Func<TResult> F# snippets (#7607)

Author: albert-du

samples/snippets/fsharp/VS_Snippets_CLR_System/system.Func~1/fs/Delegate.fs

@@ -0,0 +1,25 @@
+module Delegate
+
+// <Snippet1>
+open System.IO
+
+type WriteMethod = delegate of unit -> bool
+
+type OutputTarget() =
+    member _.SendToFile() =
+        try
+            let fn = Path.GetTempFileName()
+            use sw = new StreamWriter(fn)
+            sw.WriteLine "Hello, World!"
+            true
+        with _ ->
+            false
+
+let output = new OutputTarget()
+let methodCall = WriteMethod output.SendToFile
+if methodCall.Invoke() then
+    printfn "Success!"
+else
+    printfn "File write operation failed."
+
+// </Snippet1>

samples/snippets/fsharp/VS_Snippets_CLR_System/system.Func~1/fs/Example.fs

@@ -0,0 +1,45 @@
+module Example
+
+// <Snippet5>
+open System
+
+type LazyValue<'T>(func: Func<'T>) =
+    let mutable value = ValueNone
+
+    member _.Value =
+        match value with
+        | ValueSome v -> v
+        | ValueNone ->
+            // Execute the delegate.
+            let v = func.Invoke()
+            value <- ValueSome v
+            v
+
+let expensiveOne () =
+    printfn "\nExpensiveOne() is executing."
+    1
+
+let expensiveTwo (input: string) =
+    printfn "\nExpensiveTwo() is executing."
+    int64 input.Length
+
+// Note that each lambda expression has no parameters.
+let lazyOne = LazyValue(fun () -> expensiveOne ())
+let lazyTwo = LazyValue(fun () -> expensiveTwo "apple")
+
+printfn "LazyValue objects have been created."
+
+// Get the values of the LazyValue objects.
+printfn $"{lazyOne.Value}"
+printfn $"{lazyTwo.Value}"
+
+
+// The example produces the following output:
+//     LazyValue objects have been created.
+//
+//     ExpensiveOne() is executing.
+//     1
+//
+//     ExpensiveTwo() is executing.
+//     5
+// </Snippet5>

samples/snippets/fsharp/VS_Snippets_CLR_System/system.Func~1/fs/Func1.fs

@@ -0,0 +1,24 @@
+module Func1
+
+// <Snippet2>
+open System
+open System.IO
+
+type OutputTarget() =
+    member _.SendToFile() =
+        try
+            let fn = Path.GetTempFileName()
+            use sw = new StreamWriter(fn)
+            sw.WriteLine "Hello, World!"
+            true
+        with _ ->
+            false
+
+let output = OutputTarget()
+let methodCall = Func<bool> output.SendToFile
+if methodCall.Invoke() then
+    printfn "Success!"
+else
+    printfn "File write operation failed."
+
+// </Snippet2>

samples/snippets/fsharp/VS_Snippets_CLR_System/system.Func~1/fs/Lambda.fs

@@ -0,0 +1,23 @@
+module Lambda
+
+// <Snippet4>
+open System
+open System.IO
+
+type OutputTarget() =
+    member _.SendToFile() =
+        try
+            let fn = Path.GetTempFileName()
+            use sw = new StreamWriter(fn)
+            sw.WriteLine "Hello, World!"
+            true
+        with _ ->
+            false
+
+let output = OutputTarget()
+let methodCall = Func<bool>(fun () -> output.SendToFile())
+if methodCall.Invoke() then
+    printfn "Success!"
+else
+    printfn "File write operation failed."
+// </Snippet4>

samples/snippets/fsharp/VS_Snippets_CLR_System/system.Func~1/fs/fs.fsproj

@@ -0,0 +1,12 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="Delegate.fs" />
+    <Compile Include="Func1.fs" />
+    <Compile Include="Lambda.fs" />
+    <Compile Include="Example.fs" />
+  </ItemGroup>
+</Project>

xml/System/Func`1.xml

@@ -77,25 +77,28 @@
  You can use this delegate to represent a method that can be passed as a parameter without explicitly declaring a custom delegate. The encapsulated method must correspond to the method signature that is defined by this delegate. This means that the encapsulated method must have no parameters and must return a value.  
   
 > [!NOTE]
->  To reference a method that has no parameters and returns `void` (or in Visual Basic, that is declared as a `Sub` rather than as a `Function`), use the <xref:System.Action> delegate instead.  
+>  To reference a method that has no parameters and returns `void` (`unit`, in F#) (or in Visual Basic, that is declared as a `Sub` rather than as a `Function`), use the <xref:System.Action> delegate instead.  
   
  When you use the <xref:System.Func%601> delegate, you do not have to explicitly define a delegate that encapsulates a parameterless method. For example, the following code explicitly declares a delegate named `WriteMethod` and assigns a reference to the `OutputTarget.SendToFile` instance method to its delegate instance.  
   
  :::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/system.Func~1/cs/Delegate.cs" interactive="try-dotnet" id="Snippet1":::
+ :::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR_System/system.Func~1/fs/Delegate.fs" id="Snippet1":::
  :::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR_System/system.Func~1/vb/Delegate.vb" id="Snippet1":::
   
  The following example simplifies this code by instantiating the <xref:System.Func%601> delegate instead of explicitly defining a new delegate and assigning a named method to it.  
   
  :::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/system.Func~1/cs/Func1.cs" interactive="try-dotnet" id="Snippet2":::
+ :::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR_System/system.Func~1/fs/Func1.fs" id="Snippet2":::
  :::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR_System/system.Func~1/vb/Func1.vb" id="Snippet2":::
   
  You can use the <xref:System.Func%601> delegate with anonymous methods in C#, as the following example illustrates. (For an introduction to anonymous methods, see [Anonymous Methods](/dotnet/csharp/programming-guide/statements-expressions-operators/anonymous-methods).)  
   
  :::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/system.Func~1/cs/Anon.cs" interactive="try-dotnet" id="Snippet3":::
   
- You can also assign a lambda expression to a <xref:System.Func%602> delegate, as the following example illustrates. (For an introduction to lambda expressions, see [Lambda Expressions](/dotnet/visual-basic/programming-guide/language-features/procedures/lambda-expressions) and [Lambda Expressions](/dotnet/csharp/programming-guide/statements-expressions-operators/lambda-expressions).)  
+ You can also assign a lambda expression to a <xref:System.Func%602> delegate, as the following example illustrates. (For an introduction to lambda expressions, see [Lambda Expressions (VB)](/dotnet/visual-basic/programming-guide/language-features/procedures/lambda-expressions), [Lambda Expressions (C#)](/dotnet/csharp/programming-guide/statements-expressions-operators/lambda-expressions), and [Lambda Expressions (F#)](/dotnet/fsharp/language-reference/functions/lambda-expressions-the-fun-keyword/).)  
   
  :::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/system.Func~1/cs/Lambda.cs" interactive="try-dotnet" id="Snippet4":::
+ :::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR_System/system.Func~1/fs/Lambda.fs" id="Snippet4":::
  :::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR_System/system.Func~1/vb/Lambda.vb" id="Snippet4":::
   
  The underlying type of a lambda expression is one of the generic `Func` delegates. This makes it possible to pass a lambda expression as a parameter without explicitly assigning it to a delegate. In particular, because many methods of types in the <xref:System.Linq> namespace have `Func` parameters, you can pass these methods a lambda expression without explicitly instantiating a `Func` delegate.  
@@ -110,13 +113,16 @@
  The example creates two methods and instantiates two `LazyValue` objects with lambda expressions that call these methods. The lambda expressions do not take parameters because they just need to call a method. As the output shows, the two methods are executed only when the value of each `LazyValue` object is retrieved.  
   
  :::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/system.Func~1/cs/Example.cs" interactive="try-dotnet" id="Snippet5":::
+ :::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR_System/system.Func~1/fs/Example.fs" id="Snippet5":::
  :::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR_System/system.Func~1/vb/Example.vb" id="Snippet5":::
   
  ]]></format>
     </remarks>
     <related type="Article" href="/dotnet/csharp/programming-guide/statements-expressions-operators/lambda-expressions">Lambda Expressions (C# Programming Guide)</related>
+    <related type="Article" href="/dotnet/fsharp/language-reference/functions/lambda-expressions-the-fun-keyword">Lambda Expressions: The fun Keyword (F#)</related>
     <related type="Article" href="/dotnet/visual-basic/programming-guide/language-features/procedures/lambda-expressions">Lambda Expressions</related>
     <related type="Article" href="/dotnet/csharp/programming-guide/delegates/">Delegates (C# Programming Guide)</related>
+    <related type="Article" href="/dotnet/fsharp/language-reference/delegates/">Delegates (F#)</related>
     <related type="Article" href="/dotnet/visual-basic/programming-guide/language-features/delegates/">Delegates in Visual Basic</related>
   </Docs>
 </Type>