In an actual program, it would be able to apply defaulting rules and monomorphize `b0` to Integer. The somewhat unusual ghci environment, in particular how it tries to figure out whether to evaluate an expression and `print` the result or instead run an IO action --- coupled with the monomorphism restriction being disabled in the interactive context in ghci 7.8 and later --- is working against you here. As a result, you need to specify a result type when invoking `main1`, which is what the error message is telling you.