CSE 130 Spring 2011 - Style Exercise 1

Problem 1

Consider the following function has_one_element that attempts to take a list as input and return true if its length is exactly one and false otherwise.

let has_one_element_a l =
  if List.tl l = [] 
  then true 
  else false

This code has a bug, however, and might fail with a runtime exception when certain lists are provided as input. Briefly describe the problem.

Problem 2

Here are four other versions of the above function. Briefly explain how each improves upon the previous version. Or if you think a version is inferior to the one before, explain why.

Hint: variants A and B may also fail at runtime.

let has_one_element l =
  List.tl l = []

let has_one_element l =
  match l with
  | x::[] -> true

let has_one_element l =
  match l with
  | [] -> false
  | x::[] -> true
  | h::t -> false

let has_one_element l =
  match l with
  | [_] -> true
  | _   -> false

Problem 3

Based on the style used in variant D, implement the following function that returns true if its input list has length two and false otherwise.

val has_two_elements : 'a list -> bool