I am trying to set the message formatter for a message in F#. In C# I can have:
foreach (System.Messaging.Message message in messages)
{
message.Formatter = new XmlMessageFormatter(new String[] { "System.String,mscorlib" });
string body = message.Body.ToString();
Console.WriteLine(body);
}
which works just fine. I now want to do the same thing in F# and have:
let mList = messageQueue.GetAllMessages()
let xt = [| "System.String,mscorlib" |]
for m in mList do
m.Formatter = XmlMessageFormatter(xt)
which causes this error at compile time:
Error 2 This expression was expected to have type
IMessageFormatter
but here has type
XmlMessageFormatter
I suspect I am missing a basic concept in F#. What am I doing wrong?
--EDIT--
latkin's answer worked perfectly. Just in case anyone else is interested, here is the full working program in F#:
open System.Messaging
[<EntryPoint>]
let main argv =
printfn "%A" argv
let messageQueue = new MessageQueue(".\private$\Twitter")
messageQueue.MessageReadPropertyFilter.SetAll();
let mList = messageQueue.GetAllMessages()
let xt = [| "System.String,mscorlib" |]
for m in mList do
m.Formatter <- XmlMessageFormatter(xt)
printfn "%s " (m.Body.ToString())
0 // return an integer exit code
When you are assigning a mutable value, the operator is <-, not =. In F# = is only used for initial bindings, otherwise it's used as the Boolean equality operator (like C-family ==). Some docs here.
You want
let mList = messageQueue.GetAllMessages()
let xt = [| "System.String,mscorlib" |]
for m in mList do
m.Formatter <- XmlMessageFormatter(xt)
No casting is needed in this case.
The error comes up because the compiler thinks you are trying to compare a IMessageFormatter to a XmlMessageFormatter.
F# doesn't have implicit casts like C# does, so it doesn't automatically upcast your XmlMessageFormatter to the IMessageFormatter used by the Formatter property.
There was a similar question a couple of days ago with more information about this:
F# return ICollection
Related
I am new in F# and I wonder whether is there a possibility (unsing NUnit) to construct a test class multiple parameters in its constructor with some similar construction - following end up with
Message:
OneTimeSetUp: No suitable constructor was found
// if data with one parameter, no problem to run the tests
// the data not only the constants at the compile time, so need to work TestFixtureSoource attribute.
type SimpleFixtureArgs =
static member Source = [| (String.Empty, String.Empty); ("hello", "hello") |]
[<TestFixtureSource(typeof<SimpleFixtureArgs>, "Source")>]
type ``simple tests class``(text, text2) =
[<Test>]
member this.``simple test``() =
let expexted = text
let actual = text2
Assert.AreEqual(expexted, actual)
Since removing the one parameter (e.g. the text2) and having appropriate one-parameter TestFixtureSource it started to work...
So the question is how to write the NUnit test to work with TestFixtureSource with multiple parameters?
TIA,
Mojmir
Individual items of text fixture source should be object arrays or derive from the TestFixtureParameters class (NUnit documentation). But tuple is not an object array - it's a single object. So change source property to return IEnumerbale (or array) of arrays:
type SimpleFixtureArgs =
static member Source = seq {
[| String.Empty; String.Empty|]
[| "hello"; "hello"|]
}
Later, I needed to deal with different types of parameters.
There is slightly modified code in F# from the previous answer, which worked for me.
type SimpleFixtureArgs2 =
static member Source : seq<obj []> =
seq {
yield [| String.Empty; String.Empty; 1; 1 |]
yield [| "hello"; "hello"; 2; 2 |]
}
[<TestFixtureSource(typeof<SimpleFixtureArgs2>, "Source")>]
type ``simple tests2 class``(text1, text2, num1, num2) =
[<Test>]
member this.``simple strings and integers test``() =
let expextedText = text1
let actualText = text2
Assert.AreEqual(expextedText, actualText)
Assert.AreEqual(num1, num2)
A couple days ago, I posted a question about deserialization with enums in F#.
The question is here: Deserialization in F# vs. C#
The answer pointed to some code written by Isaac Abraham, at: https://gist.github.com/isaacabraham/ba679f285bfd15d2f53e
However I am facing another problem:
If the object to deserialize to has an object of type 'enum option', the deserialization will fail, whereas it'll work if the type is just 'enum'.
A minimal example:
type TestType =
| A = 0
| B = 1
type TestObjectA =
{
test : TestType
}
type TestObjectB =
{
test : TestType option
}
let x = "{\"test\":\"A\"}"
let TestA = Deserialize<TestObjectA> x // will work
let TestB = Deserialize<TestObjectB> x // will fail
and the large deserialization code is at: https://pastebin.com/95JZLa6j
I put the whole code in a fiddle: https://dotnetfiddle.net/0Vc0Rh
but it can't be run from there since the F# version they support will not accept the 'object' keyword.
So, my question is: why can't I use the option type on an enum, but it works on other types? As a side note, since I'm quite new to F#, I'm not fully understanding Isaac's code, although I spent some time going through it and trying to troubleshoot it.
My understanding is that this line:
|> Seq.map (fun (value, propertyInfo) -> Convert.ChangeType(value, propertyInfo.PropertyType))
will try to convert the type to the right enum, but not to the enum option.
As a bonus question, is there a working solution that does full idiomatic deserialization with enums? (without going through null types)
open System.IO
type TestType =
| A = 0
| B = 1
type TestObjectB =
{
test : TestType option
}
let jsonSerializeToString obj =
use writer = new StringWriter()
let ser = new Newtonsoft.Json.JsonSerializer()
ser.Formatting <- Newtonsoft.Json.Formatting.Indented
ser.Serialize(writer, obj)
writer.ToString()
let jsonDeserializeFromString str =
Newtonsoft.Json.JsonConvert.DeserializeObject<TestObjectB>(str)
let Test obj =
let str = jsonSerializeToString obj
let obj' = jsonDeserializeFromString str
obj'
[<EntryPoint>]
let main argv =
{ test = Some TestType.B } |> Test |> ignore
{ test = None } |> Test |> ignore
0
Note: if you need to serialize a large collection of objects, then stream them to a file instead of an in-memory string to avoid an OutOfMemoryException. Like use writer = File.CreateText(filePath).
As a bonus question, is there a working solution that does full
idiomatic deserialization with enums?
I use the Microsoft.FsharpLu.Json package in production and find it works quite well for serializing and deserializing between "plain" javascript and idiomatic F#. Note Microsoft.FsharpLu.Json relies on Newtonsoft.Json under the hood.
Below is an example with your types and your test string, using Expecto for tests.
namespace FsharpLuJsonTest
open Newtonsoft.Json
open Microsoft.FSharpLu.Json
open Expecto
open Expecto.Flip
// Setup for FSharpLu.Json
type JsonSettings =
static member settings =
let s = JsonSerializerSettings(
NullValueHandling = NullValueHandling.Ignore,
MissingMemberHandling = MissingMemberHandling.Ignore)
s.Converters.Add(CompactUnionJsonConverter())
s
static member formatting = Formatting.None
type JsonSerializer = With<JsonSettings>
// Your example
type TestType =
| A = 0
| B = 1
type TestObjectA = { test : TestType }
type TestObjectB = { test : TestType option }
module Tests =
let x = """{"test":"A"}"""
[<Tests>]
let tests =
testList "Deserialization Tests" [
testCase "To TestObjectA" <| fun _ ->
JsonSerializer.deserialize x
|> Expect.equal "" { TestObjectA.test = TestType.A }
testCase "To TestObjectB" <| fun _ ->
JsonSerializer.deserialize x
|> Expect.equal "" { TestObjectB.test = Some TestType.A }
]
module Main =
[<EntryPoint>]
let main args =
runTestsInAssembly defaultConfig args
As you can see FsharpLu.Json supports Discriminated Unions and option types out of the box in the way you prefer. FsharpLu.Json is a less flexible solution than some others like Chiron (which allow for much more customisation) but I tend to prefer the opinionated approach of FsharpLu.Json.
I haven't used it personally, but the new FSharp.SystemText.Json library with the JsonUnionEncoding.ExternalTag setting should work roughly the same way FsharpLu.Json does. That library uses Microsoft's new System.Text.Json library under the hood rather than Newtonsoft.Json.
I would like to have a mutable state in an F# object expression.
The first approach is to use ref cells as follows:
type PP =
abstract member A : int
let foo =
let a = ref 0
{ new PP with
member x.A =
let ret = !a
a := !a + 1
ret
}
printfn "%A" foo.A
printfn "%A" foo.A
printfn "%A" foo.A
printfn "%A" foo.A
A different approach would be as follows:
type State(s : int) =
let mutable intState = s
member x.state
with get () = intState
and set v = intState <- v
[<AbstractClass>]
type PPP(state : State) =
abstract member A : int
member x.state
with get () = state.state
and set v = state.state <- v
let bar n =
{ new PPP(State(n)) with
member x.A =
let ret = x.state
x.state <- ret + 1
ret
}
let barA1 = bar 0
printfn "%A" barA1.A
printfn "%A" barA1.A
printfn "%A" barA1.A
printfn "%A" barA1.A
Which version would be likely more performing (I need the state updating x.state <- ret + 1
in performance critical sections)? My guess is that the State object is also allocated on the heap so there is no reason why the second version should be faster. However it is slightly more appealing to use.
Thanks for any feedback and suggestions
As Daniel said, the last approach is essentially equivalent to using built-in ref.
When using ref, you're allocating two objects - the one that you're returning and the reference cell itself. You can reduce this to just a single allocated object by using a concrete implementation (but I don't think this will matter in practice):
type Stateful(initial:int) =
let mutable state = initial
interface PP with
member x.A =
let ret = state
state <- state + 1
ret
let foo =
Statefull(0) :> PP // Creates a single object that keeps the state as mutable field
Aside, you are using read-only property that modifies internal state of the object and returns a new state each time. This is a dangerous pattern that could be quite confusing - properties with getter shouldn't modify the state, so you should probably use a method (unit -> int) instead.
Your State class is identical to ref. They're both reference types (you can't capture a mutable value type from an object expression). I would prefer a built-in type when possible. ref is the idiomatic way to represent a heap-allocated mutable value.
If ever in doubt about performance, benchmark it.
Suppose I have type A with indexer implemented, e.g. type A is a library. Now I want to extend the indexer of it, e.g. here I want to add float number into the indexer.
I worked out the following code:
type A(a:int array) =
member this.Item
with get(x) = a.[x]
and set(x) value = a.[x] <- value
type A with
member m.Item with
get(x:float) = m.[x |> int]
and set(x:float) v = m.[x |> int] <- v
But it seems not working:
let a = A([| 1;2;3 |])
a.[1]
a.[1] <- 10
a.[1.0]
For the last line, I get:
Script1.fsx(243,4): error FS0001: This expression was expected to have type
int
but here has type
float
Is extending indexer possible in F#? Thanks!
This behaves differently when the type extension is defined in a separate assembly (or separate module) and when it is in the same module as the type definition.
When both are in the same module, F# compiles them into a single class and Item becomes a standard overloaded indexer - In this case, your code works as expected (and this is how you actually wrote it here).
When they are in separate modules, F# compiles the indexer as an extension member. In this case, I get the error message you described.
Adding new overloads using extension members (e.g. new method) is possible. As far I can see, the specificaton doesn't say that this shouldn't work for indexers, so I think it is a bug (can you report it to fsbugs at microsoft dot com?)
I just tried this in FSI and it seems to work.
What compiler are you using?
This is what I fed to FSI:
type A(a:int array) =
member this.Item
with get(x) = a.[x]
and set(x) value = a.[x] <- value
type A with
member m.Item
with get(x:float) = m.[x |> int]
and set(x:float) v = m.[x |> int] <- v
let a = A([| 1;2;3 |])
a.[1] <- 10
printfn "%A" a.[1.2]
This prints '10'
Is there a way to have mutable function arguments in F#, that would allow something like
let mutable i = 9
let somefun n = n <- 12; ()
somefun i
(* *not* a real-world example *)
I do understand that this can be made to work by wrapping it into a record type
type SomeRec = { mutable i: int }
let ri = { i = 9 }
let someotherfun r = r.i <- 12; ()
and that this can be done in a similar fashion for class members. However, even after browsing through the whole F# Language Specification (yes, I did!), there seems to be no syntax to allow the first case, and the compiler appears to be quite unhappy about my trying this. I was hoping there would be some sort of type annotation, but mutable cannot be used in such.
I also know that I should not be doing this sort of thing in the first place, but the first case (int binding) and the second (record type) are semantically identical, and any such objection would hold for both cases equally.
So I think that I am missing something here.
You can use ref as arguments
let v = ref 0
let mutate r =
r := 100
mutate v
printfn "%d" !v
Or byref keyword
let mutable v = 0
let mutate (r : byref<_>) =
r <- 100
mutate &v
printfn "%d" v
Use byref keyword which is equivalent to C# ref.
See Passing by reference.