Where and Let

Where and Let

Haskell has two ways to introduce local bindings: where and let ... in.

where

where goes at the end of a function definition and scopes over the whole body:

bmiCategory :: Double -> String
bmiCategory weight
  | bmi < 18.5  = "Underweight"
  | bmi < 25.0  = "Normal"
  | otherwise   = "Overweight"
  where
    bmi = weight / (1.75 * 1.75)

let ... in

let ... in is an expression — it can appear anywhere an expression is expected:

circleArea :: Double -> Double
circleArea r =
  let pi_ = 3.14159
      r2  = r * r
  in pi_ * r2

In do-blocks

Inside do, use just let (without in):

main = do
  let x = 10
      y = 20
  print (x + y)

Your Task

Define hypotenuse that computes the hypotenuse sqrt(a² + b²) using a where clause for intermediate values. Print hypotenuse 3 4.