I'm confused as to the uses of "as" keyword.
Is it a cast operator or alias operator?
I encountered the following code on the internet which looked like a cast operator:
var list = json['images'] as List;
What does this mean?
as means different things in different contexts.
It's primarily used as a type cast operator. From the Dart Language Tour:
as: Typecast (also used to specify library prefixes)
It links to an explanation of how as is also used to add a prefix to an imported library to avoid name collisions. (as was reused to do different things to avoid needing extra keywords.)
just to add the as keyword is now flagged by the linter and they prefer you to use a check like is
if (pm is Person)
pm.firstName = 'Seth';
you can read more here https://github.com/dart-lang/linter/issues/145
As the language tour says:
Use the as operator to cast an object to a particular type if and only if you are sure that the object is of that type.
Following with your example:
var list = json['images'] as List;
You would use as here to cast or convert json['images'] into a <List> object.
From another SO post (talking about explicit cast vs. as):
as ... is more like an assertion, if the values type doesn't match as causes a runtime exception.
You can also use it when importing packages. A common example is the dart:convert as JSON which then can be reached final foo = JSON.jsonDecode(baz)
It's casting, your code is similar as:
List list = json['images'];
Related
So the following code snippet
Set mySet = {1,2,3};
is an instance of type Set which is permissible, however what would the class of the set literal be. I have tried to search for this, however I have found no answer in the dart documentation.
A literal exists only in your source code. Asking for its "class" doesn't make a lot of sense.
Using a Set, Map, or List literal is just syntactic sugar for invoking a corresponding constructor. The Set factory constructor constructs a LinkedHashSet.
However, you'll see that LinkedHashSet is also abstract. Its factory constructor returns an instance of a private, internal class. You can see its typename via print(Set().runtimeType); the actual type might be different for different platforms and is unlikely to be useful to you.
I'm trying to pass a type in order to make use of the type information, but that types doesn't appear to be pass through.
I went back to the docs to double check that Dart generics are in fact reified and according to the docs, they are:
I call hydrate on a response which morphs the content of response object:
response.hydrate<BoqVO>();
I'm expecting T to be of type BoqVO:
class Response {
...
void hydrate<T>() {
print(T.runtimeType); // always prints _Type
if (T is BoqVO) {
print("IF");
} else {
print("ELSE"); // always goes into ELSE block
}
}
...
}
... but it's not.
Replacing response.hydrate<BoqVO>(); with response.hydrate(new BoqVO()); and changing the method signature to
void hydrate(T t) {
works if i now use lowercase t, but one shouldn't have to instantiate the object in order for reified generics to be available.
Any ideas why Dart is doing this or what i'm missing for reified generics to work correctly?
PS: I'm not on Dart 2 yet, currently on Dart 1.24.3
As Günther Zöchbauer has said, the type parameter doesn't work in Dart 1.24.
The following explains what would happen if you tried the same code in Dart 2.0, where it would also not work, because it uses the type parameter incorrectly.
The code T.runtimeType treats T as an expression. When a type, including a type parameter, is used as an expression, it evaluates to an instance of the class Type. What you print is the runtime type of that Type object (where _Type is an internal platform implementation of Type).
To print the real type, just print(T) (that still converts T to a Type object, but one representing the type BoqVO and with a toString that includes the BoqVO name).
Likewise for T is BoqVO, you evaluate T to a Type object, and since Type doesn't implement BoqVO, that test is always false. There is no simple way to test if the type of a type parameter implements a specific other type, but you can hack around it as <T>[] is List<BoqVO>.
Generic collections were supported from the beginning and they got some type support, but generic methods were only experimental in Dart 1 and reified type parameters were only added in Dart 2 pre releases.
I just started to study F# and accidentally wrote this binding
let List = 1
Now when I try to obtain List methods such as 'filter' I get this error
error FS0039: The field, constructor or member 'filter' is not defined.
Of course using method with full type name like Microsoft.FSharp.Collections.List.filter is still working.
I'm wondering why it is possible to use type name as identifier in F# and how I can set back name List to type List from Microsoft.FSharp.Collections.
When I tried to reassign like this
type List = Microsoft.FSharp.Collections.List<'T>
I get
Error FS0039: The type parameter 'T is not defined.
Thank you!
In F# you can redefine almost everything and shadow existing definitions. This applies to both types (well actually types have a different behavior regarding shadowing, they shadow their values as you open the namespaces) and values but not interchangeably since values and type (and also modules) can somehow coexist at the same time in the scope. The compiler will do his best to find out which one is.
You are not forced to, but it's a common good practice in F# not to use let bindings in uppercase.
Regarding your second question, you are using a type parameter in the right side which doesn't exist in the left side of the assignment, it should be:
type List<'T> = Microsoft.FSharp.Collections.List<'T>
But notice that filter doesn't belong to the type. It's rather defined in the List module.
You should just rename your let binding from List to something sensible - as Gustavo mentioned, your definition is shadowing the core List module from F# and there is no way to use List to refer both to your integer and to the module. Shadowing core functions will make your code pretty confusing. It's also a good idea to use camelCase for let bindings, but that's a matter of taste.
If you insist on shadowing List, then you won't be able to call List.filter using List.filter. If you wanted something shorter, you could define module alias:
module FsList = Microsoft.FSharp.Collections.List
Note that your attempt to do something similar with List<'T> does not do the same thing, because functions such as filter are in a module named List rather than being static members of the type. With this, you can call filter using FsList.filter.
In a fsyacc based project, I have this line:
type 'a cucomment = string
This is the full error description I'm getting:
CALast.fs(117,9): error FS0035: This construct is deprecated: This
type abbreviation has one or more declared type parameters that do not
appear in the type being abbreviated. Type abbreviations must use all
declared type parameters in the type being abbreviated. Consider
removing one or more type parameters, or use a concrete type
definition that wraps an underlying type, such as 'type C<'a> = C of
...'.
Any idea how to solve this?
F# no longer allows type aliases that add generic type parameters to a type without declaring a new type. If you want to define a generic type that wraps some other type, you have to use some constructor. For example, you can use single-case discriminated union:
type 'a Cucomment = CC of string
Unfortunately, this means that you'd have to change all code that uses the type to unwrap the value using pattern matching or by adding Value member to the type.
The only case where generic type aliases are allowed is when you declare a version of type with units of measure, which requires a special attribute. However, this is probably not going to work for you (because units behave quite differently):
[<MeasureAnnotatedAbbreviation>]
type 'a Cucomment = string
If this is in some code generated by fsyacc, then that's a bug in fsyacc that should be fixed (I think this was quite recent change). In that case, report it to fsbugs at microsoft dot com.
I need to set a version number to be used in the AssemblyVersion attribute by several related projects.
In C# I use the following
public class Constants {
public const string Version = "1.2.3.4";
}
then it can be used as follows:
[assembly:AssemblyVersion(Constants.Version)]
What would be the equivalent construct in F#. All my attempts to come up with a binding which can be accepted as an attribute argument did not work.
Use the attribute Literal:
[<Literal>]
let version = "1.2.3.4"
[<assembly:AssemblyVersion(version)>]
Since I stepped into this trap myself I thought I'd share for anyone following.
A 'Literal' requires that the letter starts with a capital letter. This will hit you when you try to use the literal in a pattern matching construct.
Reference:
Literal attribute not working