用幺半群实现 FizzBuzz：一种优雅的重构思路Fizz Buzz Through Monoids

We have previously seen the guard-sequence pattern which sits at the core of this implementation. The expression

In[3]:

"fizz" <$ guard (rem i 3 == 0)

will evaluate to Just "fizz" whenever i is divisible by three, and in all other cases it evaluates to Nothing. Thus, if a number is not divisible by any of 3, 5, or 7, the list will evaluate to

In[4]:

[Nothing, Nothing, Nothing]

If a number is divisible by only 5, but not 3 or 7, the list will be

In[5]:

[Nothing, Just "buzz", Nothing]

And if a number is divisible by, say, 3 and 7, but not 5, the list will be

In[6]:

[Just "fizz", Nothing, Just "zork"]

These are smushed together by mconcat from the Monoid interface, which applies the generic smushing operation <> over the list. This operation behaves as expected for our strings-that-might-not-exist: it concatenates them together if they do exist, otherwise it returns Nothing.

Whatever we get out of mconcat, we pass it to fromMaybe (show i), which replaces any Nothing values with the string representation of the number coming into the function, but passes through any actual values it receives intact. That is the full fizzbuzz function that converts a number to the correct textual representation.

To make it an actual program, we loop through all numbers [1..100], convert them with fizzbuzz, and print the result.