Bad behavior when running ormolu on a file and piping to that file itself

[regr4]

I can't have been the only one who (missing the -i flag on the help page) has tried ormolu FILE.hs > FILE.hs to format a file in place. However, when you do that, ormolu overwrites the file with nothing.

I'm not sure if it's possible to detect this situation, but if so, it would be nice to warn the user and somehow avoid destroying the contents (e.g. make a backup to /tmp/ or dump the original contents of the file to stderr).

To reproduce:

echo "module Example where" > Example.hs
ormolu Example.hs > Example.hs
cat Example.hs # empty

[avdv]

Note that it is not ormolu writing an empty file, but your shell that sets up the redirection and truncating the file before calling ormolu.

Maybe if ormolu sees an empty file it could warn about that situation and suggest to use the -i flag? But it definitely is not possible to avoid truncating the file, since that is what the > shell redirection already does before ormolu is able to access it.

BTW, there's the sponge tool (from moreutils) which could be used exactly for this purpose: ormolu Example.hs | sponge Example.hs.