Of course GHCi correctly infers the type as: revFn :: [a] -> [a] -> c
Adding the base case:
revFn  xs = xs
Now gives an error;
“Equations for ‘revF’ have different numbers of arguments”
Of course this can be “fixed” by either adding the cancelled argument to the first clause, or converting the base case to only have one explicit argument, and a RHS of a lambda or identity function.
But since the interpreter already correctly inferred that the first clause has two arguments (with only one explicit), why does it then ignore this and give an error when the second clause shows two explicit arguments? The types are all correct in either case – why require explicit arguments?