First, let me say that I agree that eval(repr(some_string)) is potentially a bad idea. But it is a thing that exists, and I have a specific question about it.
Why doesn't this work?
my_type = int
my_type_str = repr(int)
my_type_from_str = eval(my_type_str)
To clarify, I know specifically why the eval call fails. The command repr(int) produces a string which cannot be automatically interpreted. I guess my issue is that this isn't behaving as I expect... so either my expectation is faulty or the implementation is faulty. Which is it?
Side Note
There are some tricks that can be used to get around this default behavior, some obvious, some not. For instance, this:
my_type = int
my_type_str = my_type.__name__
my_type_from_str = eval(my_type_str)
assert my_type is my_type_from_str
This sets off my "hack" alert hardcore, and I don't like it (or other similar hacks, for instance parsing the string "<class 'int'>" with some home made function).
The documentation explains:
Return a string containing a printable representation of an object. For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to eval(), otherwise the representation is a string enclosed in angle brackets that contains the name of the type of the object together with additional information often including the name and address of the object. A class can control what this function returns for its instances by defining a repr() method.
(emphasis mine)
So, it doesn't say that repr must always return valid Python code - sometimes it's not the case. Just another argument for not relying on it.
To illustrate the following example I created a litte spock test (but it's about groovy itself, not spock):
void "some spock test"() {
given: String value = null
expect: someMethod(value) == 3
}
int someMethod(String s) {
return 3
}
int someMethod(Map s) {
return 5
}
There are two methods who's signatures only differ by the type of the given parameter. I thought that when I give it a null value that is explicitly typed as a string, the string-method will be called.
But that doesn't happen; the test fails, because the map-method is called! Why?
I guess groovy ignores the type and treats all nulls the same. There seems to be some kind of priority of types: When I use Object instead of Map as the parameter type of the wrong-method, its all the same, but when I for instance use Integer, the test succeeds.
But than again: If groovy really ignores the type of nulls, why can the following fix the original test:
expect: someMethod((String) value) == 3
If you read my answer to the question Tim already mentioned you will see that I talk there about runtime types. The static type plays normally no role in this. I also described there how the distance calculation is used and that for null the distance to Object is used to determine the best fitting method. What I did not mention is that you can force method selection by using a cast. Internally Groovy will use a wrapper for the object, that also transports the type. Then the transported type is used instead. But you surely understand, that this means a one additional object creation per method class, which is very inefficient. Thus it is not the standard. In the future Groovy maybe change to include that static type information, but this requires a change to the MOP as well. And that is difficult
I am making a simple debugger window in ActionScript for myself where I can add and remove variables I want to track. I was to be able to add variables to the list by just doing something like
DebuggerMonitor.trackVar(variable).
My question is, is there any way I can turn "variable" itself (the name, not the value) into a String to be added into a text field?
Depending on how "intelligent" your debugger should be, you could just pass the name along:
DebuggerMonitor.trackVar( variable, "variable" );
since obviously, when used in a context like this, the name should be known at the time you are writing the program.
You can also do some reflection magic to get instance variable names, but it won't work for temp variables (their names are dropped at compilation time):
public function getVariableName( instance:*, match:* ):String {
var typeDescription:XML = describeType( instance );
var variables:XMLList = typeDescription..variable;
var accessors:XMLList = typeDescription..accessor;
for each(var variable:XML in variables)
if(matchesXMLName( instance, variable, match ))
return variable.#name;
for each(var accessor:XML in accessors)
if(matchesXMLName( instance, accessor, match ))
return accessor.#name;
return "No name found.";
}
private function matchesXMLName( instance:*, xml:XML, match:* ):Boolean {
return match == instance[xml.#name.toString()];
}
var varName:String = getVariableName ( myObject, variable );
Using reflections like this will also be quite costly, if used often - you will have to think of a way to cache the type descriptions.
I recommend you check out the as3commons reflections package - there is a lot of useful functionality in there...
Short answer - No :(
You can access the type name but not individual instance names, as these are lost at run-time.
There is a confusion caused by the keyword 'var' because it is used to create several types of bindings.
Lexical bindings (the keyword 'var' was used inside a function).
Dynamic bindings (the keyword 'var' was used to declare a class' field).
Lexical bindings are interpreted by the compiler at compile time as addresses of the registers of the registers space occupied by the function. The names given to lexical bindings perish at this time and it is not possible to restore them at runtime - therefore you can't get the "name" of the variable.
Dynamic bindings are a kind of "public API" of the objects that declare them, they may be accessed from the code that was not compiled together with the code that created them, this is why, for the purpose of reflection the names of these bindings are stored in compiled code. However, ActionScript has no way of referencing LHS values, so you cannot, even if you know the name of the variable and the object declaring it, pass it to another function. But you can look it up in the debugger or by calling describeType on the object declaring the variable. Note that describeType will not show information on private variables even if you are calling it from the scope of the object in question.
Hypothetic and academic question.
pseudo-code:
<pre><code>
class Book{
read(theReader)
}
class BookWithMemory extends Book {
read(theReader, aTimestamp = null)
}
</pre></code>
Assuming:
an interface (if supported) would prohibit it
default value for parameters are supported
Notes:
PHP triggers an strict standards error for this.
I'm not surprised that PHP strict mode complains about such an override. It's very easy for a similar situation to arise unintentionally in which part of a class hierarchy was edited to use a new signature and a one or a few classes have fallen out of sync.
To avoid the ambiguity, name the new method something different (for this example, maybe readAt?), and override read to call readAt in the new class. This makes the intent plain to the interpreter as well as anyone reading the code.
The actual behavior in such a case is language-dependent -- more specifically, it depends on how much of the signature makes up the method selector, and how parameters are passed.
If the name alone is the selector (as in PHP or Perl), then it's down to how the language handles mismatched method parameter lists. If default arguments are processed at the call site based on the static type of the receiver instead of at the callee's entry point, when called through a base class reference you'd end up with an undefined argument value instead of your specified default, similarly to what would happen if there was no default specified.
If the number of parameters (with or without their types) are part of the method selector (as in Erlang or E), as is common in dynamic languages that run on JVM or CLR, you have two different methods. Create a new overload taking additional arguments, and override the base method with one that calls the new overload with default argument values.
If I am reading the question correctly, this question seems very language specific (as in it is not applicable to all dynamic languages), as I know you can do this in ruby.
class Book
def read(book)
puts book
end
end
class BookWithMemory < Book
def read(book,aTimeStamp = nil)
super book
puts aTimeStamp
end
end
I am not sure about dynamic languages besides ruby. This seems like a pretty subjective question as well, as at least two languages were designed on either side of the issue (method overloading vs not: ruby vs php).
I've often wondered why languages with a null representing "no value" don't differentiate between the passive "I don't know what the value is" and the more assertive "There is no value.".
There have been several cases where I'd have liked to differentiate between the two (especially when working with user-input and databases).
I imagine the following, where we name the two states unknown and null:
var apple;
while (apple is unknown)
{
askForApple();
}
if (apple is null)
{
sulk();
}
else
{
eatApple(apple);
}
Obviously, we can get away without it by manually storing the state somwhere else, but we can do that for nulls too.
So, if we can have one null, why can't we have two?
Isn't is bad enough that we have one null?
In my programming, I recently adopted the practice of differentiating "language null" and "domain null".
The "language null" is the special value that is provided by the programming language to express that a variable has "no value". It is needed as dummy value in data structures, parameter lists, and return values.
The "domain null" is any number of objects that implement the NullObject design pattern. Actually, you have one distinct domain null for each domain context.
It is fairly common for programmers to use the language null as a catch-all domain null, but I have found that it tends to make code more procedural (less object oriented) and the intent harder to discern.
Every time to want a null, ask yourself: is that a language null, or a domain null?
In most programming languages null means "empty" or "undefined". "Unknown" on the other hand is something different. In essence "unknown" describes the state of the object. This state must have come from somewhere in your program.
Have a look at the Null Object pattern. It may help you with what you are trying to achieve.
javascript actually has both null and undefined (http://www.w3schools.com/jsref/jsref_undefined.asp), but many other languages don't.
It would be easy enough to create a static constant indicating unknown, for the rare cases when you'd need such a thing.
var apple = Apple.Unknown;
while (apple == Apple.Unknown) {} // etc
Existence of value:
Python: vars().has_key('variableName')
PHP: isset(variable)
JavaScript: typeof(variable) != 'undefined'
Perl: (variable != undef) or if you wish: (defined variable)
Of course, when variable is undefined, it's not NULL
Why stop at two?
When I took databases in college, we were told that somebody (sorry, don't remember the name of the researcher or paper) had looked at a bunch of db schemas and found that null had something like 17 different meanings: "don't know yet", "can't be known", "doesn't apply", "none", "empty", "action not taken", "field not used", and so on.
In haskell you can define something like this:
data MaybeEither a b = Object a
| Unknown b
| Null
deriving Eq
main = let x = Object 5 in
if x == (Unknown [2]) then putStrLn ":-("
else putStrLn ":-)"
The idea being that Unknown values hold some data of type b that can transform them into known values (how you'd do that depends on the concrete types a and b).
The observant reader will note that I'm just combining Maybe and Either into one data type :)
The Null type is a subtype of all reference types - you can use null instead of a reference to any type of object - which severely weakens the type system. It is considered one of the a historically bad idea by its creator, and only exists as checking whether an address is zero is easy to implement.
As to why we don't have two nulls, is it down to the fact that, historically in C, NULL was a simple #define and not a distinct part of the language at all?
Within PHP Strict you need to do an isset() check for set variables (or else it throws a warning)
if(!isset($apple))
{
askForApple();
}
if(isset($apple) && empty($apple))
{
sulk();
}
else
{
eatApple();
}
In .net langages, you can use nullable types, which address this problem for value types.
The problem remains, however, for reference types. Since there's no such thing as pointers in .net (at least in 'safe' blocks), "object? o" won't compile.
Note null is an acceptable, yet known condition. An unknown state is a different thing IMO. My conversation with Dan in the comments' section of the top post will clarify my position. Thanks Dan!.
What you probably want to query is whether the object was initialized or not.
Actionscript has such a thing (null and undefined). With some restrictions however.
See documentation:
void data type
The void data type contains only one value, undefined. In previous versions of ActionScript, undefined was the default value for instances of the Object class. In ActionScript 3.0, the default value for Object instances is null. If you attempt to assign the value undefined to an instance of the Object class, Flash Player or Adobe AIR will convert the value to null. You can only assign a value of undefined to variables that are untyped. Untyped variables are variables that either lack any type annotation, or use the asterisk (*) symbol for type annotation. You can use void only as a return type annotation.
Some people will argue that we should be rid of null altogether, which seems fairly valid. After all, why stop at two nulls? Why not three or four and so on, each representing a "no value" state?
Imagine this, with refused, null, invalid:
var apple;
while (apple is refused)
{
askForApple();
}
if (apple is null)
{
sulk();
}
else if(apple is invalid)
{
discard();
}
else
{
eatApple(apple);
}
It's been tried: Visual Basic 6 had Nothing, Null, and Empty. And it led to such poor code, it featured at #12 in the legendary Thirteen Ways to Loathe VB article in Dr Dobbs.
Use the Null Object pattern instead, as others have suggested.
The problem is that in a strongly typed language these extra nulls are expected to hold specific information about the type.
Basically your extra null is meta-information of a sort, meta-information that can depend on type.
Some value types have this extra information, for instance many numeric types have a NaN constant.
In a dynamically typed language you have to account for the difference between a reference without a value (null) and a variable where the type could be anything (unknown or undefined)
So, for instance, in statically typed C# a variable of type String can be null, because it is a reference type. A variable of type Int32 cannot, because it is a value type it cannot be null. We always know the type.
In dynamically typed Javascript a variable's type can be left undefined, in which case the distinction between a null reference and an undefined value is needed.
Some people are one step ahead of you already. ;)
boolean getAnswer() throws Mu
Given how long it took Western philosophy to figure out how it was possible to talk about the concept of "nothing"... Yeah, I'm not too surprised the distinction got overlooked for a while.
I think having one NULL is a lower-common denominator to deal with the basic pattern
if thing is not NULL
work with it
else
do something else
In the "do something else" part, there are a wide range of possibilities from "okay, forget it" to trying to get "thing" somewhere else. If you don't simply ignore something that's NULL, you probably need to know why "thing" was NULL. Having multiple types of NULL, would help you answering that question, but the possible answers are numerous as hinted in the other answers here. The missing thing could be simply a bug, it could be an error when trying to get it, it may not be available right now, and so on. To decide which cases apply to your code -- which means you have to handle them -- it domain specific. So it's better to use an application defined mechanism to encode these reasons instead of finding a language feature that tries to deal with all of them.
It's because Null is an artifact of the language you're using, not a programmer convenience. It describes a naturally occurring state of the object in the context in which it is being used.
If you are using .NET 3.0+ and need something else, you might try the Maybe Monad. You could create whatever "Maybe" types you need and, using LINQ syntax, process accordingly.
AppleInformation appleInfo;
while (appleInfo is null)
{
askForAppleInfo();
}
Apple apple = appleInfo.apple;
if (apple is null)
{
sulk();
}
else
{
eatApple(apple);
}
First you check if you have the apple info, later you check if there is an apple or not. You don't need different native language support for that, just use the right classes.
For me null represents lack of value, and I try to use it only to represent that. Of course you can give null as many meanings as you like, just like you can use 0 or -1 to represent errors instead of their numerical values. Giving various meanings to one representation could be ambiguous though, so I wouldn't recommend it.
Your examples can be coded like apple.isRefused() or !apple.isValid() with little work; you should define beforehand what is an invalid apple anyway, so I don't see the gain of having more keywords.
You can always create an object and assign it to same static field to get a 2nd null.
For example, this is used in collections that allow elements to be null. Internally they use a private static final Object UNSET = new Object which is used as unset value and thus allows you to store nulls in the collection. (As I recall, Java's collection framework calls this object TOMBSTONE instead of UNSET. Or was this Smalltalk's collection framework?)
VB6
Nothing => "There is no value."
Null = > "I don't know what the value is" - Same as DBNull.Value in .NET
Two nulls would be the wrongest answer around. If one null is not enough, you need infinity nulls.
Null Could mean:
'Uninitialized'
'User didn't specify'
'Not Applicable here, The color of a car before it has been painted'
'Unity: This domain has zero bits of information.'
'Empty: this correctly holds no data in this case, for example the last time the tires were rotated on a new car'
'Multiple, Cascading nulls: for instance, the extension of a quantity price when no quantity may be specified times a quantity which wasn't specified by the user anyway'
And your particular domain may need many other kinds of 'out of band' values. Really, these values are in the domain, and need to have a well defined meaning in each case. (ergo, infinity really is zero)