that right now, though.
There are two ways of spawning an isolate: spawnFunction, which runs an existing function from the existing code in a new isolate, so nothing you are looking for, and spawnUri, which downloads code from given URI and runs it in new isolate. That is essentially dynamic code loading -- but the dynamically loaded code is isolated from the existing code. It runs in a new isolate, so the only means of communicating with it is via message passing (through ports).
You can run a string as Dart code by first constructing a data URI from it and then passing it into Isolate.spawnUri.
import 'dart:isolate';
void main() async {
final uri = Uri.dataFromString(
'''
void main() {
print("Hellooooooo from the other side!");
}
''',
mimeType: 'application/dart',
);
await Isolate.spawnUri(uri, [], null);
}
Note that you can only do this in JIT mode, which means that the only place you might benefit from it is Dart VM command line apps / package:build scripts. It will not work in Flutter release builds.
To get a result back from it, you can use ports:
import 'dart:isolate';
void main() async {
final name = 'Eval Knievel';
final uri = Uri.dataFromString(
'''
import "dart:isolate";
void main(_, SendPort port) {
port.send("Nice to meet you, $name!");
}
''',
mimeType: 'application/dart',
);
final port = ReceivePort();
await Isolate.spawnUri(uri, [], port.sendPort);
final String response = await port.first;
print(response);
}
I wrote about it on my blog.
Eval(), in Ruby at least, can execute anything from a single statement (like an assignment) to complete involved programs. There is a substantial time penalty for executing many small snippets over most any other form of execution that is possible.
Looking at the problem closer, there are at least three different functions that were at the base of the various schemes where eval might be used. Dart handles at least 2 of these in at least minimal ways.
Dart does not, nor does it look like there is any plan to support "general" script execution.
However, the NoSuchMethod method can be used to effectively implement the dynamic "injection" of variables into your local class environment. It replaces an eval() with a string that would look like this: eval( "String text = 'your first name here';" );
The second function that Dart readily supports now is the invocation of a method, that would look like this: eval( "Map map = SomeClass.some_method()" );
After messing about with this it finally dawned on me that a single simple class can be used to store the information needed to invoke a method, for a class, as a string which seems to have general utility. I can replace a big maintenance prone switch statement that might otherwise be necessary to invoke a series of methods. In Ruby this was almost trivial, however in Dart there are some fairly less than intuitive calls so I wanted to get this "trick" in one place, which fits will with doing ordering and filtering on the strings such as you may need.
Here's the code to "accumulate" as many classes (a whole library?) into a map using reflection such that the class.methodName() can be called with nothing more than a key (as a string).
Note: I used a few "helper methods" to do Map & List functions, you will probably want to replace them with straight Dart. However this code is used and tested only using the functions..
Here's the code:
//The used "Helpers" here..
MAP_add(var map, var key, var value){ if(key != null){map[key] = value;}return(map);}
Object MAP_fetch(var map, var key, [var dflt = null]) {var value = map[key];if (value==null) {value = dflt;}return( value );}
class ClassMethodMapper {
Map _helperMirrorsMap, _methodMap;
void accum_class_map(Object myClass){
InstanceMirror helperMirror = reflect(myClass);
List methodsAr = helperMirror.type.methods.values;
String classNm = myClass.toString().split("'")[1]; ///#FRAGILE
MAP_add(_helperMirrorsMap, classNm, helperMirror);
methodsAr.forEach(( method) {
String key = method.simpleName;
if (key.charCodeAt(0) != 95) { //Ignore private methods
MAP_add(_methodMap, "${classNm}.${key}()", method);
}
});
}
Map invoker( String methodNm ) {
var method = MAP_fetch(_methodMap, methodNm, null);
if (method != null) {
String classNm = methodNm.split('.')[0];
InstanceMirror helperMirror = MAP_fetch(_helperMirrorsMap, classNm);
helperMirror.invoke(method.simpleName, []);
}
}
ClassMethodMapper() {
_methodMap = {};
_helperMirrorsMap = {};
}
}//END_OF_CLASS( ClassMethodMapper );
============
main() {
ClassMethodMapper cMM = new ClassMethodMapper();
cMM.accum_class_map(MyFirstExampleClass);
cMM.accum_class_map(MySecondExampleClass);
//Now you're ready to execute any method (not private as per a special line of code above)
//by simply doing this:
cMM.invoker( MyFirstExampleClass.my_example_method() );
}
Actually there some libraries in pub.dev/packages but has some limitations because are young versions, so that I can recommend you this library expressions to dart and flutter.
A library to parse and evaluate simple expressions.
This library can handle simple expressions, but no blocks of code, control flow statements and so on. It supports a syntax that is common to most programming languages.
There I create an example of code to evaluate arithmetic operations and comparations of data.
import 'package:expressions/expressions.dart';
import 'dart:math';
#override
Widget build(BuildContext context) {
final parsing = FormulaMath();
// Expression example
String condition = "(cos(x)*cos(x)+sin(x)*sin(x)==1) && respuesta_texto == 'si'";
Expression expression = Expression.parse(condition);
var context = {
"x": pi / 5,
"cos": cos,
"sin": sin,
"respuesta_texto" : 'si'
};
// Evaluate expression
final evaluator = const ExpressionEvaluator();
var r = evaluator.eval(expression, context);
print(r);
return Scaffold(
body: Container(
margin: EdgeInsets.only(top: 50.0),
child: Column(
children: [
Text(condition),
Text(r.toString())
],
),
),
);
}
I/flutter (27188): true