I am a bit confused about why this casting am I performing is throwing an exception. This is the code:
var myJson = json.decode(response.body);
List<Map<String,dynamic>> childrenJson = myJson['children'] as List<Map<String,dynamic>>;
But this works as expected:
var myJson = json.decode(response.body);
List<dynamic> childrenJson = myJson['children'];
Map<String, dynamic> item = childrenJson[0];
If the children entry is a list of maps of (String, dynamic) why am I not able to cast it that way?
I think you'll want to use:
var childrenJson = List.cast<Map<String,dynamic>>(myJson['children]);
which should throw an exception at runtime if any of the children cannot be casted. The "as" is more of a compile-time thing, and can only be used if the compiler can infer that as a type.
Related
I have a class
UserModel{
late final List<UserEmail> emails;
}
I trying to retrieve an array from Firestore.
UserModel.fromJson( emails = json[emailsKey] != null
? (json[emailsKey] as List<Map<String,dynamic>>).map((Map<String, dynamic> email) => UserEmail.fromJson(email)).toList()
: <UserEmail>[],
The emailsKey is the key to emails array in Firestore..
The error I'm getting back is
Failed: type 'List<dynamic>' is not a subtype of type 'List<Map<String, dynamic>>' in type cast<…>
I'm not sure why my casting isn't working (at runtime).
Any ideas
Try the following:
var temp = json[emailsKey];
List<String> emailKeys = List<String>.from(temp);
I wrote this code to convert dynamic list to Word list but linter says:
Omit type annotations for local variables. on 2nd line.
However if I omit type annotations, I get an error A value of type 'List<dynamic>' can't be returned from method 'convert' because it has a return type of 'List<Word>'.
It there any smarter way to convert?
static List<Word> convert(List<dynamic> words) {
final List<Word> wordsList = [];
words.forEach((v) {
final map = Map<String, dynamic>.from(v as Map<dynamic, dynamic>);
wordsList.add(Word.fromMap(map));
});
return wordsList;
}
Word.fromMap is:
Word.fromMap(Map<String, dynamic> map)
: text = map['text'] as String,
count = map['count'] as int;
To avoid the warning, and put the type on the right-hand side as the lint wants, just write:
final wordsList = <Word>[];
I assume words is JSON data, so the maps are already Map<String, dynamic>. Then you can also do everything in one line:
static List<Word> convert(List<dynamic> words) =>
[for (var v in words) Word.fromMap(v)];
Use the cast() method like this:
class Word {
final String text;
final int count;
static List<Word> convert(List<dynamic> words) {
final List<Word> wordsList = [];
words.cast<Map<dynamic, dynamic>>().forEach((v) { // <-- look here
final map = Map<String, dynamic>.from(v);
wordsList.add(Word.fromMap(map));
});
return wordsList;
}
Word.fromMap(Map<String, dynamic> map)
: text = map['text'] as String,
count = map['count'] as int;
}
It will ensure the casting are done on each element. Make sure the type are correct since it will else result in a type-cast error.
I'm having a problem getting my DU working as expected. I've defined a new DU which either has a result of type <'a> or any Exception derived from System.Exception
open System
// New exceptions.
type MyException(msg : string) = inherit Exception(msg)
type MyOtherException(msg : string) = inherit MyException(msg)
// DU to store result or an exception.
type TryResult<'a, 't> =
| Result of 'a
| Error of 't :> Exception
//This is fine.
let result = Result "Test"
// This works, doing it in 2 steps
let ex = new MyOtherException("Some Error")
let result2 = Error ex
// This doesn't work. Gives "Value Restriction" error.
let result3 = Error (new MyOtherException("Some Error"))
I can't understand why it is allowing me to create an "Error" if I do it in 2 steps, but when i'm doing the same thing on a single line, I get a Value Restriction error.
What am i missing?
Thanks
UPDATE
Looking at the post by #kvb, adding type information each time I need to create an Error seemed a bit verbose, so I wrapped it up into an additional method which creates an Error and is a bit more succinct.
// New function to return a Result
let asResult res : TryResult<_,Exception> = Result res
// New function to return an Error
let asError (err : Exception) : TryResult<unit,_> = Error(err)
// This works (as before)
let myResult = Result 100
// This also is fine..
let myResult2 = asResult 100
// Using 'asError' now works and doesn't require any explicit type information here.
let myError = asError (new MyException("Some Error"))
I'm not sure if specifying an Error with 'unit' will have any consequences I haven't foreseen yet.
TryResult<unit,_> = Error(err)
Consider this slight variation:
type MyOtherException(msg : string) =
inherit MyException(msg)
do printfn "%s" msg
let ex = new MyOtherException("Some Error") // clearly, side effect occurs here
let result2 = Error ex // no side effect here, but generalized value
let intResults = [Result 1; result2]
let stringResults = [Result "one"; result2] // can use result2 at either type, since it's a generalized value
let result3 = Error (MyOtherException("Some Error")) // result would be of type TryResult<'a, MyOtherException> for any 'a
// In some other module in a different compilation unit
let intResults2 = [Result 1; result3] // why would side effect happen here? just using a generic value...
let stringResults2 = [Result "one"; result3] // likewise here...
The issue is that it looks like result3 is a value, but the .NET type system doesn't support generic values, it only supports values of concrete types. Therefore, the MyOtherException constructor needs to be called each time result3 is used; however, this would result in any side effects occurring more than once, which would be surprising. As Ringil suggests, you can work around this by telling the compiler to treat the expression as a value anyway:
[<GeneralizableValue>]
let result3<'a> : TryResult<'a,_> = Error(new MyOtherException("Some Error"))
This is fine as long as the constructor doesn't have side effects.
You can do:
let result3<'a> = Error (new MyOtherException("Some Error"))
EDIT:
As for why you can't do it in one step, first note that this results in the same error:
let result4 = Result (new MyOtherException("Some Error"))
As does this:
let result4 = Result ([|1;|])
But that this works:
let result4 = Result ([1;])
What's similar about Exception and Arrays, but not Lists? It's their mutability. The value restriction will bother you when you try to do make a TryResult with a type that is mutable in a single step.
Now as for why the two step process solves this, it's because the constructor make the whole function not generalizable because you're applying a function to the constructor. But splitting it into two steps solves that. It is similar to Case 2 here on MSDN.
You can read more about it at the above MSDN article and the why this happens in this more indepth blog post.
This is what I am trying to do with the dictionary:
if let deliveries = dictionary["deliveries"] as? NSDictionary {
var castedDeliveries = [Double: Double]()
for delivery in deliveries {
if let value = delivery.value as? Double {
castedDeliveries[Double(delivery.key as! NSNumber)] = value //Could not cast value of type 'NSTaggedPointerString' (0x1a1e3af20) to 'NSNumber' (0x1a1e458b0).
}
}
settings!.deliveries = castedDeliveries
}
And this is what I try to cast, as a part of JSON response from server:
deliveries = {
2 = 0;
5 = "2.59";
7 = "3.59";
};
It doesnt work, because there is an error at commented line:
Could not cast value of type 'NSTaggedPointerString' (0x1a1e3af20) to 'NSNumber' (0x1a1e458b0).
You are trying to cast dictionary directly but instead you need to cast each key - value pair. If you want generic solution to this problem take a look at SwiftyJSON library which address JSON parsing problem for you.
Casting doens't mean data transformation from a type to another.
Your dictionary seems to be composed by Integer keys and String values.
If you want to transform in something else you ca use the map function.
let converted = deliveries.map{[Double($0) : Double($1)]}
But pay attention.
Here we are saying, iterate over the dictionary (in the $0 there is the dictionary key in the $1 there is the value) and create a new dictionary that has as a key a Double initialized at the key value and as a new value a Double initialized as the old dictionary value. The last conversion can fail, so the returned data is an optional.
As I noted in the comments, this isn't casting. You want a data conversion. You need to do that explicitly, especially in this case since it might fail.
Looking at the error, I think you really have a dictionary of [String:String] here (in NSDictionary form). That suggests the JSON is badly encoded, but such is life. Assuming that dictionary looks something like this:
let dictionary: NSDictionary = ["deliveries": ["2":"0", "5": "2.59", "7": "3.59"]]
You would convert it to [Double:Double] like this:
if let jsonDeliveries = dictionary["deliveries"] as? [String:String] {
var deliveries: [Double: Double] = [:]
for (key, value) in jsonDeliveries {
if let keyDouble = Double(key),
valueDouble = Double(value) {
deliveries[keyDouble] = valueDouble
}
}
// Use deliveries
}
This silently ignores any values that can't be converted to Double. If you would rather generate errors, use a guard let rather than an if let.
I have a use case where I want to assign an array of objects of a specific type to a variable of type [Any]. Considering these references:
var genericArray : [Any]
let specificArray : [MyClass]
This throws a compiler error:
genericArray = specificArray
//Cannot assign value of type '[MyClass]' to value of type '[Any]'
And so does this:
genericArray = specificArray as [Any]
//'Any' is not a subtype of 'MyClass'
This works:
genericArray = specificArray.map { $0 }
But it seems less than elegant. Is there a better way to handle this assignment?
If your genericArray hold instances of class types only (not including function types) then just use var genericArray : [AnyObject]. That will work because specificArray type is set to MyClass which is class I guess