I just checked, and lining up the | with the open parentheses does work in modern F#. I believe they revamped the indentation rules a few years back, because I definitely remember things like that not working back when. So perhaps now your idea would be feasible. However, there was definitely a time in the past when F#'s indentation rules would have been problematic for tabs, and that fact is why the F# compiler still forbids tab characters outside of string literals or comments. (And there are still indentation styles recommended, though not required, by the official F# style guidelines that would require the use of spaces. Which is another reason to avoid tabs in F# code, because they're less flexible than spaces. Usually the reason to prefer tabs is because they're more flexible, but the slightly-peculiar nature of F#'s indentation rules means tabs are less flexible in F# code.)