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Why printing None prints

When printing None via printfn (or any of the other print functions for that matter), the result is . This example shows why.

Printing None results in <null>

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None |> printfn "Value: %A"
// Value: <null>

Custom implementation of Option<'T> - Printing None results in None

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type MyOption<'T> =
| Some of 'T
| None

MyOption.None |> printfn "Value: %A"
// Value: None

Revised MyOption<'T> - Printing None results in <null>

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// Here, MyOption2<'T> is decorated with CompilationRepresentationAttribute and
// the UseNullAsTrueValue flag is set just like it is for Option<'T> in the F#
// library. This is how None is represented in the compiled code.
// See http://bit.ly/1cA0QfS for more info about CompilationRepresentation
[<CompilationRepresentation(CompilationRepresentationFlags.UseNullAsTrueValue)>]
type MyOption2<'T> =
| Some of 'T
| None

MyOption2.None |> printfn "Value: %A"
// Value: <null>

union case Option.None: Option<'T>

val printfn : format:Printf.TextWriterFormat<'T> -> 'T

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.printfn

type MyOption<'T> =
| Some of 'T
| None

Full name: Script.MyOption<_>

union case MyOption.Some: 'T -> MyOption<'T>

union case MyOption.None: MyOption<'T>

Multiple items
type CompilationRepresentationAttribute =
  inherit Attribute
  new : flags:CompilationRepresentationFlags -> CompilationRepresentationAttribute
  member Flags : CompilationRepresentationFlags

Full name: Microsoft.FSharp.Core.CompilationRepresentationAttribute

--------------------
new : flags:CompilationRepresentationFlags -> CompilationRepresentationAttribute

CompilationRepresentationFlags.UseNullAsTrueValue: CompilationRepresentationFlags = 8

type MyOption2<'T> =
| Some of 'T
| None

Full name: Script.MyOption2<_>

union case MyOption2.Some: 'T -> MyOption2<'T>

union case MyOption2.None: MyOption2<'T>

Raw view Test code New version

More information

Link:	http://fssnip.net/kL
Posted:	10 years ago
Author:	Dave Fancher
Tags:	option , compilationrepresentationattribute

Why printing None prints

Printing None results in &lt;null&gt;

Custom implementation of Option&lt;'T&gt; - Printing None results in None

Revised MyOption&lt;'T&gt; - Printing None results in &lt;null&gt;

More information

Printing None results in <null>

Custom implementation of Option<'T> - Printing None results in None

Revised MyOption<'T> - Printing None results in <null>