case and list shapes in Haskell

Reads a corridor of traffic signals with case expressions over a sum type.

-- case tries each pattern top to bottom and takes the first match.
module Signals where

data Signal = Green | Yellow | Red | Dark

speedLimit :: Signal -> Int
speedLimit signal = case signal of
  Green  -> 45
  Yellow -> 25
  Red    -> 0
  _      -> 15

-- Lists match by shape: [] is empty, (x:xs) splits head from tail.
firstGreen :: [Signal] -> Int
firstGreen [] = 0
firstGreen (x:xs) = case x of
  Green -> speedLimit x
  _     -> firstGreen xs

How it works

1. case signal of matches the Green, Yellow, and Red constructors in turn.
2. A _ wildcard branch catches Dark and falls back to the cautious limit.
3. firstGreen [] and firstGreen (x:xs) split the empty list from head and tail.

Keywords and builtins used here

• Dark
• Green
• Int
• Red
• Signal
• Yellow
• _
• case
• data
• firstGreen
• module
• of
• speedLimit
• where

The run, in numbers

Lines

18

Characters to type

438

Tokens

67

Three-star pace

75 tpm

At the three-star pace of 75 tokens a minute, this run takes about 54 seconds.

Type this snippet

Step 1 of 3 in Pattern matching, step 7 of 27 in Language basics.

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