Why TypeProvider supports static parameters, how is this done, and does the f# compiler specifically support this feature?
I write a class myself, do not implement the ITypeProvider interface, can do the same effect?
The syntax of static parameters is not described in the f# language reference, is there any relevant help?
Thank you very much!
I think you are asking if the static parameters mechanism used in type providers can be used in ordinary F# code.
The short answer is no.
Type providers are special: they are compiler plug-ins that provides types, properties, and methods for use in your code. Special rules apply to them.
Related
When F# 4.5 was announced, it was stated that:
The F# feature set is comprised of
[...]
The ability to produce IsByRefLike structs (examples of such structs: Span<'T>and ReadOnlySpan<'T>).
How to "produce" these types? I tried the [<IsByRefLike>] attribute but is was not found in .NET Standard 2.0.
The attribute is found in System.Runtime.CompilerServices
open System.Runtime.CompilerServices
[<Struct; IsByRefLike>]
type Apa =
{ A: Span<int>
B: int }
Phillip Carter talks about this in What's new in F# 4.5 (about 21 min. in).
It is available for .NET Core and .NET Framework, but not .NET Standard 2.0.
Starting with .NET SDK 6.0.200 (available in Visual Studio 2022 17.1), the F# compiler recognizes user-defined IsByRefLikeAttributes. The following code will transparently enable defining ref structs on .NET Standard 2.0 as well as later frameworks:
#if NETSTANDARD2_0
namespace System.Runtime.CompilerServices
open System
[<Sealed; AttributeUsage(AttributeTargets.Struct)>]
type IsByrefLikeAttribute() = inherit Attribute()
#endif
namespace MyLibrary
open System
open System.Runtime.CompilerServices
[<IsByRefLike>]
type MyRefStruct(span: Span<int>) = struct end
Technically, this is not an answer.
First, according to the specs, IsByRefLike is for the compiler, not for the developers to use: https://learn.microsoft.com/en-us/dotnet/api/system.runtime.compilerservices.isbyreflikeattribute?view=netframework-4.7.2
Second, when we do want something from a compiler, then it is crucial that we do understand what we want from it. So a correct question could be: I need ABC because I need to do XYZ, where ABC would be something related to IsByRefLike and then XYZ would be something based on IsByRefLike. The question does not explain why IsByRefLike is needed.
I think that a minimalist approach should be always weighted in when considering which language features to use / not to use: do we really need some particular language feature to accomplish what we need? F# Option.bind comes to mind: if the function is a one-liner then Option.bind is great. However, if there is some tree of how to parse the result, then it might be better to do that explicitly without using Option.bind.
So the questions are:
Do you really need IsByRefLike?
If you think that you do, could you, please, post an example of where you actually do need it.
According to the source of Kotlin's lexer, there is a typealias keyword, and it's not "reserved for future use" like yield and typeof.
Also, the grammar reference suggests that typealias should be a valid keyword for a class member declaration, and when I type typealias in IntelliJ IDEA (Android Studio) with Kotlin plugin it recognizes it as a keyword but I get expecting member declaration error. I have also tried using it with the "usual" syntax, for example like it is implemented in Swift, however with no success.
So, is the typealias feature actually implemented in Kotlin (as of 1.0), and if so, what is the syntax for it? Is there any documentation that describes its use?
Update
Kotlin 1.1 supporting type aliases is now out!
With a typealias, you can provide an alternative name for an existing type since Kotlin 1.1:
typealias Multimap<K, V> = MutableMap<K, MutableList<V>>
For more information, see the official documentation or the KEEP proposal.
I've been programming in F# for some years and there's an "issue" that's been bothering me for some time and I have not been able to solve. It is not a bug, I think it is a design decision, but anyway, the problem is this: is there a way to delay (maybe that's not the correct word for this) the implementation of interfaces?, that is, not implementing them in the initial definition, but later, maybe in the same file after I have implemented a module for the type. I'll explain with a simplified example:
Suppose I have the following data structure:
type 'T MyCollection =
(*type definition*)
interface IEnumerable<'T> with
member this.GetEnumerator () =
(* I don't want to implement it here
because I still don't have the module
with a toSeq function *)
If I implemented the method right there, I would have to also implement all the functions as methods of the type and then the module would be just a "proxy" for calling the methods. This way I'm creating a OO-first data structure and then creating a module (overloaded with type annotations) to allow for a functional-first usage. I would prefer to write a functional-first data structure (cleaner since the type inference can work better) and then create a OO wrapper to allow a better intellisense support for languages like C#. That approach complies with what the design guidelines for F# tells us, but the interfaces can't be implemented anywhere but in the initial definition of the type. That restriction forces me to write the whole data structure with members.
I've been looking for examples and I've found that the list implementation in FSharp.Core list does exactly what I want, but I can't do that, the compiler won't let me.
I'm almost sure that this is a design decision, maybe to avoid encouraging bad practices, I don't know, but I don't consider my wish to be a bad practice. Also I'm well aware of the linear nature of the fsharp compiler.
Please if any of you know how to do what I want, I'll be glad if you tell me. Also, if any of you know why I should not follow this approach I'll be glad to know too. There must be a reason why this is not a problem for anyone else.
Thanks in advance.
I completely agree that this is unfortunate problem. The trick that is used in the source code of 'a list in the F# Core library is to define the implementation of the interface in a type augmentation. The compiler does not complain when you add members to a type in this way, but it says that adding implementation of an interface in this way is deprecated. However, it does not prevent you from doing this. The following compiles fine for me:
open System.Collections
open System.Collections.Generic
type MyCollection<'T> =
{ Data : 'T list }
interface IEnumerable<'T>
interface IEnumerable
let getEnumerator { Data = d } =
(d :> seq<_>).GetEnumerator()
type MyCollection<'T> with
interface IEnumerable<'T> with
member this.GetEnumerator() = getEnumerator this
interface IEnumerable with
member this.GetEnumerator() = (getEnumerator this) :> _
The fact that this is deprecated is a bit unfortunate. I quite like this style and I use it when it makes sense. You can start a discussion about this on F# user voice and perhaps it could be turned back into a normal accepted feature :-)
I am very new to F# and I started to write my functional wrapper on top of OpenGL. I also intend to use it to write a graphics engine which should have interop with all .Net languages.
But it is hard to find information about which code constructs in F# are not CLS compliant.
For example I already know of a few that are not CLS compliant:
static type constrains
tupleless functions
probably 'T list and Seq
maybe even union types
How do I know what features of F# are not CLS compliant?
Use the CLSCompliant attribute. It will guarantee that your code is CLS-Compliant at compile time.
Like this:
module myProject.AssemblyInfo
open System
[<assembly: CLSCompliant(true)>]
do()
Source: Mike-Ward.Net: Learning F#–Assembly Level Attributes
For a more complete discussion of the CLSCompliant attribute, see C# Corner: Making Your Code CLS Compliant
Do F# type providers work by using the DLR under the hood? That is to say, do they work in the way that the dynamic keyword in C# does? How is this related to expando objects?
How does codegen fit in?
Type providers are plugin to the compilation process. Internally a type provider may use DLR or anything but when the compiler ask it for a type it needs to return a type that is statically resolved at compile time. Think of it like rather than a human creating a type (class in C#) you have a assembly (type provider) which the compiler can ask to create a new type at compile time.
Ex: In case of SQL type provider the type representing the tables will be generated at compile time and put in the assembly as static types.
Type providers solve similar problem as the dynamic keyword in C# - both of them were designed to make it easier to access data that have some structure that is not described in your programming langauge and so you need to somehow infer it later.
The dynamic keyword just lets you access any member (i.e. data filed) or method at compile time and then decides how to handle the operation at runtime. If you're using it to access .NET object, then it will use DLR, but if you're accessing some other object (like JSON data) then it will perform some simple dictionary lookup.
F# type providers are quite different - they infer the structure at compile time and pass it to the F# compiler. The compiler will then check all your code. The type provider also decides how the access to a field or method should be compiled. Typically, it will either replace it with an ordinary .NET type (so it will be compiled as normal .NET invocation) or it will replace the object with some dictionary lookup. A type provider may use DLR under the cover, but I don't think it is very common case.