Thomas Davie wrote:
But, as there is only one value in the Unit type, all values we have no information about must surely be that value
The flaw in your logic is your assumption that the Unit type has only one value. Consider bottom :: () bottom = undefined Oviously, bottom is not (), but its type, nonetheless, is Unit. Unit actually has both () and _|_. More generally, _|_ inhabits every Haskell type, even types with no constructors (which itself requires a GHC extension, of course): data Empty bottom' :: Empty bottom' = undefined If you only ever use total functions then you can get away with not accounting for _|_. Perhaps ironically a function that doesn't account for _|_ may be viewed philosophically as a partial function since its contract doesn't accommodate all possible values. - Jake