What is the difference between let and where in Haskell?

In Haskell, both ‘let‘ and ‘where‘ are used to introduce local bindings to be used within expressions. The primary difference between them is their syntax and scope, though they are functionally quite similar.

1. ‘let‘ expressions:

The ‘let‘ expression introduces local bindings to an expression. It follows the syntax:

let <bindings> in <expression>

Here, `<bindings>` are the local bindings whose scope is limited to `<expression>`. The syntax for the bindings is similar to the top-level declarations. You can bind values, functions, and also use pattern matching. A simple example:

sumSquares :: Int -> Int -> Int
sumSquares x y = total
  where
    total = xSquare + ySquare
    xSquare = x * x
    ySquare = y * y

3. Comparison:

The primary difference between ‘let‘ and ‘where‘ is their position in the code (the scope) and a bit of syntax. The ‘where‘ clause is part of the function declaration (or guard statements), whereas ‘let‘ is itself an expression (inside the function body).

Here’s a comparison using a simple example of calculating the area of a rectangle:

with ‘where‘:

Rectangle :: Float -> Float -> Float
area x y = totalArea
  where
    totalArea = x * y

with `let`:

area :: Float -> Float -> Float
area x y =
  let totalArea = x * y
  in totalArea

In summary, it’s mostly a matter of preference and style when deciding between ‘let‘ and ‘where‘. Some people prefer ‘let‘ as it provides a more straightforward top-down flow of code, while others like ‘where‘ as it often makes the primary expression more readable without being interrupted by the bindings it relies on.