Reviewing a question recently but could not post response due to lacking any reputation.
In this question regarding a compiletime error coming from using List<Map<String,Object>> there was a compile time error when trying to pull the value of the Object which was known to be either a String or a Widget.
My resolution was to use as when calling using the values 'as String' or 'as Widget' in the appropriate spots.
Another more elegant solution was to replace 'Object' with 'dynamic'.
I remember reading 'as' was discouraged where possible. I don't know why, and I feel it resolve the issue. Is this simply because it should be cast as a specific type when created?
When trying to recreate in Dartpad I had no issues, potentially just a flutter issue?
Why does dynamic work, but Object doesn't in this scenario? I mean everything will be a subtype of object right?
Thanks,
Can copy and paste code across if required, felt context of attached question was valuable.
I remember reading 'as' was discouraged where possible. I don't know why, and I feel it resolve the issue. Is this simply because it should be cast as a specific type when created? When trying to recreate in Dartpad I had no issues, potentially just a flutter issue?
Explicit type casts are a code smell. They're not necessarily wrong or bad, but they're frequently indicative of APIs that are unnecessarily awkward and perhaps could be designed better to use the correct types in the first place.
Explicit casts are also brittle. They introduce potential runtime failure points. That is, if the actual (runtime) type of the object changed, the cast could fail, and that failure wouldn't be noticed at compilation time.
Why does dynamic work, but Object doesn't in this scenario? I mean everything will be a subtype of object right?
dynamic is a special type that disables static (compile-time) type-checking. Object (like every other non-dynamic type) is statically type-checked, so any methods that you call on it must be statically known to exist on that type.
Note that using dynamic is brittle too since failures involving dynamic types are inherently runtime failures. Using dynamic also can be less readable in general; except when there's some obvious context, readers won't know what type you expect the object to be and therefore won't know what behavior you expect the called method to have.
Also see the Effective Dart recommendation: AVOID using dynamic unless you want to disable static checking.
In hist recent talk at Strange Loop Martin Odersky shed the light on his vision of Scala's future version called Dotty. I understand this is work-in-progress and it even may not flow into Scala (at least not very fast) due to many possible backward-compatibility issues. But if it happens, how should we program in Scala today to be forward-compatible with Dotty? I didn't get all the ideas from the talk so I'd like someone more profound to summarize the changes and describe how can we prepare to them.
The only other advice I would give is to also stay away from structural types containing a type members. Those type members are the elements that make type-checking in Scala turing complete, and I am not yet sure whether we want to support that in Dotty. Structural types that contain only vals and defs will likely stay supported.
Towards the end of the talk, he says "if your program is compiled without any language feature imports for higher-kinded and existentials then you should be ok".
So if I understand it correctly the only thing you would need to be aware of is any possible usage of existential types or higher-kinded types in your code.
When I return something of type Option, it seems useful to explain in the name of the function name that it is an option, not the thing itself. For example, seqs have reduceOption. Is there a standard naming convention? Things I have seen:
maybeFunctionName
functionNameOption
- neither seems to be all that great.
reduceOption and friends (headOption, etc.) are only named that way to distinguish them from their unsafe alternatives (which arguably shouldn't exist in the first place—i.e, there should just be a head that returns an Option[A]).
whateverOption isn't the usual practice in the standard library (or most other Scala libraries that I'm aware of), and in general you shouldn't need or want to use this kind of Hungarian notation in Scala.
Why would you want to make your function names longer? It doesn't contribute anything, as the fact that it returns an Option is obvious when looking at the function's type.
reduceOption is sort of a special case, since in most cases you really want to use reduce, except that it doesn't work on empty sequences.
I have read that Scala's type inference is not global so that is why people must place type annotations on the methods. (Would this be "local" type inference?)
I only a little understand that the reason is from its object-oriented nature, but clarity eludes me. Is there an explanation for "global type inference" and why Scala cannot have it that a beginner might understand?
The problem is that HM type inference is undecidable in general in a language with subtyping, overloading or similar features.Ref This means more and more stuff could be added to the inferencer to make it infer more special cases, but there will always be code where it will fail.
Scala has made the decision to make type annotations in method arguments and some other places mandatory. This might seem like a hassle first, but consider that this helps to document the code and provides the compiler with information it can understand in one place. Additionally, languages with HM inference often suffer from the problem that programming errors are sometimes detected in code far away from the original mistake, because the HM algorithm just went along and happened (by chance) to infer other parts of the code with the faulty type it inferred before it failed.
Scala's inference basically works from the outside (method definition) to the inside (code inside the method) and therefore limits the impact of a wrong type annotation.
Languages with HM inference work from the inside to the outside (ignoring the possibility to add type annotations) which means there is a chance that a small code change in one single method can change the meaning of the whole program. This can be good or bad.
Ref: Lower bounds on type inference with subtypes
The typical example for a global type inference is Hindley-Milner: It takes a given program and "calculates" all the necessary types. However in order to achieve this, the given language needs to have some properties (there are extensions to HM, which try to overcome some of these restrictions). Two things HM doesn't like are inheritance and method overloading. As far as I understand these are the main obstacles for Scala to adopt HM or some variant of it. Note that in practice even languages which heavily rely on HM never reach a 100% inference, e.g. even in Haskell you need a type annotation from time to time.
So Scala uses a more limited (as you say "local") form of type inference, which is still better than nothing. As far as I can tell the Scala team tries to improve the type inference from release to release when it is possible, but so far I've seen only smaller steps. The gap to a HM style type inferencer is still huge, and can't be closed completely.
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C# 'var' keyword versus explicitly defined variables
EDIT:
For those who are still viewing this, I've completely changed my opinion on var. I think it was largely due to the responses to this topic that I did. I'm an avid 'var' user now, and I think its proponents comments below were absolutely correct in pretty much all cases. I think the thing I like most about var is it REALLY DOES reduce repetition (conforms to DRY), and makes your code considerably cleaner. It supports refactoring (when you need to change the return type of something, you have less code cleanup to deal with, and NO, NOT everyone has a fancy refactoring tool!), and anecdotally, people don't really seem to have a problem not knowing the specific type of a variable up front (its easy enough to "discover" the capabilities of a type on-demand, which is generally a necessity anyway, even if you DO know the name of a type.)
So here's a big applause for the 'var' keyword!!
This is a relatively simple question...more of a poll really. I am a HUGE fan of C#, and have used it for over 8 years, since before .NET was first released. I am a fan of all of the improvements made to the language, including lambda expressions, extension methods, LINQ, and anonymous types. However, there is one feature from C# 3.0 that I feel has been SORELY misused....the 'var' keyword.
Since the release of C# 3.0, on blogs, forums, and yes, even Stackoverflow, I have seen var replace pretty much every variable that has been written! To me, this is a grave misuse of the feature, and leads to very arbitrary code that can have many obfuscated bugs due to the lack in clarity of what type a variable actually is.
There is only a single truly valid use for 'var' (in my opinion at least). What is that valid use, you ask? The only valid use is when you are incapable of knowing the type, and the only instance where that can happen:
When accessing an anonymous type
Anonymous types have no compile-time identity, so var is the only option. It's the only reason why var was added...to support anonymous types.
So...whats your opinion? Given the prolific use of var on blogs, forums, suggested/enforced by tools like ReSharper, etc. many up and coming developers will see it as a completely valid thing.
Do you think var should be used so prolifically?
Do you think var should ever be used for anything other than an anonymous type?
Is it acceptable to use in code posted to blogs to maintain brevity...terseness? (Not sure about the answer this one myself...perhaps with a disclaimer)
Should we, as a community, encourage better use of strongly typed variables to improve code clarity, or allow C# to become more vague and less descriptive?
I would like to know the communities opinions. I see var used a lot, but I have very little idea why, and perhapse there is a good reason (i.e. brevity/terseness.)
var is a splendid idea to help implement a key principle of good programming: DRY, i.e., Don't Repeat Yourself.
VeryComplicatedType x = new VeryComplicatedType();
is bad coding, because it repeats VeryComplicatedType, and the effects are all negative: more verbose and boilerplatey code, less readability, silly "makework" for both the reader and the writer of the code. Because of all this, I count var as a very useful enhancement in C# 3 compared to Java and previous versions of C#.
Of course it can be mildly misused, by using as the RHS an expression whose type is not clear and obvious (e.g., a call to a method whose declaration may be far away) -- such misuse may decrease readability (by forcing the reader to hunt for the method's declaration or ponder deeply about some other subtle expression's type) instead of increasing it. But if you stick to using var to avoid repetition, you'll be in its sweet spot, and no misuse.
I think it should be used in those situations where the type is clearly specified elsewhere in the same statement:
Dictionary<string, List<int>> myHashMap = new Dictionary<string, List<int>>();
is a pain to read. This could be replaced by the following with no loss of clarity:
var myHashMap = new Dictionary<string, List<int>>();
Pop quiz!
What type is this:
var Foo = new string[]{"abc","123","yoda"};
How about this:
var Bar = {"abc","123","yoda"};
It takes me roughly no longer to determine what types those are than with the explicity redundant specification of the type. As a programmer I have no issues with letting a compiler figure out things that are obvious for me. You may disagree.
Cheers.
Never say never. I'm pretty sure there are a bunch of questions where people have expounded their views on var, but here's mine once more.
var is a tool; use it where it's appropriate, and don't use it when it's not. You're right that the only required use of var is when addressing anonymous types, in which case you have no type name to use. Personally, I'd say any other use has to be considered in terms of readability and laziness; specifically, when avoiding use of a cumbersome type name.
var i = 5;
(Laziness)
var list = new List<Customer>();
(Convenience)
var customers = GetCustomers();
(Questionable; I'd consider it acceptable if and only if GetCustomers() returns an IEnumerable)
Read up on Haskell. It's a statically typed language in which you rarely have to state the type of anything. So it uses the same approach as var, as the standard "idiomatic" coding style.
If the compiler can figure something out for you, why write the same thing twice?
A colleague of mine was at first very opposed to var, just as you are, but has now started using it habitually. He was worried it would make programs less self-documenting, but in practice that's caused more by overly long methods.
var MyCustomers = from c in Customers
where c.City="Madrid"
select new { c.Company, c.Mail };
If I need only Company and Mail from Customers collection. It's nonsense define new type with members what I need.
If you feel that giving the same information twice reduces errors (the designers of many web forms that insist you type in your email address twice seem to agree), then you'll probably hate var. If you write a lot of code that uses complicated type specifications then it's a godsend.
EDIT: To exapand this a bit (incase it sounds like I'm not in favour of var):
In the UK (at least at the time I went), it was standard practice to make Computer Science students learn how to program in Standard ML. Like other functional languages it has a type system that puts languages in the C++/Java mould to shame.
Anyway, what I noticed at the time (and heard similar remarks from other students) was that it was a nightmare to get your SML programs to compile because the compiler was so increadibly picky about types, but once the did compile, they almost always ran without error.
This aspect of SML (and other functional languages) seems to be one the questioner sees as a 'good thing' - i.e. that anything that helps the compiler catch more errors at compile time is good.
Now here's the thing with SML: it uses type inference exclusively when assigning. So I don't think type inference can be inherently bad.
I agree with others that var eliminates redundancy. I have decided to use var where it eliminates redundancy as much as possible. I think consistency is important. Choose a style and stick with it through a project.
As Earwicker indicated, there are some functional languages, Haskell being one and F# being another, where such type inference is used much more pervasively -- the C# analogy would be declaring the return types and parameter types of methods as "var", and then having the compiler infer the static type for you. Static and explicit typing are two orthogonal concerns.
In fact, is it even correct to say that use of "var" is dynamic typing? From what I understood, that's what the new "dynamic" keyword in C# 4.0 is for. "var" is for static type inference. Correct me if I am wrong.
I must admit when i first saw the var keyword pop up i was very skeptical.
However it is definitely an easy way to shorten the lines of a new declaration, and i use it all the time for that.
However when i change the type of an underlying method, and accept the return type using var. I do get the occasional run time error. Most are still picked up by the compiler.
The secound issue i run into is when i am not sure what method to use (and i am simply looking through the auto complete). IF i choose the wrong one and expect it to be type FOO and it is type BAR then it takes a while to figure that out.
If i had of literally specified the variable type in both cases it would have saved a bit of frustration.
overall the benefits exceed the problems.
I have to dissent with the view that var reduces redundancy in any meaningful way. In the cases that have been put forward here, type inference can and should come out of the IDE, where it can be applied much more liberally with no loss of readability.