Understanding the pure and impure functions with c# examples

Pure functions

Pure function is that:

1. Always returns the same output given the same inputs.
2. Does not modify the state of object or any external state.
3. Does not have any side effects such as I/O operations, exceptions or modifying external variables.

• In other words, a pure function takes input, process it and returns the output without modifying anything else.
• Pure functions are easy to test and debug.

Examples

1. Mathematical calculations

public int Add(int a, int b)
{
    return a + b;
}

2. String manipulation

public string Concatenate(string a, string b)
{
    return a + b;
}

3. Array operations

public int[] SortArray(int[] array)
{
    return array.OrderBy(x => x).ToArray();
}

4. Data transformations

public string ConvertToString(int value)
{
    return value.ToString();
}

5. Validation

public bool IsValidEmail(string email)
{
    return email.Contains("@");
}

Impure Functions

Impure function is that

1. Modifies the state of the object or any external state.
2. Has side effects, such as I/O operation, db operations, throwing exceptions, writing to file,db or console etc.
3. May return different outputs given the same inputs, due to external factors.

Examples of impure functions

• I/O operations

public void SaveFile(string data)
{
    File.WriteAllText("data.txt", data);
}

• Database operations

public void InsertUser(User user)
{
    // database insert operation
}

• Web service calls

public string GetWeatherData(string city)
{
    // web service call
}

• Modifying external collections

public void AddItem(List<string> list, string item)
{
    list.Add(item);
}

public void ValidateInput(string input)
{
    if (input == null)
    {
        throw new ArgumentNullException();
    }
}

• Sending an email

public void SendWelcomeEmail(User user)
{
    // send email using SMTP or email service
}

• Writing to a log file

public void LogError(Exception ex)
{
    File.AppendAllText("error.log", ex.Message);
}

• Printing to console

public void PrintReceipt(Order order)
{
    Console.WriteLine("Receipt:");
    Console.WriteLine(order.ToString());
}

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