There are two differences between the formal and informal approaches. First, they look different. If the abstraction is a simple one, as this one is, then it is probably easier to read an expression that has the loop explicitly written out than to read one that calls i n t e r a c t i ve - i n terpreter, since that requires finding the definition of i n t e r a c t i v e - i n t e r p r e t e r and understanding it as well.