I have a name clashing problem while opening some .Net assemblies in my F# code using
open System.IO
The only option I found and use now is to provide the full names of types for conflicting types, but this is a little bit boring.
Is there any possibility to define an alias for .Net namespace in F# to prevent name clashing?
F# does not support aliasing of namespaces - only modules and types. So, to resolve the conflicts between .NET assemblies, you will, unfortunatelly, need to define aliases for all the types you're using.
This may be slightly easier thanks to the fact that F# type aliases are viewed as normal type declarations (by the F# compiler, not by the runtime). This means that, unlike with C# using keyword, you can define them in a spearate file:
// Aliases.fs
namespace SysIO
// Open the 'System' namespace to get a bit shorter syntax
// ('type File = File' is interpreted as discriminated union)
open System
type File = IO.File
type Directory = IO.Directory
In the rest of your application, you can now use SysIO.File. You still have to write the aliases, but at least you don't have to do that in every file...
Related
(previously "using type provider from C#")
This should be easy to answer, and the underlying issue has be asked before, but I'm slightly confused about it, having dug into it, there's something I'm missing.
The Xml type provider in
https://fsharp.github.io/FSharp.Data/library/XmlProvider.html
is erased?
so the types in it can't be used from another assembly?
"Erasing Type Providers produce types that can only be consumed in the assembly or project they are generated from" (says the MS docs).
but if I define in one assembly this
module Xml
type Xml = FSharp.Data.XmlProvider<"""
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>""">
let foo () = Xml.Parse ("")
and in another assembly, reference the above assembly and go...
let x = Xml.foo ()
let s = x.From
this compiles...so if the MS docs are correct, the provider must be generative?
if I implement the same code in C#,
var x = Xml.foo();
var y = x.From;
then this code doesnt compiler
'XmlElement' does not contain a definition for 'From' and no accessible extension method 'From' accepting a first argument of type 'XmlElement'
which seems to imply its erased...or I need to do something else (I've included the same dependencies).
My hunch is its erased, but then how does the F# assembly compile?
(I want it to be generative to use from C# and VB).
As far as I can tell by looking at the source code, all the types provided by FSharp.Data are erased.
The XmlProvider type is defined by this line at https://github.com/fsharp/FSharp.Data/blob/master/src/Xml/XmlProvider.fs#L26
let xmlProvTy = ProvidedTypeDefinition(asm, ns, "XmlProvider", None, hideObjectMethods=true, nonNullable=true)
As you can see, the instantiation of the ProvidedTypeDefinition type does not include isErased=false.
The ProvidedTypeDefinition type has two constructors. See the signatures at https://github.com/fsprojects/FSharp.TypeProviders.SDK/blob/master/src/ProvidedTypes.fsi#L268-L275
Both constructors have an implementation including this line, which means that the provide type is erased by default:
let isErased = defaultArg isErased true
Without redesigning the XmlProvider, the only way you can consume it from C# is to wrap it in an F# class that exposes C#-friendly types. Not sure if that would result in a better design than just avoiding the type provider altogether.
Assuming by "erased" you mean being removed from the FSharp.Data Library? then XMLProvider its probably not removed from it because I just tried your code and it works in F# project (as you have also noticed).
Now about the C# project the thing is TypeProviders are a Fsharp specific thing, whatever magic F# compiler is doing to type check xml and access the From xml-node here is probably only exclusive to F#.
The type of you variable x here is FSharp.Data.Runtime.BaseTypes.XmlElement, looking at the documentation here and the source code here.
There doesn't seem to be any way of accessing the xml nodes in an OOP way. Its probably not meant to be accessed that way either. If you want to parse xml and read its nodes in C# there are plenty of other ways for that. One being XmlReader in System.Xml
I have a union type that has a single, empty case.
type Default =
| Default
This type has a purpose, but it's not meant to be visible or usable.
Unfortunately, I have to use it in an inline function that does need to be visible. This prevents me from making the type or the case private.
The solution I came up with is using the CompilerMessageAttribute on it to signal an error whenever it's used. This would be fine, but now I can't compile my own assembly because IT uses it.
Is there a way to signal an error only when it's used by an assembly that references my assembly?
Let me reiterate the requirements to make sure I understand them:
The type needs to be public, so that that other assemblies can reference it implicitly via inline.
But if other assemblies reference it explicitly, then that is an error.
I don't know of any way of doing this using standard tooling.
I can see two possible solutions.
If only one calling assembly needs to use the inline function, what about making the type internal and then have the calling assembly be a friend assembly, using the InternalsVisibleToAttribute.
The only other alternative I can think of is security by obscurity. Hide the type in some awkwardly named module and require module qualification. This will stop accidental use of the type, if nothing else.
You could even add a build step to check that no source code references the module name.
[<RequireQualifiedAccessAttribute>]
module ``Dont Use This`` =
type Default =
| Default
let x = ``Dont Use This``.Default
And yes, it's very kludgy.
Is there any way to open a namespace of types provided by a type provider implementation? I ask, because some of the generated type name paths are very long and ugly in code. I have used type abbreviations to alleviate this, but obviously this is a manual process. The F# open keyword does not support this. Is there another way? Update: as pointed out in the answer and comments this is wrong. You can open a type provided namespace. I had not realised I was looking at deeply nested types, not a namespace.
This is tricky - parameterized type providers (like the standard SQL providers or the F# Data providers for XML and JSON) need to put all types that they generate (representing tables, XML nodes, etc) inside the main generated type. So all types that you might want to use are hidden somewhere as nested types of the main type (with parameters specified).
This makes sense - if you use the type provider with multiple parameters, the types generated for each configuration have to be separate.
As #kvb points out, you cannot open a nested type, but you can use type aliases to make this a bit less painful. For example, using F# Data, I can define an alias R that lets me access all the generated domain types with just two additional characters:
#r #"..\packages\FSharp.Data.1.1.10\lib\net40\FSharp.Data.dll"
open FSharp.Data
type RssFeed = XmlProvider<"http://rss.nytimes.com/services/xml/rss/nyt/HomePage.xml">
type R = RssFeed.DomainTypes
let printTitle (itm:R.Item) = printfn "%A" itm.Title
If I use the F# Type Providers from the assembly FSharp.Data.TypeProviders 4.3.0.0, I am able to create types in a very simple F# library. I am then able to use those types without any dependency on the assembly FSharp.Data.TypeProviders. That is pretty sweet! Here is an example:
I created an F# library project called TryTypeProviders. I put this in the .fs:
module TryTypeProviders
type Northwind = Microsoft.FSharp.Data.TypeProviders.ODataService
I then am able to use the F# library from a C# project:
public static void Main()
{
var c = new TryTypeProviders.Northwind();
foreach (var cust in c.Customers)
Console.WriteLine("Customer is: " + cust.ContactName);
Console.ReadKey(true);
}
I haven't been able to find any working examples of how to create a type provider like this. The type providers in FSharpx.TypeProviders are not accessible from C#. My guess is that they are erased types and not generated types. I'm still a little fuzzy on which is which, but it is defined here as:
Generated types are real .NET types that get embedded into the assembly that uses the type provider (this is what the type providers that wrap code generation tools like sqlmetal use)
Erased types are simulated types which are represented by some other type when the code is compiled.
The samples from the F# 3.0 Sample Pack mentioned in the MSDN tutorial are not working for me. They build, but when I try to use them I get errors.
open Samples.FSharp.RegexTypeProvidertype PhoneNumberRegEx = CheckedRegexProvider< #"(?<AreaCode>^\d{3})-(?<PhoneNumber>\d{3}-\d{4}$)">
open Samples.FSharp.MiniCsvProvidertype csv = MiniCsvProvider<"a.csv">
It was last released in March of 2011 and my guess is that they don't yet reflect the final version of type providers that shipped with Visual Studio 2012.
F# Type Providers look like a great technology, but we need help building them. Any help is appreciated.
The reason why standard type providers (for OData, LINQ to SQL and WSDL) work with C# is that they generate real .NET types behind the cover. This is called generative type provider. In fact, they simply call the code generation tool that would be called if you were using these technologies from C# in a standard way. So, these type providers are just wrappers over some standard .NET tools.
Most of the providers that are newly written are written as erasing type providers. This means that they only generate "fake" types that tell the F# compiler what members can be called (etc.) but when the compiler compiles them, the "fake" types are replaced with some other code. This is the reason why you cannot see any types when you're using the library from C# - none of the types actually exist in the compiled code.
Unless you're wrapping existing code-generator, it is easier to write erased type provider and so most of the examples are written in this way. Erasing type providers have other beneftis - i.e. they can generate huge number of "fake" types without generating excessively big assemblies.
Anyway, there is a brief note "Providing Generated Types" in the MSDN tutorial, which has some hints on writing generative providers. However, I'd expect most of the new F# type providers to be written as erasing. It notes that you must have a real .NET assembly (with the generated types) and getting that is not simplified by the F# helpers for building type providers - so you'll need to emit the IL for the assembly or generate C#/F# code and compile that (i.e. using CodeDOM or Roslyn).
This works
open System
let f = Action(fun () -> Unchecked.defaultof<_>)
But this
let f = System.Action(fun () -> Unchecked.defaultof<_>)
produces the compilation error
Multiple types exist called 'Action', taking different numbers of generic parameters. Provide a type instantiation to disambiguate the type resolution, e.g. 'Action<,,_,,,_,,,_>'.
I know I can fix it by adding a type parameter placeholder (System.Action<_>(...)), but any idea why they behave differently?
EDIT
Found this in the spec, section 14.1.9:
When a module or namespace declaration group F is opened, items are added to the name environment as follows:
Add the type to the TypeNames table. If the type has a CLI-mangled generic name such as List'1 then an entry is added under both List and List'1.
Is this behavior replicated for fully-qualified types (with omitted type parameters)? It doesn't appear so.
I agree with #James that this is related to the bug submitted on Connect, but I think it is a slightly different case. Anyway, I think this is not the intended behaviour. Could you report it to fsbugs at microsoft dot com?
Anyway - I did some debugging and here is what I found so far:
It seems that the compiler uses different code paths to resolve the name Action and the name System.Action. When resolving the other, it searches all loaded modules (i.e. assemblies) for a type named System.Action (see ResolveLongIndentAsModuleOrNamespaceThen function in the nameres.fs file of the open-source release).
This finds the two definitions of Action (one in mscorlib and another in System.Core). I think the issue comes from the fact that the name resolution simply iterates over the results - it finds the first one (from System.Core), which doesn't have a usable overload (because it ranges from Action<_,_,_,_,_> to a version with about 15 type parameters). After finding this type, it reports an error without even looking whether there is another type (in another assembly) that could be used.
If you don't reference system assemblies, then the F# compiler resolves the overload just fine. Running the compiler without parameters references the default assembly set, so this doesn't work:
fsc test.fs
but if I add the --noframework flag, then it compiles without issues:
fsc --noframework test.fs