But you would never define it like this in Haskell...
You'd probably do something like:
let n - m = ( n + (-m) ) `mod` 6
'let 3 - 4 = 5' is just a non-sense example.
For much the same reason, one would probably never want to write "let 1 = 1", but it's still a valid statement. It may not be interesting, but it's not nonsense.