forked from orion/obsidian
57 lines
1.1 KiB
Markdown
57 lines
1.1 KiB
Markdown
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## What
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[[Typeclasses|Typeclass]] defining the [[Functions|function]] `map`, [[Infix Operators|operators]] `<$>`, and `<#>`.
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```haskell
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class Functor f where
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map :: forall a b. (a -> b) -> f a -> f b
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```
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## Why
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Modify the data contained in a [[Data Structures|data structure]]
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### Abstracts
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- For every element in [[Collections|collection]] ...
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- When [[Maybe|nullable]] value is non-null ...
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- When _(potentially async)_ [[Effect|IO]] resolves ...
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## Examples
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### [[Array]] of [[Int]]
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```haskell
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add 1 <$> [1, 2, 3]
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-- [2, 3, 4]
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```
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### [[Maybe]] of [[Int]]
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```haskell
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add 1 <$> Just 1
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-- Just
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add 1 <$> Nothing
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-- Nothing
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```
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### [[Effect]] of [[Int]]
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See also:
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- [[Bind]]
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- [[Show]]
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- [[compose]]
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- [[do notation]]
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```haskell
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import Node.FS.Sync (readTextFile, writeTextFile)
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import Node.Encoding (Encoding(..))
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import Data.Int as Int
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writeNum :: Int -> Effect Unit
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writeNum n = writeTextFile "num.txt" (show n) UTF8
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readNum :: Effect Int
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readNum =
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readTextFile "num.txt" UTF8
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>>= (Int.fromString >>> liftMaybe (error "invalid integer"))
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main = do
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writeNum
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n <- add 1 <$> readNum
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log $ show n
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-- 2
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```
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