Custom instances in Haskell

Hand-written Show and Eq instances decide how a value prints and what counts as equal.

module Tags where

import Data.Char (toLower)

-- A tag keeps the spelling the author typed.
newtype Tag = Tag String

-- Writing Show by hand controls exactly how a value is rendered.
instance Show Tag where
  show (Tag name) = '#' : name

-- Domain equality: normalize both sides, then compare.
instance Eq Tag where
  Tag a == Tag b = map toLower a == map toLower b

-- So two spellings of one tag are the same tag.
sameTopic :: Bool
sameTopic = Tag "Haskell" == Tag "haskell"

How it works

1. newtype Tag = Tag String wraps a string so it can carry its own instances.
2. The Show instance defines show (Tag name) = '#' : name, so a tag renders with a hash in front.
3. The Eq instance compares map toLower a == map toLower b, which makes sameTopic true for two spellings of one tag.

Keywords and builtins used here

• Bool
• Eq
• Show
• String
• Tag
• import
• instance
• module
• newtype
• sameTopic
• toLower
• where

The run, in numbers

Lines

18

Characters to type

475

Tokens

61

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 52 seconds.

Type this snippet

Step 2 of 3 in Typeclasses, step 20 of 27 in Language basics.

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