CODEWITHSUNDEEP
f#monadssyntactic-sugarcomputation-expression
Chechy Levas
Chechy Levas

Reputation: 2312

Why can't I use match! in a computation expression yet?

Starting with F# 4.5, the match! keyword allows you to inline a call to another computation expression and pattern match on its result

As far as I can tell, I am using F# 4.5.

screen shot of project settings

However, VS2017 does not like the following code

let someAsyncIntOption = async{
    return Some 1
}

let someAsyncString = async{
    match! someAsyncIntOption with
    |Some 1 -> return "Some one"
    |_ -> return "None"
}

match! is not recognized as valid syntax.

What do I need to do to make this work?

Upvotes: 1

Views: 107

Answers (2)

just-joel
just-joel

Reputation: 61

According to...

https://blogs.msdn.microsoft.com/dotnet/2018/07/26/announcing-f-4-5-preview/

If you haven't already...

First, install:

  • A preview of the .NET SDK 2.1.400 to ensure you have the latest bits for for F# in .NET Core
  • Visual Studio 2017 update 15.8 Preview 5 if you are on Windows

If you create a .NET desktop F# project in Visual Studio (from the F# desktop development component), then you will need to update your FSharp.Core package to 4.5.2 with the NuGet UI.

Once you have installed the necessary bits, you can start developing F# 4.5 code with Visual Studio, Visual Studio for Mac, or Visual Studio Code with Ionide.

When .NET Core SDK 2.1.400 and Visual Studio 2017 version 15.8 are released, the referenced FSharp.Core will be 4.5.2 for all new projects and you will not need to perform this second step.

It may be that you need to be targeting .NET Core for this to work and only after updating your FSharp.Core package to 4.5.2.

I haven't tried any of this, but hope that helps.

Upvotes: 1

rmunn
rmunn

Reputation: 36738

F# 4.5 was released in VS 2017 v15.8 preview 5. It's not enough to target the F# 4.5 runtime; you also have to have the F# 4.5 compiler in order for that syntax to be accepted. And the F# 4.5 compiler comes with VS 15.8 preview 5; if you're running a Visual Studio version older than 15.8 preview 5, then you have an older F# compiler that doesn't know about the match! syntax.

Upvotes: 2

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