arliss_obsidian/fp/Classes/Functor/Functor.md

Typeclasses defining the Functions map, Infix Operators <$>, and <#>.

class Functor f where
  map :: forall a b. (a -> b) -> f a -> f b

map

For a Data Structures containing some data a, change the data using some function a -> b.

[!info] Another way of thinking of map is that it lifts a function of a -> b to f a -> f b:

addTwo :: Int -> Int
addTwo = add 2

maybeAddTwo :: Maybe Int -> Maybe Int
maybeAddTwo = map addTwo

Replaces the patterns:

• Modify every element in Collections ...
• When Maybe value is non-null ...
• When (potentially async) Effect resolves ...

Examples

Array of Int

add 1 <$> [1, 2, 3]
-- [2, 3, 4]

Maybe of Int

add 1 <$> Just 1
-- Just

add 1 <$> Nothing
-- Nothing

Effect of Int

See also:

• Bind
• Show
• Functions/Composition
• do notation

import Node.FS.Sync (readTextFile, writeTextFile)
import Node.Encoding (Encoding(..))
import Data.Int as Int

writeNum :: Int -> Effect Unit
writeNum n = writeTextFile "num.txt" (show n) UTF8

readNum :: Effect Int
readNum =
  readTextFile "num.txt" UTF8
  >>= (Int.fromString >>> liftMaybe (error "invalid integer"))

main = do
  writeNum
  n <- add 1 <$> readNum
  log $ show n
  -- 2