Haskell Program to Return a String from a Function

In this article, we are going to learn how to return a string from a function using user-defined function along with record syntax and let binding. In the first example, we are going to use (myFunction = "Hello, World!") function and in the second example, we are going to use (myFunction = stringValue myData). And in third example, we are going to use let binding, (myFunction = let str = "Using let binding!" in str).

Method 1: Returning a string from the user-defined function

In this method, the user-defined functions are defined that will contain the function definition and return a string once the function is being called.

Algorithm

• Step 1 ? The user defined function is defined by writing its definition that will return a string after computation.

• Step 2 ? Program execution will be started from main function. The main() function has whole control of the program. It is written as main = do. In the main function, the user defined function is being called.

• Step 3 ? The resultant string is printed to the console, after the function is being called.

Example 1

In this example, myFunction is a function that returns the string, "Hello, World!". The main function calls myFunction and prints the result to the console.

myFunction :: String
myFunction = "Hello, World!"
main = do
   let result = myFunction
   putStrLn result

Output

Hello, World!

Example 2

In this example, a Haskell function myFunction is defined using a case expression. The case expression case str of str -> str matches the value of str and returns it. The value of str is defined in the where clause as "Hello, World!".

myFunction :: String
myFunction = case str of
   str -> str
   where str = "Hello, World!"
main = do
   let result = myFunction
   putStrLn result

Output

Hello, World!

Method 2: Returning a string from the user-defined function using record syntax

In this method, the user-defined functions are defined using record syntax that will contain the function definition and return a string once the function is being called.

Algorithm

• Step 1 ? The user defined function is defined by writing its definition using record syntax that will return a string after computation.

• Step 2 ? Program execution will be started from main function. The main() function has whole control of the program. It is written as main = do. In the main function, the user defined function is being called.

• Step 3 ? The resultant string is printed to the console, after the function is being called.

Example 

In this example, a record data type MyData is used with a single field stringValue of type String, which holds the value "Hello, World!". The myFunction then uses pattern matching to extract the stringValue from the MyData value myData and returns it.

data MyData = MyData { stringValue :: String }

myFunction :: String
myFunction = stringValue myData
   where myData = MyData { stringValue = "Using Record Syntax!" }
main = do
   let result = myFunction
   putStrLn result

Output

Using Record Syntax!

Method 3: Returning a string from the user-defined function using let binding

In this method, the user-defined functions are defined using let binding that will contain the function definition and return a string once the function is being called.

Algorithm

• Step 1 ? The user defined function is defined by writing its definition using the let binding that will return a string after computation.

• Step 2 ? Program execution will be started from main function. The main() function has whole control of the program. It is written as main = do. In the main function, the user defined function is being called.

• Step 3 ? The resultant string is printed to the console, after the function is being called.

Example 

In this example, a Haskell function myFunction is defined using a let binding. The let binding str = "Using let binding!" defines a local variable str with the value "Using let binding!". The myFunction then returns the value of str.

myFunction :: String
myFunction = let str = "Using let binding!" in str
main = do
   let result = myFunction
   putStrLn result

Output

Using let binding!

Conclusion

In Haskell, a string is a sequence of characters, represented as a list of characters. A string can be defined using double quotes (").

The user-defined functions are functions created by the programmer to perform specific operations. The users can define functions as per their need by passing any desired arguments and returning some value in the function definition. The arguments passed can be integers, strings or any array of the values and the corresponding resultant string is returned and printed to console.