In most .NET project I can use folder to organise the code files. In C++, I can't, but filters end up playing the same role. However, in F# with Visual Studio 2010, I can't. Every code file is shown directly in the project dir. Why is this feature not available?
And what is the optimal strategy for organizing a project with a lot of files?
Actually, you can add folders to F# projects but it's not supported directly through Visual Studio (you have to edit the project file yourself): http://fsprojectextender.codeplex.com/ (edit: old link was broken, updated to F# Project Extender home page which has links to the original blog posts which were moved) (which I found in this answer).
I do this myself, but it is cumbersome and you end up avoiding it until keeping sanity really demands it. I think the feature simply slipped, or perhaps there wasn't as much a culture for folder organization with the F# designers in the first place. You can see in the F# source code that they favor huge source files with no directories, with separate projects as an organization boundary.
I imagine the F# project template could be modified to support this, and it is certainly something I'd like to see happen. At the same time the linear compilation order F# enforces causes your code to be somewhat self-organized, and so folder grouping plays a less significant role.
Manually editing the .fsproj file as described in Stephen's answer is one option (and I used it when I wanted to organize one larger project).
However, you have to be a bit careful and I think you cannot add new files to the folders (creating a file by hand and then adding an existing file works). However, if you like to keep things organized (like I do), then it should work for you.
Additionally, there is also a tool called F# Project Extender that should make things a bit easier for you . I have not tried it yet, but it looks like it supports adding folders (and perhaps other useful things). See for example this blog post by the project author.
