The following code throws a MessagingException with message At most one parameter (or expression via method-level #Payload) may be mapped to the payload or Message. Found more than one on method [public abstract java.lang.Integer org.example.PayloadAndGatewayHeader$ArithmeticGateway.add(int,int)].
#MessagingGateway
interface ArithmeticGateway {
#Gateway(requestChannel = "add.input", headers = #GatewayHeader(name = "operand", expression = "#args[1]"))
Integer add(#Payload final int a, final int b);
}
The desired functionality could be achieved with something like:
#MessagingGateway
interface ArithmeticGateway {
#Gateway(requestChannel = "add.input", headers = #GatewayHeader(name = "operand", expression = "#args[1]"))
#Payload("#args[0]")
Integer add(final int a, final int b);
}
Should the first version also work? Nevertheless I believe the error message could be improved.
A sample project can be found here. Please check org.example.PayloadAndGatewayHeader and org.example.PayloadAndGatewayHeaderTest.
EDIT
The purpose of #GatewayHeader was to show why one may want to have additional parameters that will not be part of the payload but I am afraid it created confusion. Here is a more streamlined example:
#MessagingGateway
interface ArithmeticGateway {
#Gateway(requestChannel = "identity.input")
Integer identity(#Payload final int a, final int unused);
}
Shouldn't the unused parameter be ignored since there is already another one that is annotated with #Payload?
You can't mix parameter annotations (which are static) with expressions (which are dynamic) because the static code analysis can't anticipate what the dynamic expression will resolve to at runtime. It is probably unlikely, but there theoretically could be conditions in the expression. In any case, it can't determine at analysis time that the expression will provide a value for #args[1] at runtime (it could, of course for this simple case, but not all cases are this simple).
Use one or the other; use your second approach or
Integer add(#Payload final int a, #Header("operand") final int b);
Related
Can i implement switch statements(by passing string arguments) in java 5 without making use of enums?I tried doing it using hashcode but i got an error
package com.list;
import java.util.Scanner;
public class SwitchDays implements Days {
static final int str = "sunday".hashCode();
public static void main(String[] args) {
Scanner in=new Scanner(System.in);
String day= in.nextLine();
switch (day.hashCode()) {
case str:
System.out.println(day);
break;
default:
break;
}
}
}
str in case str given an error:
case expressions must be constant expressions
Please guide.
The problem is that str is referring to the expression "sunday".hashCode(), which is not a compile time constant expression as described by the JLS:
A constant expression is an expression denoting a value of primitive
type or a String that does not complete abruptly and is composed using
only the following: ... Qualified names (§6.5.6.2) of the form
TypeName . Identifier that refer to constant variables (§4.12.4).
When you check the definition of constant variables:
A constant variable is a final variable of primitive type or type
String that is initialized with a constant expression (§15.28).
Whether a variable is a constant variable or not may have implications
with respect to class initialization (§12.4.1), binary compatibility
(§13.1, §13.4.9), and definite assignment (§16 (Definite Assignment)).
Since "sunday".hashCode() does not meet this requirements, you get the error.
If you would change "sunday".hashCode() to a real compile time constant like 3 it would compile.
The most straight forward solution is to make an enum, ie.
enum Days{
sunday, monday;
}
Then it could be used as:
Day d = Day.valueOf("sunday");
switch(d){
case sunday:
System.out.println("ONE");
break;
case monday:
System.out.println("TWO");
break;
}
private static int counter = 0; public static final int
CLIENT_REQUEST_TIME = counter++;
...
switch (command) { case Command.CLIENT_REQUEST_TIME:
...
but here it comes the
"Error:(20, 25) java: constant expression required",
for the case statement.
but why on earth? since CLIENT_REQUEST_TIME is constant
I know this is something that should be rather addressed to Oracle and I can choose from millions of workarounds... but just in case someone can see some logic behind, it will certainly make me sleep better tonight :)
This error comes because you set not constant property to constant field. I am not understand what this code must to do, but possibly you can change it to look like this:
private static int counter = 0;
public static int CLIENT_REQUEST_TIME = counter++;
How do you create public constants in Haxe? I just need the analog of good old const in AS3:
public class Hello
{
public static const HEY:String = "hey";
}
The usual way to declare a constant in Haxe is using the static and inline modifiers.
class Main {
public static inline var Constant = 1;
static function main() {
trace(Constant);
trace(Test.Constant);
}
}
If you have a group of related constants, it can often make sense to use an enum abstract. Values of enum abstracts are static and inline implicitly.
Note that only the basic types (Int, Float, Bool) as well as String are allowed to be inline, for others it will fail with this error:
Inline variable initialization must be a constant value
Luckily, Haxe 4 has introduced a final keyword which can be useful for such cases:
public static final Regex = ~/regex/;
However, final only prevents reassignment, it doesn't make the type immutable. So it would still be possible to add or remove values from something like static final Values = [1, 2, 3];.
For the specific case of arrays, Haxe 4 introduces haxe.ds.ReadOnlyArray which allows for "constant" lists (assuming you don't work around it using casts or reflection):
public static final Values:haxe.ds.ReadOnlyArray<Int> = [1, 2, 3];
Values = []; // Cannot access field or identifier Values for writing
Values.push(0); // haxe.ds.ReadOnlyArray<Int> has no field push
Even though this is an array-specific solution, the same approach can be applied to other types as well. ReadOnlyArray<T> is simply an abstract type that creates a read-only "view" by doing the following:
it wraps Array<T>
it uses #:forward to only expose fields that don't mutate the array, such as length and map()
it allows implicit casts from Array<T>
You can see how it's implemented here.
For non-static variables and objects, you can give them shallow constness as shown below:
public var MAX_COUNT(default, never):Int = 100;
This means you can read the value in the 'default' way but can 'never' write to it.
More info can be found http://adireddy.github.io/haxe/keywords/never-inline-keywords.
I have a list of KeyValuePairs. I normally would use ToDictionary.
However I just noted that the error message (shown below) has something about explicit cast, which implies I can actually cast the list to Dictionary<...>. How can I do this?
Cannot implicitly convert type 'System.Linq.IOrderedEnumerable<System.Collections.Generic.KeyValuePair<int,string>>' to 'System.Collections.Generic.Dictionary<int, string>'. An explicit conversion exists (are you missing a cast?)
Sample code:
Dictionary<int, string> d = new Dictionary<int, string>() {
{3, "C"},
{2, "B"},
{1, "A"},
};
var s = d.OrderBy(i => i.Value);
d = s;
Implies I can actually cast list to dictionary
Well, it implies that the cast would be valid at compile-time. It doesn't mean it will work at execution time.
It's possible that this code could work:
IOrderedEnumerable<KeyValuePair<string, string>> pairs = GetPairs();
Dictionary<string, string> dictionary = (Dictionary<string, string>) pairs;
... but only if the value returned by GetPairs() were a class derived from Dictionary<,> which also implemented IOrderedEnumerable<KeyValuePair<string, string>>. It's very unlikely that that's actually the case in normal code. The compiler can't stop you from trying, but it won't end well. (In particular, if you do it with the code in your question and with standard LINQ to Objects, it will definitely fail at execution time.)
You should stick with ToDictionary... although you should also be aware that you'll lose the ordering, so there's no point in ordering it to start with.
To show this with the code in your question:
Dictionary<int, string> d = new Dictionary<int, string>() {
{3, "C"},
{2, "B"},
{1, "A"},
};
var s = d.OrderBy(i => i.Value);
d = (Dictionary<int, string>) s;
That compiles, but fails at execution time as predicted:
Unhandled Exception: System.InvalidCastException: Unable to cast object of type 'System.Linq.OrderedEnumerable`2[System.Collections.Generic.KeyValuePair`2[System.Int32,System.String],System.String]' to type 'System.Collections.Generic.Dictionary`2[System.Int32,System.String]'.
at Test.Main()
As a bit of background, you can always cast from any interface type to a non-sealed class ("target"), even if that type doesn't implement the interface, because it's possible for another class derived from "target" to implement the interface.
From section 6.2.4 of the C# 5 specification:
The explicit reference conversions are:
...
From any class-type S to any interface-type T, provided S is not sealed and provided S does not implement T.
...
(The case where S does implement T is covered by implicit reference conversions.)
If you try to implicitly convert a value and there's no implicit conversion available, but there's an explicit conversion available, the compiler will give you the warning in your question. That means you can fix the compiler-error with a cast, but you need to be aware of the possibility of it failing at execution time.
Here's an example:
using System;
class Test
{
static void Main()
{
IFormattable x = GetObject();
}
static object GetObject()
{
return DateTime.Now.Second >= 30 ? new object() : 100;
}
}
Error message:
Test.cs(7,26): error CS0266: Cannot implicitly convert type 'object' to
'System.IFormattable'. An explicit conversion exists (are you missing a cast?)
So we can add a cast:
IFormattable x = (IFormattable) GetObject();
At this point, the code will work about half the time - the other half, it'll throw an exception.
I'm new to D, and I was wondering whether it's possible to conveniently do compile-time-checked duck typing.
For instance, I'd like to define a set of methods, and require that those methods be defined for the type that's being passed into a function. It's slightly different from interface in D because I wouldn't have to declare that "type X implements interface Y" anywhere - the methods would just be found, or compilation would fail. Also, it would be good to allow this to happen on any type, not just structs and classes. The only resource I could find was this email thread, which suggests that the following approach would be a decent way to do this:
void process(T)(T s)
if( __traits(hasMember, T, "shittyNameThatProbablyGetsRefactored"))
// and presumably something to check the signature of that method
{
writeln("normal processing");
}
... and suggests that you could make it into a library call Implements so that the following would be possible:
struct Interface {
bool foo(int, float);
static void boo(float);
...
}
static assert (Implements!(S, Interface));
struct S {
bool foo(int i, float f) { ... }
static void boo(float f) { ... }
...
}
void process(T)(T s) if (Implements!(T, Interface)) { ... }
Is is possible to do this for functions which are not defined in a class or struct? Are there other/new ways to do it? Has anything similar been done?
Obviously, this set of constraints is similar to Go's type system. I'm not trying to start any flame wars - I'm just using D in a way that Go would also work well for.
This is actually a very common thing to do in D. It's how ranges work. For instance, the most basic type of range - the input range - must have 3 functions:
bool empty(); //Whether the range is empty
T front(); // Get the first element in the range
void popFront(); //pop the first element off of the range
Templated functions then use std.range.isInputRange to check whether a type is a valid range. For instance, the most basic overload of std.algorithm.find looks like
R find(alias pred = "a == b", R, E)(R haystack, E needle)
if (isInputRange!R &&
is(typeof(binaryFun!pred(haystack.front, needle)) : bool))
{ ... }
isInputRange!R is true if R is a valid input range, and is(typeof(binaryFun!pred(haystack.front, needle)) : bool) is true if pred accepts haystack.front and needle and returns a type which is implicitly convertible to bool. So, this overload is based entirely on static duck typing.
As for isInputRange itself, it looks something like
template isInputRange(R)
{
enum bool isInputRange = is(typeof(
{
R r = void; // can define a range object
if (r.empty) {} // can test for empty
r.popFront(); // can invoke popFront()
auto h = r.front; // can get the front of the range
}));
}
It's an eponymous template, so when it's used, it gets replaced with the symbol with its name, which in this case is an enum of type bool. And that bool is true if the type of the expression is non-void. typeof(x) results in void if the expression is invalid; otherwise, it's the type of the expression x. And is(y) results in true if y is non-void. So, isInputRange will end up being true if the code in the typeof expression compiles, and false otherwise.
The expression in isInputRange verifies that you can declare a variable of type R, that R has a member (be it a function, variable, or whatever) named empty which can be used in a condition, that R has a function named popFront which takes no arguments, and that R has a member front which returns a value. This is the API expected of an input range, and the expression inside of typeof will compile if R follows that API, and therefore, isInputRange will be true for that type. Otherwise, it will be false.
D's standard library has quite a few such eponymous templates (typically called traits) and makes heavy use of them in its template constraints. std.traits in particular has quite a few of them. So, if you want more examples of how such traits are written, you can look in there (though some of them are fairly complicated). The internals of such traits are not always particularly pretty, but they do encapsulate the duck typing tests nicely so that template constraints are much cleaner and more understandable (they'd be much, much uglier if such tests were inserted in them directly).
So, that's the normal approach for static duck typing in D. It does take a bit of practice to figure out how to write them well, but that's the standard way to do it, and it works. There have been people who have suggested trying to come up with something similar to your Implements!(S, Interface) suggestion, but nothing has really come of that of yet, and such an approach would actually be less flexible, making it ill-suited for a lot of traits (though it could certainly be made to work with basic ones). Regardless, the approach that I've described here is currently the standard way to do it.
Also, if you don't know much about ranges, I'd suggest reading this.
Implements!(S, Interface) is possible but did not get enough attention to get into standard library or get better language support. Probably if I won't be the only one telling it is the way to go for duck typing, we will have a chance to have it :)
Proof of concept implementation to tinker around:
http://dpaste.1azy.net/6d8f2dc4
import std.traits;
bool Implements(T, Interface)()
if (is(Interface == interface))
{
foreach (method; __traits(allMembers, Interface))
{
foreach (compareTo; MemberFunctionsTuple!(Interface, method))
{
bool found = false;
static if ( !hasMember!(T, method) )
{
pragma(msg, T, " has no member ", method);
return false;
}
else
{
foreach (compareWhat; __traits(getOverloads, T, method))
{
if (is(typeof(compareTo) == typeof(compareWhat)))
{
found = true;
break;
}
}
if (!found)
{
return false;
}
}
}
}
return true;
}
interface Test
{
bool foo(int, double);
void boo();
}
struct Tested
{
bool foo(int, double);
// void boo();
}
pragma(msg, Implements!(Tested, Test)());
void main()
{
}