For me, the alternative solution makes much more sense for a language with significant indentation, i.e. to use only tabs for indentation and to completely prohibit spaces before the first printable character of a line.
I do not know why I have never seen this solution in practice.
It depends on the language's indentation requirements. That can work in Python, where you never need to line things up with characters above that aren't a multiple of an indent level. But in F#, at least a few years ago when I tried it, there are times when you would need (or strongly want) to line things up with other characters. For example, this example from https://learn.microsoft.com/en-us/dotnet/fsharp/style-guide/...:
Note how the | needs one extra space in order to line up with the word "function", due to the open parenthesis. If you're using tabs for indentation, that means you need a series of tabs plus one trailing space after the tabs, in order to line up the | correctly.Any situation where you must have a mixture of tabs and spaces for code to work right leads to a nightmare. Because it makes you have to turn on visible whitespace in your editor and peer closely at the lines. I have a simple rule: "Do NOT make me care whether there are tabs or spaces in this file!" If it's significant whether a line is indented with a tab or space, then that rule is broken, and you're probably in for a bad time. (Case in point: Makefile syntax).
I did not take this in consideration, because for such code, with brackets and separators, I prefer to align the separators with the opening and closing brackets, like this:
orI 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.)