Comment made by: dmiller
spit with a string argument defaults to opening a FileStream (wrapped in a StreamWriter) in mode FileMode.CreateOrNew. This will work whether or not the file exists. To default to Truncate will fail if the file does not exist. For this reason, I don't want to make it the default. I'm also not sure how to play with tests of file existence versus given file-mode to determine what action to take.
If you know the file exists and want to truncate, call spit with that file mode.
(spit "filename" "test" :file-mode System.IO.FileMode/Truncate)