I've got a class written in F# that I'm consuming in C#, that defines a method Render:
member this.Render template (context: IContext) =
let tokens = Lexer.tokenize template
let parser = new DefaultParser([for filter in _filters -> filter])
let resp = new StringBuilder()
for node in parser.Parse tokens None do
ignore <| resp.Append(node.render context)
resp.ToString()
The signature of this method is template:string -> (IContext -> string), which of course reads as "member Render takes a string parameter, then returns a function that takes an IContext and produces a string.
If I change the declaration from "member" to a let binding, defining it as a function local to the class definition:
let Render template (context: IContext) = ...
Then the signature becomes what you would expect it to be - string -> IContext -> string, which reads "Render takes a string, then an IContext and produces a string".
Is there a way to make a member behave like the let binding? This is causing issues consuming this member from C#, as the signature becomes Render(string, FastFunc<IContext, string>), which is not overly usable.
If you want to expose to C#, you should write it tupled style:
> type Foo =
- member this.Bar (param1, param2) = param1 + param2;;
type Foo =
class
member Bar : param1:int * param2:int -> int
end
That'll expose a normal .NET style method.
Related
I'm calling a C# API which uses overloads and optional parameters.
Unfortunately, one of the overloads is a params object[] and F# selects it over a more specific overload which I intend to call. How do I make F# select the overload I want?
Here's a small repro. And here is a link to the actual API.
open System
open System.Linq.Expressions
type S =
static member Foo(expression: Expression<Func<int>>, ?gg: string) =
"expression"
static member Foo([<ParamArray>] args: obj[]) =
"params array"
[<EntryPoint>]
let main argv =
// This prints "params array", but I want it to print "expression"
let result = S.Foo(fun () -> 3, "")
printfn "%s" result
0
To call the expression version with two arguments, you need:
let result = S.Foo((fun () -> 3), "")
In your code, you've actually defined a function that returns a (3, "") tuple, which is only a single argument.
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 have the below code:
type IQuery =
abstract List<'T> : unit -> IList<'T>
let create (str)=
let getList () : IList<'T> = upcast List<'T>()
{ new IQuery with
member this.List<'T>() = getList<'T>()
And for the last line it gives me a warning stating that:
The method or function 'getList' should not be given explicit type argument(s) because it does not declare its type parameters explicitly
However if I remove <'T> from getList call then I get a compilation error as :
The member 'List<'T> : unit -> IList<'a>' does not have the correct type to override the corresponding abstract method. The required signature is 'List<'T> : unit -> IList<'T>'.
What can I do ?
You can declare getList with an explicit type parameter:
let getList<'T> () : IList<'T> = upcast List<'T>()
You then get an error:
Explicit type parameters may only be used on module or member bindings
If you then move the let binding to the top-level at the same scope as the type, it all works:
type IQuery =
abstract List<'T> : unit -> IList<'T>
let getList<'T> () : IList<'T> = upcast List<'T>()
let create (str) =
{ new IQuery with
member this.List<'T>() = getList<'T>()
}
If your real code has getList using values only in scope in create, like str, you'll need to add them as explicit parameters to getList.
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
I have a FileReader class whose job is to read and process text files using a StreamReader. To facilitate unit testing, I'd like to provide a type parameter to this class so that I can swap the StreamReader for a FakeReader that doesn't actually interact with the file system (and maybe throws exceptions such as OutOfMemory, so I can test the error handling in FileReader).
Ideally, I'd like to define FileReader something like this (trivialized for clarity):
type FileReader<'Reader> =
member this.Read file =
use sr = new 'Reader(file)
while not sr.EndOfStream do
printfn "%s" <| sr.ReadLine()
and simply define FakeReader to have a constructor that takes the file name, the EndOfStream property getter, the ReadLine() method, and the (empty) Dispose() method. However, F# has several complaints about this type definition, including "Calls to object constructors on type parameters cannot be given arguments." Since StreamReader has no default constructor, this approach seems like a no-go.
So far the only way I've gotten this to work is to inherit FakeReader from StreamReader:
type FakeReader() =
inherit StreamReader("") with
override this.ReadLine() = "go away"
member this.EndOfStream = false
member this.Dispose() = ()
and use a factory method that returns either a new FakeReader or a new StreamReader as appropriate.
type ReaderType = Fake | SR
let readerFactory (file : string, readerType) =
match readerType with
| Fake -> new FakeReader() :> StreamReader
| SR -> new StreamReader(file)
type FileReader(readertype) =
member this.Read file =
use sr = readerFactory(file, readertype)
while not sr.EndOfStream do
printfn "%s" <| sr.ReadLine()
This seems a lot less elegant. Is there a way to do this with a type parameter? Thanks to all.
Using a function that creates a reader object (as suggested by MizardX) is the direct answer to your question. However, I'd maybe consider using a different abstraction than TextReader). As Ankur mentioned in a comment, you could use a more functional approach.
If you're just reading lines of text from the input using TextReader, you could use a seq<string> type instead. The FileReader type may actually be just a function taking seq<string> (although that may be oversimplification... it depends).
This makes it more "functional" - in functional programming, you're often transforming data structures using functions, which is exactly what this example does:
open System.IO
/// Creates a reader that reads data from a file
let readFile (file:string) = seq {
use rdr = new StreamReader(file)
let line = ref ""
while (line := rdr.ReadLine(); !line <> null) do
yield !line }
/// Your function that processes the input (provided as a sequence)
let processInput input =
for s in input do
printfn "%s" s
readFile "input.txt" |> processInput
To test the processInput function, you could then create a new seq<string> value. This is significantly easier than implementing a new TextReader class:
let testInput = seq {
yield "First line"
yield "Second line"
raise <| new System.OutOfMemoryException() }
testInput |> processInput
You could pass in a function that constructs and returns an object of your desired type.
type FileReader(f : string -> TextReader) =
member this.Read file =
use sr = f file
while sr.Peek() <> -1 do
printfn "%s" <| sr.ReadLine()
type FakeReader() =
inherit StringReader("")
override this.ReadLine() = "go away"
override this.Peek() = 0
let reader1 = new FileReader(fun fn -> new StreamReader(fn) :> _)
let reader2 = new FileReader(fun fn -> new FakeReader() :> _)
Cast was necessary because I dropped the generic type-argument, but the actual type can be inferred.