Haskell Let Expressions

A let expression binds names inside an expression and gives a result that uses them. The bindings last only for that one expression.

A let value can be used straight away

The let binds x to 5 inside the print call. The result after in doubles it.

main :: IO ()
main = print (let x = 5 in x * 2)

One let can hold several bindings

This let binds x to 10 and y to 5. The result after in adds the two names.

total =
  let x = 10
      y = 5
  in x + y

main :: IO ()
main = print total

A let can sit inside a function

The function binds discount inside its own let. The result takes that discount away from the price input.

finalPrice price =
  let discount = 5
  in price - discount

main :: IO ()
main = print (finalPrice 30)

Example

discountedTotal =
  let subtotal = 4 * 15
      discount = 10
  in subtotal - discount

main :: IO ()
main = print (let extra = 5 in discountedTotal + extra)

Key points

Let expression: a binding written inside an expression, before the word in.
in: the word in separates the bindings from the result that uses them.
Result: the value of the whole let expression is the value of the part after in.
Scope: a name bound by let can be used only in the result part of that same let expression.
Several bindings: one let can hold several bindings, and the result can use all of them.

In this example, the first let works out a subtotal of 60 and takes off a discount of 10, giving 50. A second let inside print adds an extra 5, so the program prints 55.

Note

a let expression is a value, so it can sit inside a larger expression such as a print call.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Haskell Playground

discountedTotal =
  let subtotal = 4 * 15
      discount = 10
  in subtotal - discount

main :: IO ()
main = print (let extra = 5 in discountedTotal + extra)

Exercise

What is the value of let x = 4 in x * x?

8 16 4 20