When I move UiRoutes module into a separate file it gives me an error when compiling. This is the first time using Saturn and Giraffe so I'm not sure what is happening? What is the difference having the module in a separate file?
The error I get is:
"Value restriction. The value 'uiRouter' has been inferred to have generic type\n val uiRouter : (HttpFunc -> '_a -> HttpFuncResult) when '_a :> AspNetCore.Http.HttpContext \nEither make the arguments to 'uiRouter' explicit or, if you do not intend for it to be generic, add a type annotation."
Program.fs
open Saturn
// MongoDB.FSharp.Serializers.Register()
module ApiRoutes =
open Giraffe
let apiRoutes =
router {
//Routers in here
}
module UiRoutes =
open Giraffe
open Giraffe.ViewEngine
open DBApi
let carsView =
html [] [
//Some HTML
]
let uiRouter = htmlView carsView //HttpFunc -> AspNetCore.Http.HttpContext -> HttpFuncResult
let appRouter =
router {
forward "/api" ApiRoutes.apiRoutes
forward "" UiRoutes.uiRouter
}
let myApp = application { use_router appRouter }
run myApp
Solution:
//Changed
let uiRouter = htmlView carsView
//to:
let (uiRouter: HttpFunc -> Microsoft.AspNetCore.Http.HttpContext -> HttpFuncResult) = htmlView carsView
F# compiles each file separately. When you move the definition of uiRouter into a different file from the one where it's used, the F# compiler can no longer infer that it has a non-generic type. In other words, when compiling UiRoutes.fs, the compiler can't tell that you want uiRouter to have the exact type HttpFunc -> AspNetCore.Http.HttpContext -> HttpFuncResult. Since F# doesn't allow uiRouter to have a generic value, you get the value restriction error.
Here are two ways to avoid this situation:
Ensure that uiRouter is constrained to a non-generic type by a subsequent use of the value in the same file where it is defined.
Provide an explicit type annotation to manually constrain the type of uiRouter.
See similar SO question here.
Related
Running the following in a dotnet core 3.1 console application fails to compile the assembly:
module Test
open FSharp.Compiler.SourceCodeServices
open FSharp.Compiler.Text
let source = """module Test
type TestType() =
interface System.IAsyncDisposable with
member __.DisposeAsync() =
System.Threading.Tasks.ValueTask()
"""
let references =
[
"Microsoft.Bcl.AsyncInterfaces"
]
[<EntryPoint>]
let main(args) =
let checker = FSharpChecker.Create()
let options =
{ FSharpParsingOptions.Default with
SourceFiles = [|"empty.fs"|]
}
let sourceText = SourceText.ofString source
let parseResult =
checker.ParseFile("empty.fs", sourceText, options) |> Async.RunSynchronously
let input = parseResult.ParseTree.Value
let thisAssembly =
System.Reflection.Assembly.GetExecutingAssembly().GetName().Name
let allReferences =
thisAssembly :: references
let errors, _, assemblyOpt =
checker.CompileToDynamicAssembly([input], "TestProject", allReferences, None)
|> Async.RunSynchronously
0
assemblyOpt is None and the errors array contains the following messages:
The type 'IAsyncDisposable' is not defined in 'System'. Maybe you want one of the following:
IDisposable
The type 'obj' is not an interface type
The type 'IAsyncDisposable' is not defined in 'System'. Maybe you want one of the following:
IDisposable
The type 'IAsyncDisposable' is not defined in 'System'. Maybe you want one of the following:
IDisposable
This type is not an interface type
No abstract or interface member was found that corresponds to this override
The value, constructor, namespace or type 'ValueTask' is not defined.
p_tyar_spec: from error
p_tyar_spec: from error
However, both Microsoft.Bcl.AsyncInterfaces, and System should contain IAsyncDisposable. What's causing the missing reference error?
I suspect its the assembly list is incomplete in some way, you can confirm this by altering the dynamic assembly generation function to the following:
checker.CompileToDynamicAssembly([input], "TestProject", allReferences, None, noframework = true)
noframework = true
Which works, (I just tested it)
So, I was trying to get this simple test working in an F# console app:
open System.Reflection
open System.ComponentModel.Composition
open System.ComponentModel.Composition.Hosting
[<Export(typeof<int -> string>)>]
let toString(i: int) = i.ToString()
[<EntryPoint>]
let main argv =
use catalog = new AssemblyCatalog(Assembly.GetEntryAssembly())
use container = new CompositionContainer(catalog)
let myFunction = container.GetExportedValue<int -> string>()
let result = myFunction(5)
0
I expected MEF to get the function properly resolved, but it doesn't.
Instead, I get this:
An unhandled exception of type
'System.ComponentModel.Composition.CompositionContractMismatchException'
occurred in System.ComponentModel.Composition.dll
Additional information:
Cannot cast the underlying exported value of type 'Program.toString (ContractName="Microsoft.FSharp.Core.FSharpFunc(System.Int32,System.String)")' to type 'Microsoft.FSharp.Core.FSharpFunc``2[System.Int32,System.String]'.
What am I missing here?
What is the difference between FSharpFunc(System.Int32, System.String) and FSharpFunc``2[System.Int32, System.String]?
What is the correct way to import/export F# functions via MEF?
The compiler turns top-level F# functions into methods, so your example will be compiled as:
[Export(FSharpFunc<int,string>)]
public string toString(int i) { return i.ToString(); }
This is probably causing the error. You can force the compiler to produce a property getter of FSharpFunc type by calling some operation that returns a function - even a simple identity function will don:
let makeFunc f = f
[<Export(typeof<int -> string>)>]
let toString = makeFunc <| fun (i:int) ->
i.ToString()
I have not tested this, but I think it could work. That said, it is probably safer to go with a simple single-method interface in this case.
In my first attempt to create a type provider, I have a ProvidedTypeDefinition for a message:
// Message type
let mTy = ProvidedTypeDefinition(asm, ns, message.Key, Some(typeof<ValueType>),
HideObjectMethods = true, IsErased = false)
// Direct buffer
let bufferField = ProvidedField("_directBuffer", typeof<IDirectBuffer>)
mTy.AddMember bufferField
let mCtor1 =
ProvidedConstructor(
[ProvidedParameter("buffer", typeof<IDirectBuffer>)],
InvokeCode = fun args ->
match args with
| [this;buffer] ->
Expr.FieldSet (this, bufferField, <## %%buffer:IDirectBuffer ##>)
| _ -> failwith "wrong ctor params"
)
mTy.AddMember mCtor1
Then I need to create an instance of that type in a method of another provided type. I am doing this:
let mMethod = ProvidedMethod(message.Key, [ProvidedParameter("buffer", typeof<IDirectBuffer>)], mTy)
mMethod.InvokeCode <- (fun [this;buffer] ->
let c = mTy.GetConstructors().Last()
Expr.NewObject(c, [ buffer ])
)
ILSpy shows the following C# code equivalent for the method:
public Car Car(IDirectBuffer buffer)
{
return new Car(buffer);
}
and it also shows that the Car struct is present in the test assembly (this test assembly builds OK unless I access the Car method):
But when I try to create the Car via the method like this:
type CarSchema = SbeProvider<"Path\to\SBETypeProvider\SBETypeProvider\Car.xml">
module Test =
let carSchema = CarSchema()
let car = carSchema.Car(null)
I get the following errors:
The module/namespace 'SBETypeProvider' from compilation unit 'tmp5CDE' did not contain the namespace, module or type 'Car'
A reference to the type 'SBETypeProvider.Car' in assembly 'tmp5CDE' was found, but the type could not be found in that assembly
What I am doing wrong? The picture shows that the type is here. Why I cannot create it?
I looked through many type providers on GitHub and cannot find a clear example how to generate a ProvidedTypeDefinition from another one.
This might not be the problem, but at a quick glance it looks like the line you linked might actually be the issue:
let mTy = ProvidedTypeDefinition(asm, ns, message.Key, Some(typeof<ValueType>),
HideObjectMethods = true, IsErased = false)
This type is being added to the ty provided type (the one that will actually be written to the temporary assembly) and so shouldn't have the assembly and namespace specified itself.
let mTy = ProvidedTypeDefinition(message.Key, Some(typeof<ValueType>),
HideObjectMethods = true, IsErased = false)
Might work better. Generated types are a bit of a black art though, with very little documentation, so it's possible (probable?) that there will be other issues you might find.
On a more general note, for creating provided types what I normally end up doing is returning the provided constructor as a value which can then be embedded in the invoke code for other properties/functions using Expr.Call. This is especially important for erased types, as reflection will not work on them anyway.
I am an F# noob, so am struggling a bit getting some basic stuff to work. The context here is a FAKE build script.
I am trying to use the AssemblyInfoFile namespace to generate an AssemblyInfo.cs without including a generated internal class. I know I need to call the CreateCSharpAssemblyInfoWithConfig function and pass through an AssemblyInfoFileConfig that turns off the GenerateClass property. I have this:
Target "Build" (fun _ ->
CreateCSharpAssemblyInfoWithConfig (srcDir + "TestAssemblyInfo.cs")
[Attribute.Version version
Attribute.Configuration configuration]
{ GenerateClass = false }
)
But the compiler is complaining as follows:
The type 'AssemblyInfoParams' is not compatible with the type 'AssemblyInfoFileConfig'.
At first I thought I just needed to provide the specific type to the compiler so that it didn't resolve to AssemblyInfoParams. However, I couldn't figure out how to do that inline so I instead tried this:
Target "Build" (fun _ ->
let config : AssemblyInfoFileConfig = { GenerateClass = false }
CreateCSharpAssemblyInfoWithConfig (srcDir + "TestAssemblyInfo.cs")
[Attribute.Version version
Attribute.Configuration configuration)]
config
)
But now the compiler complains as follows:
The type 'AssemblyInfoFileConfig' does not contain a field 'GenerateClass'
Looking at the definition of AssemblyInfoFileConfig, it clearly contains a property called GenerateClass:
type AssemblyInfoFileConfig
(
generateClass : bool,
?useNamespace : string ) =
member x.GenerateClass = generateClass
member x.UseNamespace =
match useNamespace with
| Some n -> n
| None -> "System"
static member Default = AssemblyInfoFileConfig(true)
Can anyone tell me what I'm doing wrong here?
AssemblyInfoFileConfig is not a record - it is a more standard type with a constructor.
You could just call it like this:
let config = AssemblyInfoFileConfig(false)
I'm trying to create an abstraction for a lightweight data storage module using an F# signature file. Here is my signature file code, let's say it's called repository.fsi
namespace DataStorage
/// <summary>Lightweight Repository Abstraction</summary>
module Repository =
/// <summary> Insert data into the repository </summary>
val put: 'a -> unit
/// <summary> Fetch data from the repository </summary>
val fetch: 'a -> 'b
/// <summary> Remove data from the repository </summary>
val remove: 'a -> unit
Here is the corresponding implementation, let's call it repository.fs
namespace DataStorage
module Repository =
(* Put a document into a database collection *)
let put entity = ()
(* Select a document from a database collection *)
let fetch key = ("key",5)
(* Remove a document from a database collection *)
let remove entity = ()
In my visual studio project file I have the signature file (repository.fsi) above
my implementation file (repository.fs). The put and remove functions are being parsed and verified correctly with no errors (in the implementation file) but the fetch function keeps giving me the red squiggly in visual studio with the following error message:
Module 'DataStorage.Repository' contains
val fetch: s:string -> string * int
but its signature specifies
val fetch<'a,'b> : 'a -> 'b
The respective type parameter counts differ
Can someone tell me what I'm doing wrong? Is my fetch function value defined wrong in
my signature file? I'm just trying to create a generic function ('a -> 'b) in my signature file and have the implementation take one type as input and return a different type as output.
I am not very strong in F# yet but I think here you are using signature files in wrong way.
First. Here is how you can fix compilation errors:
Replace:
let fetch key = ("key",5)
with:
let fetch key = ("key",5) :> obj :?> 'b
and you won't get compilation errors.
But this fix doesn't actually makes sense in many cases. For example if next works:
let a = Repository.fetch(56)
if you specify type explicity it will crash (in most cases):
let b = Repository.fetch<int, string>(56)
The case is that generic implementation should operate with generic types. If I understod correctly from what are you trying to do you should use OOP and polymophism when signature files are used to hide implementation aspects. For example:
namespace DataStorage
[<AbstractClass>]
type Repository<'TKey,'T>() =
abstract member put: 'T -> unit
abstract member fetch: 'TKey -> 'T
abstract member remove: 'T -> unit
type IntRepository() =
inherit Repository<int, int>()
override self.put item = ()
override self.fetch index = 5
override self.remove item = ()
type StringRepository() =
inherit Repository<int, string>()
override self.put item = ()
override self.fetch index = "Hello"
override self.remove item = ()
One (some what limiting) alternative that I've recently tried is sort of one step away from generics but seems to work for my scenario. Basically the signature file for the fetch function now looks like this.
'a -> RepositoryRecord
and the implementation of the RepositoryRecord is an algebraic datatype.
type RepositoryRecord = | SomeVal1 of int * string | SomeVal2 of string