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Fibonacci

A classic recursive Fibonacci function showing JAPL's expression-oriented design, where if/else blocks return values directly.

Source Code

fn fib(n: Int) -> Int {
  if n <= 1 { n }
  else { fib(n - 1) + fib(n - 2) }
}

fn main() {
  println(show(fib(0)))
  println(show(fib(1)))
  println(show(fib(5)))
  println(show(fib(10)))
}

What This Demonstrates

• Expression-oriented if/else -- The if/else block returns a value directly, no return keyword needed.
• Type annotations -- Function signatures declare parameter and return types with name: Type syntax.
• Recursive functions -- Functions can call themselves naturally; the compiler verifies the types at every call site.
• Built-in show -- Converts integers to strings for printing.

Line-by-Line Breakdown

fn fib(n: Int) -> Int

Declares a function named fib that takes an Int and returns an Int. The return type is explicit.

if n <= 1 { n }

Base case: when n is 0 or 1, the function returns n itself. The block { n } is an expression that evaluates to n.

else { fib(n - 1) + fib(n - 2) }

Recursive case: computes the sum of the two preceding Fibonacci numbers. Both branches must return the same type (Int).

println(show(fib(10)))

Calls fib(10), converts the result to a string with show, and prints it. Result: 55.

Try Modifying

• Add memoization by passing an accumulator: write fib_acc(n, a, b) with tail recursion.
• Change the base case to return a different starting sequence (e.g., Lucas numbers: 2, 1, 3, 4, 7...).
• Add a fib_list function that builds a list of the first N Fibonacci numbers.