In using LanguageExt, I am trying to display just the error message of a failed validation from using Validation<Error,Unit> in LanguageExt.

However, I get the following string:

ValidationData(Fail, 0, [Starting number is not a valid number])

I can't seem to find a way to get just the error message which is

Starting number is not a valid number

Can anyone provide any advice on how to do this?

Here's a snippet of the code

ValidateStartingNumberError(int number)
    .Match(
        () => None,
        error => DisplayError(error.ToString())
    );

private Validation<Error, Unit> ValidateStartingNumberError(int number)
    => int.TryParse(this.number, out var parsedNumber) ?
         unit :
         Fail<Error,Unit>("Starting number is not a valid number");

private void DisplayError(string message)=> MessageBox.Show(message);

In using LanguageExt, I am trying to display just the error message of a failed validation from using Validation<Error,Unit> in LanguageExt.

However, I get the following string:

ValidationData(Fail, 0, [Starting number is not a valid number])

I can't seem to find a way to get just the error message which is

Starting number is not a valid number

Can anyone provide any advice on how to do this?

Here's a snippet of the code

ValidateStartingNumberError(int number)
    .Match(
        () => None,
        error => DisplayError(error.ToString())
    );

private Validation<Error, Unit> ValidateStartingNumberError(int number)
    => int.TryParse(this.number, out var parsedNumber) ?
         unit :
         Fail<Error,Unit>("Starting number is not a valid number");

private void DisplayError(string message)=> MessageBox.Show(message);

• c#
• error-handling
• functional-programming
• language-ext
• language-extension

• What is DisplayError and what is Fail<Error,Unit>? – shingo Commented Nov 20, 2024 at 9:39
• Sample code for DisplayError included. Fail<Error,Unit> is a class part of Language-Ext library – Jeremy Loh Commented Nov 20, 2024 at 11:22
• How can you call a class? Or did you miss the new keyword? – shingo Commented Nov 20, 2024 at 11:31

2 Answers 2

Looking at the documentation I think that you need the Message property:

ValidateStartingNumberError(int number)
    .Match(
        () => None,
        error => DisplayError(error.Message)
    );

Found the reason: I implemented the wrong .Match(...). It should be

ValidateStartingNumberError(int number)
    .Match(
        _ => None,
        error => DisplayError(error.Head.Message)
    );