REVISED: Sunday, September 14, 2025
1. OCAML FUNCTIONS
The OCaml Fibonacci Sequence script shown in the previous tutorial is logically divided into three main functions and an entry point.
A. let rec fib n = ...
This is the core computational function of the script. Its purpose is to calculate the Fibonacci number for a given integer index n.
let rec fib n: This declares a function named fib that takes a single argument, n. The rec keyword is crucial; it signifies that this is a recursive function, meaning it is allowed to call itself within its own definition.
match n with: This is OCaml's powerful pattern-matching construct. It inspects the value of n and executes the code corresponding to the first pattern it matches.
| 0 -> 0: This is the first "base case" of the recursion. If n is 0, the function immediately returns 0.
| 1 -> 1: This is the second base case. If n is 1, it returns 1. These base cases are essential to prevent the function from calling itself infinitely.
| _ -> fib (n - 1) + fib (n - 2): This is the "recursive step". The underscore _ is a wildcard that matches any value not caught by the preceding patterns (i.e., any integer greater than 1). In this case, the function calls itself twice: once with n - 1 and once with n - 2, and returns the sum of their results. This perfectly models the definition of the Fibonacci sequence.
B. let print_fib_sequence start_num end_num = ...
This function handles the logic for displaying a segment of the Fibonacci sequence to the user.
let print_fib_sequence start_num end_num: This defines a function that takes two arguments, start_num and end_num, which represent the desired range.
Input Validation: The first two if/else if statements act as guards. They check for invalid user input, such as negative numbers or a starting number that is larger than the ending number, and print an appropriate error message if found.
for i = start_num to end_num do ... done: If the input is valid, the function uses a standard for loop. This is an imperative-style feature in OCaml used here for its simplicity in iterating through a range of numbers. The loop variable i will take on every integer value from start_num to end_num, inclusive.
let result = fib i in: Inside the loop, for each number i, it calls the fib function we just discussed to get the corresponding Fibonacci number and stores it in a variable called result.
print_endline ("Fib...": This line constructs the output string. It uses the string_of_int function to convert the integers i and result into strings, and the ^ operator to concatenate them into a user-friendly format (e.g., "Fib(5) = 5"). Finally, print_endline prints this string to the console, automatically adding a newline at the end.
C. let rec main_loop () = ...
This function creates the interactive command-line interface that the user interacts with. It's a recursive loop that keeps running until the user decides to exit.
let rec main_loop (): This defines a recursive function that takes () (called "unit") as an argument. Unit is a special type in OCaml that indicates the absence of a meaningful value, which is common for functions that exist primarily for their side effects, like printing to or reading from the console.
Prompting the User: print_string displays the prompt, and flush stdout ensures that the prompt is immediately visible on the screen rather than being held in an output buffer.
Reading Input: let input = read_line () in pauses the program and waits for the user to type a line of text and press Enter. The entered text is stored in the input variable.
match input with: Pattern matching is used again to decide what to do with the user's input.
| "exit" -> print_endline "Goodbye!": This is the base case for this recursive loop. If the user types exactly "exit", a goodbye message is printed, and the function simply finishes without calling itself again. This breaks the chain of recursive calls and ends the program.
| _ -> ...: If the input is anything else, this block is executed.
Error Handling (try...with): Because parsing user input can fail (e.g., if they type "hello"), the parsing logic is wrapped in a try...with block.
Scanf.sscanf input "%d %d" print_fib_sequence: This is the "happy path". Scanf.sscanf is a versatile function that attempts to parse the input string according to a format string. Here, "%d %d" tells it to look for two integers separated by a space. If it successfully finds them, it passes those two integers directly as arguments to the print_fib_sequence function, which then runs.
with _ -> print_endline "...": If Scanf.sscanf fails to match the pattern, it raises an exception. The with _ block catches any exception and prints a helpful error message.
main_loop (): This is the recursive call. After either printing the sequence or handling an error, main_loop calls itself to repeat the entire process: prompt the user, read the input, and process it.
D. let () = ... (The Entry Point)
This is not a function definition but rather the main executable part of the script. In OCaml, code at the top level is executed in order.
let () = ...: This is a common OCaml idiom to mark the main entry point of a program.
print_endline "--- ... ---": It first prints a welcome banner to the console.
main_loop (): It then makes the initial call to main_loop, which kicks off the interactive loop that is the core of the program's execution.
2. CONCLUSION
The OCaml Fibonacci Sequence script shown in the previous tutorial is logically divided into three main functions and an entry point as described in the tutorial above..
3. REFERENCES
Bird, R. (2015). Thinking Functionally with Haskell. Cambridge, England: Cambridge University Press.
Davie, A. (1992). Introduction to Functional Programming Systems Using Haskell. Cambridge, England: Cambridge University Press.
Goerzen, J. & O'Sullivan, B. & Stewart, D. (2008). Real World Haskell. Sebastopol, CA: O'Reilly Media, Inc.
Hutton, G. (2007). Programming in Haskell. New York: Cambridge University Press.
Lipovača, M. (2011). Learn You a Haskell for Great Good!: A Beginner's Guide. San Francisco, CA: No Starch Press, Inc.
Pierce, Benjamin C., et al., editors. Logical Foundations. Vol. 1 of Software Foundations. Electronic Textbook, 2024. Version 6.7, http://softwarefoundations.cis.upenn.edu.
Thompson, S. (2011). The Craft of Functional Programming. Edinburgh Gate, Harlow, England: Pearson Education Limited.
