Defining functions in Haskell

A shipping quote assembled from two small functions and a literal pattern.

-- A function is an equation: name, arguments by juxtaposition, body.
boxFee :: Double -> Double -> Double
boxFee weightKg ratePerKg = 3.5 + weightKg * ratePerKg

-- Two clauses tried top to bottom: the literal 0 first, then any n.
discountFor :: Int -> Double
discountFor 0 = 0.0
discountFor n = min 0.25 (fromIntegral n * 0.05)

totalCost :: Double -> Double -> Int -> Double
totalCost weightKg ratePerKg boxes =
  boxFee weightKg ratePerKg * (1 - discountFor boxes)

main :: IO ()
main = print (totalCost 2.5 1.8 3)

How it works

1. boxFee weightKg ratePerKg takes its two arguments written side by side.
2. discountFor 0 is a separate equation from discountFor n and is tried first.
3. totalCost calls both, and main prints totalCost 2.5 1.8 3.

Keywords and builtins used here

• Double
• IO
• Int
• boxFee
• discountFor
• main
• totalCost

The run, in numbers

Lines

15

Characters to type

516

Tokens

75

Three-star pace

80 tpm

At the three-star pace of 80 tokens a minute, this run takes about 56 seconds.

Type this snippet

Step 1 of 3 in Functions, step 4 of 27 in Language basics.

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Defining functions in other languages

• Mojo
• Python