Filtering exceptions on exception messages in F#

Here’s an example of pattern matching in a with clause in F#:

let testDotNetExceptions =
    try
        raise (new System.IndexOutOfRangeException("Your list is not large enough"))
    with
        | ๐Ÿ˜• System.IndexOutOfRangeException as ex ->
            printfn "Out of range: %s" ex.Message
            0
        | ๐Ÿ˜• System.Exception as ex ->
            printfn "Something bad has happened: %s" ex.Message
            0

If you don’t understand the above code then check out this post for details.

In F# we fine tune pattern matching with the “when” keyword within a with clause. “when” must be followed by a boolean expression. If it returns true then the exception will be handled in that block otherwise the pattern matching will continue. Here’s an example:

let testDotNetExceptions =
    try
        raise (new System.InsufficientMemoryException("You should increase your RAM"))
    with
        | ๐Ÿ˜• System.IndexOutOfRangeException as ex ->
            printfn "Out of range: %s" ex.Message
            0
        | ๐Ÿ˜• System.Exception as ex when ex.Message.IndexOf("RAM") > -1 ->
            printfn "Something RAM related has happened: %s" ex.Message
            0
        | ๐Ÿ˜• System.Exception as ex ->
            printfn "Something bad has happened: %s" ex.Message
            0

Check how “when” is testing for a piece of string to be present in the exception message. If you run the above code then the exception will be handled in the second pattern matching section, i.e. where the when clause evaluates to true.

View all F# related articles here.

Advertisements

About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

ultimatemindsettoday

A great WordPress.com site

Elliot Balynn's Blog

A directory of wonderful thoughts

Robin Sedlaczek's Blog

Developer on Microsoft Technologies

HarsH ReaLiTy

A Good Blog is Hard to Find

Softwarearchitektur in der Praxis

Wissenswertes zu Webentwicklung, Domain-Driven Design und Microservices

the software architecture

thoughts, ideas, diagrams,enterprise code, design pattern , solution designs

Technology Talks

on Microsoft technologies, Web, Android and others

Software Engineering

Web development

Disparate Opinions

Various tidbits

chsakell's Blog

Anything around ASP.NET MVC,WEB API, WCF, Entity Framework & AngularJS

Cyber Matters

Bite-size insight on Cyber Security for the not too technical.

Guru N Guns's

OneSolution To dOTnET.

Johnny Zraiby

Measuring programming progress by lines of code is like measuring aircraft building progress by weight.

%d bloggers like this: