-- Binary search in Haskell
-- Precondition: the list is sorted in ascending order
-- Postcondition: the function returns the index of the target element if found, or Nothing otherwise
-- Invariant: the search space is always within the bounds of the list
-- Variant: the search space decreases by half at each recursive call
-- Internal state: the lower and upper bounds of the search space

binarySearch :: Ord a => [a] -> a -> Maybe Int
binarySearch xs x = go xs x 0 (length xs - 1)
  where
    go xs x lo hi
      | lo > hi = Nothing -- base case: search space is empty
      | x == midElem = Just mid -- base case: target element is found
      | x < midElem = go xs x lo (mid - 1) -- recursive case: search in the left half
      | otherwise = go xs x (mid + 1) hi -- recursive case: search in the right half
      where
        mid = lo + (hi - lo) `div` 2 -- calculate the middle index using a safer formula to avoid overflow
        midElem = xs !! mid -- get the middle element

-- Print results using putStrLn
main :: IO ()
main = do
  let xs = [1, 3, 5, 7, 9] -- a sorted list of integers
  let x = 5 -- a target element to search for
  let result = binarySearch xs x -- call the binary search function
  case result of -- pattern match on the result
    Nothing -> putStrLn "Element not found" -- print a message if Nothing is returned
    Just i -> putStrLn $ "Element found at index " ++ show i -- print a message with the index if Just i is returned

Haskell online compiler

Write, Run & Share Haskell code online using OneCompiler's Haskell online compiler for free. It's one of the robust, feature-rich online compilers for Haskell language, running the latest Haskell version 8.6. Getting started with the OneCompiler's Haskell editor is easy and fast. The editor shows sample boilerplate code when you choose language as Haskell and start coding.

Taking inputs (stdin)

OneCompiler's Haskell online editor supports stdin and users can give inputs to programs using the STDIN textbox under the I/O tab. Following is a sample Haskell program which takes name as input and prints hello message with your name.

main = do  
    name <- getLine  
    putStrLn ("Hello " ++ name ++ ", Happy learning!")

About Haskell

Haskell is purely a functional programming language which was introduced in 1990's.

Key Features

  • Haskell is both compiled and interpreted
  • Lazy language as the results are computed only if required
  • Pure functions
  • Pattern matching on data structures
  • Emphasizes on what to do but not on how to do
  • Glasgow Haskell Compiler (GHC), most widely used Haskell compiler also written in Haskell.
  • Data is immutable

Syntax help

Data Types

Data-typeDescription
NumbersHaskell is intelligent to identify numbers without specifying data type
CharactersHaskell is intelligent to identify characters and strings without specifying data type
TupleTo declare multiple values in a single data type. Tuples are represented in single paranthesis. For example (10, 20, 'apple')
BooleanTo represent boolean values, true or false
ListTo declare same type of values in a single data type. Lists are represented in square braces.For example [1, 2, 3] or `['a','b','c','d']

Control statements

If-Else / Nested If-Else:

When ever you want to perform a set of operations based on a condition or set of conditions, then If-Else/ Nested-If-Else are used.

Example:

main = do   
   let age = 21 
   if age > 18 
      then putStrLn "Adult" 
   else putStrLn "child"

Functions

Function is a sub-routine which contains set of statements. Usually functions are written when multiple calls are required to same set of statements which increases re-usuability and modularity. Functions play an important role in Haskell, since it is a purely functional language.

Example

multiply :: Integer -> Integer -> Integer   --declaration of a function 
multiply x1 x2 =  x1 * x2                   --definition of a function

main = do 
   putStrLn "Multiplication value is:"  
   print(multiply 10 5)    --calling a function