arliss_obsidian/fp/Class/Functor.md

## What
[[Typeclasses|Typeclass]] defining the [[Functions|function]] `map`, [[Infix Operators|operators]] `<$>`, and `<#>`.

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

## Why
Modify the data contained in a [[Data Structures|data structure]]

### Abstracts
- For every element in [[Collections|collection]] ...
- When [[Maybe|nullable]] value is non-null ...
- When _(potentially async)_ [[Effect|IO]] resolves ...

## Examples
### [[Array]] of [[Int]]
```haskell
add 1 <$> [1, 2, 3]
-- [2, 3, 4]
```

### [[Maybe]] of [[Int]]
```haskell
add 1 <$> Just 1
-- Just

add 1 <$> Nothing
-- Nothing
```

### [[Effect]] of [[Int]]
See also:
- [[Bind]]
- [[Show]]
- [[Functions/Composition|compose]]
- [[do notation]]
```haskell
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
```