Bolero is a functional, reactive web library built on top of Blazor for F# developers. It combines Elm's Model-View-Update architecture (MVU) via Elmish with powerful features pioneered by WebSharper, such as templating and easy remoting, to give developers a sound, effective and highly productive way to structure and implement full-stack .NET web applications that run on WebAssembly.

The Bolero homepage starts off with a video that shows you how to create your first client-server app with three simple commands:

Assuming you executed these commands in a simple shell (say, cmd on Windows), any issue hindering a successful run will show the following unceremonious output:

C:\fsbolero>cd MyApp3 && dotnet run -p src/MyApp3.Server
C:\fsbolero\MyApp3\src\MyApp3.Client\Main.fs(233,12): warning FS0044: This construct is deprecated. Use withHotReload instead [C:\fsbolero\MyApp3\src\MyApp3.Client\MyApp3.Client.fsproj]
C:\Users\Adam\.nuget\packages\microsoft.aspnetcore.blazor.build\3.0.0-preview7.19365.7\targets\Blazor.MonoRuntime.targets(633,5): error MSB3073: The command "dotnet "C:\Users\Adam\.nuget\packages\microsoft.aspnetcore.blazor.build\3.0.0-preview7.19365.7\targets\../tools/Microsoft.AspNetCore.Blazor.Build.dll" write-boot-json "obj\Debug\netstandard2.0\MyApp3.Client.dll" --references "C:\fsbolero\MyApp3\src\MyApp3.Client\obj\Debug\netstandard2.0\blazor\bootjson-references.txt" --embedded-resources "C:\fsbolero\MyApp3\src\MyApp3.Client\obj\Debug\netstandard2.0\blazor\embedded.resources.txt" --linker-enabled --output "C:\fsbolero\MyApp3\src\MyApp3.Client\obj\Debug\netstandard2.0\blazor\blazor.boot.json"" exited with code -2147450730. [C:\fsbolero\MyApp3\src\MyApp3.Client\MyApp3.Client.fsproj]

The build failed. Please fix the build errors and run again.

When this happens, and even in general, it's a good idea to add the -v n suffix to that last command to enable normal build output (or other debug levels as you need.)

For instance, now on an older machine I pulled out from the closet, I get the following more informative output:

C:\fsbolero\MyApp3>dotnet run -p src/MyApp3.Server -v n
Build started 9/3/2019 3:04:52 PM.
     1>Project "C:\fsbolero\MyApp3\src\MyApp3.Server\MyApp3.Server.fsproj" on node 1 (Restore target(s)).
     1>Restore:
         Committing restore...
         Committing restore...
         Assets file has not changed. Skipping assets file writing. Path: C:\fsbolero\MyApp3\src\MyApp3.Client\obj\project.assets.json
         Assets file has not changed. Skipping assets file writing. Path: C:\fsbolero\MyApp3\src\MyApp3.Server\obj\project.assets.json
         Restore completed in 46.92 ms for C:\fsbolero\MyApp3\src\MyApp3.Server\MyApp3.Server.fsproj.
         Restore completed in 46.92 ms for C:\fsbolero\MyApp3\src\MyApp3.Client\MyApp3.Client.fsproj.

         NuGet Config files used:
             C:\Users\Adam\AppData\Roaming\NuGet\NuGet.Config
             C:\Program Files (x86)\NuGet\Config\Microsoft.VisualStudio.Offline.config

         Feeds used:
             https://api.nuget.org/v3/index.json
             https://daily.websharper.com/nuget
             C:\Program Files (x86)\Microsoft SDKs\NuGetPackages\
             https://dotnetfeed.blob.core.windows.net/aspnet-aspnetcore-tooling/index.json
             https://dotnetfeed.blob.core.windows.net/aspnet-aspnetcore/index.json
             https://dotnetfeed.blob.core.windows.net/aspnet-entityframeworkcore/index.json
             https://dotnetfeed.blob.core.windows.net/aspnet-extensions/index.json
             https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json
             https://dotnetfeed.blob.core.windows.net/dotnet-windowsdesktop/index.json
     1>Done Building Project "C:\fsbolero\MyApp3\src\MyApp3.Server\MyApp3.Server.fsproj" (Restore target(s)).
   1:7>Project "C:\fsbolero\MyApp3\src\MyApp3.Server\MyApp3.Server.fsproj" on node 1 (default targets).
     1>_CheckForNETCoreSdkIsPreview:
       C:\Program Files\dotnet\sdk\3.0.100-preview5-011568\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.RuntimeIdentifierInference.targets(157,5): message NETSDK1057: You are using a preview version of .NET Core. See: https://aka.ms/dotnet-core-preview [C:\fsbolero\MyApp3\src\MyApp3.Server\MyApp3.Server.fsproj]
   1:7>Project "C:\fsbolero\MyApp3\src\MyApp3.Server\MyApp3.Server.fsproj" (1:7) is building "C:\fsbolero\MyApp3\src\MyApp3.Client\MyApp3.Client.fsproj" (2:6) on node 1 (default targets).
     2>_HandlePackageFileConflicts:
       C:\Program Files\dotnet\sdk\3.0.100-preview5-011568\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.ConflictResolution.targets(39,5): message NETSDK1041: Encountered conflict between 'Reference:C:\Users\Adam\.nuget\packages\netstandard.library\2.0.3\build\netstandard2.0\ref\System.Net.Http.dll' and 'Reference:C:\Users\Adam\.nuget\packages\system.net.http\4.3.2\ref\netstandard1.3\System.Net.Http.dll'.  NETSDK1033: Choosing 'Reference:C:\Users\Adam\.nuget\packages\netstandard.library\2.0.3\build\netstandard2.0\ref\System.Net.Http.dll' because AssemblyVersion '4.1.2.0' is greater than '4.1.1.1'. [C:\fsbolero\MyApp3\src\MyApp3.Client\MyApp3.Client.fsproj]
       GenerateTargetFrameworkMonikerAttribute:
       Skipping target "GenerateTargetFrameworkMonikerAttribute" because all output files are up-to-date with respect to the input files.
       CoreGenerateAssemblyInfo:
       Skipping target "CoreGenerateAssemblyInfo" because all output files are up-to-date with respect to the input files.
       CoreCompile:
       Skipping target "CoreCompile" because all output files are up-to-date with respect to the input files.
       _CopyFilesMarkedCopyLocal:
         Touching "C:\fsbolero\MyApp3\src\MyApp3.Client\obj\Debug\netstandard2.0\MyApp3.Client.fsproj.CopyComplete".
       _CopyOutOfDateSourceItemsToOutputDirectory:
       Skipping target "_CopyOutOfDateSourceItemsToOutputDirectory" because all output files are up-to-date with respect to the input files.
       CopyFilesToOutputDirectory:
         MyApp3.Client -> C:\fsbolero\MyApp3\src\MyApp3.Client\bin\Debug\netstandard2.0\MyApp3.Client.dll
       _GenerateLinkerDescriptor:
       Skipping target "_GenerateLinkerDescriptor" because all output files are up-to-date with respect to the input files.
       _LinkBlazorApplication:
       Skipping target "_LinkBlazorApplication" because all output files are up-to-date with respect to the input files.
       _GenerateBlazorBootJson:
         dotnet "C:\Users\Adam\.nuget\packages\microsoft.aspnetcore.blazor.build\3.0.0-preview7.19365.7\targets\../tools/Microsoft.AspNetCore.Blazor.Build.dll" write-boot-json "obj\Debug\netstandard2.0\MyApp3.Client.dll" --references "C:\fsbolero\MyApp3\src\MyApp3.Client\obj\Debug\netstandard2.0\blazor\bootjson-references.txt" --embedded-resources "C:\fsbolero\MyApp3\src\MyApp3.Client\obj\Debug\netstandard2.0\blazor\embedded.resources.txt" --linker-enabled --output "C:\fsbolero\MyApp3\src\MyApp3.Client\obj\Debug\netstandard2.0\blazor\blazor.boot.json"
         It was not possible to find any compatible framework version
         The specified framework 'Microsoft.NETCore.App', version '3.0.0-preview7-27912-14' was not found.
           - The following frameworks were found:
               2.0.6 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
               2.0.9 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
               2.1.2 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
               2.1.4 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
               2.1.9 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
               2.2.3 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
               3.0.0-preview5-27626-15 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

         You can resolve the problem by installing the specified framework and/or SDK.

         The .NET Core frameworks can be found at:
           - https://aka.ms/dotnet-download
     2>C:\Users\Adam\.nuget\packages\microsoft.aspnetcore.blazor.build\3.0.0-preview7.19365.7\targets\Blazor.MonoRuntime.targets(633,5): error MSB3073: The command "dotnet "C:\Users\Adam\.nuget\packages\microsoft.aspnetcore.blazor.build\3.0.0-preview7.19365.7\targets\../tools/Microsoft.AspNetCore.Blazor.Build.dll" write-boot-json "obj\Debug\netstandard2.0\MyApp3.Client.dll" --references "C:\fsbolero\MyApp3\src\MyApp3.Client\obj\Debug\netstandard2.0\blazor\bootjson-references.txt" --embedded-resources "C:\fsbolero\MyApp3\src\MyApp3.Client\obj\Debug\netstandard2.0\blazor\embedded.resources.txt" --linker-enabled --output "C:\fsbolero\MyApp3\src\MyApp3.Client\obj\Debug\netstandard2.0\blazor\blazor.boot.json"" exited with code -2147450730. [C:\fsbolero\MyApp3\src\MyApp3.Client\MyApp3.Client.fsproj]
     2>Done Building Project "C:\fsbolero\MyApp3\src\MyApp3.Client\MyApp3.Client.fsproj" (default targets) -- FAILED.
     1>Done Building Project "C:\fsbolero\MyApp3\src\MyApp3.Server\MyApp3.Server.fsproj" (default targets) -- FAILED.

Build FAILED.

       "C:\fsbolero\MyApp3\src\MyApp3.Server\MyApp3.Server.fsproj" (default target) (1:7) ->
       "C:\fsbolero\MyApp3\src\MyApp3.Client\MyApp3.Client.fsproj" (default target) (2:6) ->
       (_GenerateBlazorBootJson target) ->
         C:\Users\Adam\.nuget\packages\microsoft.aspnetcore.blazor.build\3.0.0-preview7.19365.7\targets\Blazor.MonoRuntime.targets(633,5): error MSB3073: The command "dotnet "C:\Users\Adam\.nuget\packages\microsoft.aspnetcore.blazor.build\3.0.0-preview7.19365.7\targets\../tools/Microsoft.AspNetCore.Blazor.Build.dll" write-boot-json "obj\Debug\netstandard2.0\MyApp3.Client.dll" --references "C:\fsbolero\MyApp3\src\MyApp3.Client\obj\Debug\netstandard2.0\blazor\bootjson-references.txt" --embedded-resources "C:\fsbolero\MyApp3\src\MyApp3.Client\obj\Debug\netstandard2.0\blazor\embedded.resources.txt" --linker-enabled --output "C:\fsbolero\MyApp3\src\MyApp3.Client\obj\Debug\netstandard2.0\blazor\blazor.boot.json"" exited with code -2147450730. [C:\fsbolero\MyApp3\src\MyApp3.Client\MyApp3.Client.fsproj]

    0 Warning(s)
    1 Error(s)

Time Elapsed 00:00:02.07

The build failed. Please fix the build errors and run again.

Installing the right .NET Core SDK

Tucked away in the source repository, in the main README file, there is a short section that details what you need to compile and run Bolero itself:

.NET Core SDK 3.0-preview7 or newer. Download it here.

Not surprisingly, this same dependency applies to your Bolero applications as well. The normal build output we saw above already listed my installed frameworks, but if I need to, I can also quickly check separately:

C:\fsbolero>dotnet --info
.NET Core SDK (reflecting any global.json):
 Version:   3.0.100-preview5-011568
 Commit:    b487ff10aa

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.17134
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\3.0.100-preview5-011568\

Host (useful for support):
  Version: 3.0.0-preview5-27626-15
  Commit:  61f30f5a23

.NET Core SDKs installed:
  2.1.104 [C:\Program Files\dotnet\sdk]
  2.1.202 [C:\Program Files\dotnet\sdk]
  2.1.302 [C:\Program Files\dotnet\sdk]
  2.1.402 [C:\Program Files\dotnet\sdk]
  2.1.505 [C:\Program Files\dotnet\sdk]
  2.1.602 [C:\Program Files\dotnet\sdk]
  2.1.700-preview-009597 [C:\Program Files\dotnet\sdk]
  2.1.700-preview-009601 [C:\Program Files\dotnet\sdk]
  2.2.202 [C:\Program Files\dotnet\sdk]
  2.2.300-preview-010046 [C:\Program Files\dotnet\sdk]
  2.2.300-preview-010050 [C:\Program Files\dotnet\sdk]
  3.0.100-preview5-011568 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  ...
  ...

Heading over to the Microsoft .NET Core download page and installing preview7 resolves the issue, and the vanilla Bolero template app now works as expected.

Happy coding!