{- This file contains the datatypes for Op and Expr.
   As well as the various methods for their display and usage. -}

-- Arithmetic operators
data Op = Add | Sub | Mul | Div

instance Show Op where
   show Add = "+"
   show Sub = "-"
   show Mul = "*"
   show Div = "/"

-- A function to check if the specified operation is valid
valid :: Op -> Int -> Int -> Bool
valid Add _ _ = True
valid Sub x y = x > y
valid Mul _ _ = True
valid Div x y = x `mod` y == 0

-- A function to perform the specified operation between two ints
apply :: Op -> Int -> Int -> Int
apply Add x y = x + y
apply Sub x y = x - y
apply Mul x y = x * y
apply Div x y = x `div` y

-- Numeric expressions
data Expr = Val Int | App Op Expr Expr

-- brak is an inline function definition that allows us to construct strings and properly place parenthesis around mathematical terms
instance Show Expr where
   show (Val n)     = show n
   show (App o l r) = brak l ++ show o ++ brak r
                      where
                         brak (Val n) = show n
                         brak e       = "(" ++ show e ++ ")"

-- Creates an Int array containing only values found inside the Expr, none of the operations 
values :: Expr -> [Int]
values (Val n)     = [n]
values (App _ l r) = values l ++ values r 

-- Evaluates an Expr down to a single item Int array 
eval :: Expr -> [Int]
eval (Val n)     = [n | n > 0]
eval (App o l r) = [apply o x y | x <- eval l,
                                  y <- eval r,

                                  valid o x y]

-- Questions 1 and 2  of In Class Practive

reverser :: Expr -> Expr
reverser (App Add i r) = App Sub (reverser i) (reverser r)
reverser (App Sub i r) = App Add (reverser i) (reverser r)
reverser (App Mul i r) = App Div (reverser i) (reverser r)
reverser (App Div i r) = App Mul (reverser i) (reverser r)

minMaxVal :: [Expr] -> [Int]
minMaxVal (xs) = [minElement, maxElement] 
 where
  maxElement = maximum 
  (valueArray(xs))
  minElement = minimum 
  (ValueArray(xs))
  
valueArray :: [Expr] -> [Int]
valueArray [] = []
valueArray (p:xs) = element: (valueArray(xs)) 
 where 
  element = head(eval p)

x :: Expr 
x = App Div (App Mul (Val 3) (Val 7)) (App Add (App Sub (Val 5) (Val 4)) (App Add (Val 2) (Val 4)))
y :: Expr
y = App Mul (App Add (Val 4) (Val 10)) (App Sub (App Div (Val 14) (Val 2)) (App Mul (Val 2) (Val 2)))
z :: Expr
z = App Sub (App Div (App Mul (Val 6) (App Add (Val 5) (App Sub (Val 9) (Val 4)))) (App Add (Val 3) (Val 2))) (App Mul (Val 3) (App Sub (App Div (Val 18) (Val 9)) (Val 1)))

main = do
 print x
 print y
 print z
 print (head (eval x))
 print (head (eval y))
 print (head (eval z))
 print (values x)
 print (values y)
 print (values z)

Haskell online compiler

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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