fizzbuzz — the classic interview question

Print 1..30 with multiples of 3 → Fizz, multiples of 5 → Buzz, multiples of 15 → FizzBuzz. Rail lacks a mod builtin, so the example writes its own using integer division.

Source (examples/fizzbuzz.rail):

-- fizzbuzz.rail — the classic interview question
--
-- Demonstrates: recursion, if/then/else, modular arithmetic, string output

mod3 n = n - ((n / 3) * 3)
mod5 n = n - ((n / 5) * 5)

fizzbuzz n =
  if mod3 n == 0 then
    if mod5 n == 0 then "FizzBuzz"
    else "Fizz"
  else if mod5 n == 0 then "Buzz"
  else show n

go i max =
  if i > max then 0
  else let _ = print (fizzbuzz i)
       go (i + 1) max

main =
  let _ = go 1 30
  0

Run:

./rail_native run examples/fizzbuzz.rail

Output:

1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz
16
17
Fizz
19
Buzz
Fizz
22
23
Fizz
Buzz
26
Fizz
28
29
FizzBuzz

The go function is the recursive loop. Because the recursive call is in tail position, Rail's TCO turns it into a tight loop (no stack growth).