Prelude> price 9.5
33.25
Prelude> price 10.5

Program error: {price {dict} {dict} 10.5}


Prelude> price 11.5
39.5
Prelude>


For arguments 9.5 and 11.5 is the result the same. For 10.5 no guard evaluates to True, and ghci gives an (unfortunately quite cryptic) error message (maybe similar to the one shown)