The Layout Rule

Before we dive into working with functions in Haskell, let's explore the layout rule. The layout rule states that each definition at the same level must begin at the same line position (column) in the script. This allows us to determine the groupings of different definitions simply from indentation. Let's define a function that adds the squares of two numbers together:

sumSquares x y = a + b
  where
    a = x ^ 2 -- (^) is the power function
    b = y ^ 2
    
ghci> sumSquares 2 5
29

From the indentation, it is obvious to Haskell that a and b are local definitions in the function sumSquares, defined using the where keyword. Local definitions exist as intermediate helper expressions for structuring our functions and making our code more readable. It is also possible to wrap the local variables a and b in curly braces to explicitly state the grouping in which case the layout does not matter (although it's considered best practice to use the layout rule to give our code better readability), but we need to also explicitly separate each local definition with ;:

sumSquares x y = a + b
  where
    {
      a = x ^ 2; -- we need to separate expressions with ';' in this case
      b = y ^ 2
    }

ghci> sumSquares 2 5
29