For F# mode in emacs, one need the following line in .emacs. The problem is that because of space between file path, C-c C-s doesn't work.
(setq fsharp-compiler "C:/Program Files/Microsoft F#/v4.0/fsc.exe")
I had to copy the F# compilers to the directory that doesn't have space in its file path.
(setq fsharp-compiler "C:/app/fsharp/fsc.exe")
How to run a process that has a space in its path name with emacs?