Currying & partial application in Haskell

A tip calculator that hands out half-applied functions instead of numbers.

-- Every function takes one argument and returns a function.
module Tipping where

addTip :: Double -> Double -> Double
addTip rate amount = amount * (1 + rate)

-- Supply only the rate and a Double -> Double is left waiting.
standardTip :: Double -> Double
standardTip = addTip 0.18

checks :: [Double]
checks = [12.0, 31.5, 8.25]

-- Partials go straight into map; (* 100) is an operator section.
withTips :: [Double]
withTips = map standardTip checks

inCents :: [Double]
inCents = map (* 100) (map (addTip 0.25) checks)

How it works

1. addTip rate amount takes its arguments one at a time rather than as a pair.
2. standardTip = addTip 0.18 fixes the rate and leaves a Double -> Double behind.
3. map standardTip checks feeds a partial to map, and (* 100) is an operator section.

Keywords and builtins used here

• Double
• addTip
• checks
• inCents
• module
• standardTip
• where
• withTips

The run, in numbers

Lines

19

Characters to type

523

Tokens

77

Three-star pace

75 tpm

At the three-star pace of 75 tokens a minute, this run takes about 62 seconds.

Type this snippet

Step 2 of 3 in Functions, step 5 of 27 in Language basics.

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