I was trying to use some of the matrix multiplication utilities from Microsoft.FSharp.Math.Matrix. This is in the PowerPack DLL these days, not part of core, so you have to add the FSharp.Powerpack.dll file to the references for the project.
This all works fine, but the interactive process fsi.exe doesn’t inherit the same settings. Attempting to use matrix code from within there just gives the following error message:
error FS0039: The namespace 'Matrix' is not defined A construct with this name was found in FSharp.PowerPack.dll, which contains some modules and types that were implicitly referenced in some previous versions of F#. You may need to add an explicit reference to this DLL in order to compile this code.
This isn’t the most helpful message, since I’ve already added the DLL but don’t know how to expose it to fsi.exe. It turns out that you can reference the DLL as you would from F# code, at the F# interactive prompt:
> #r "FSharp.Powerpack.dll";;