forked from orion/obsidian
52 lines
1005 B
Markdown
52 lines
1005 B
Markdown
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Alt is used most often as boolean OR on fallible [[Functor|functors]], but is technically whatever associative operation makes sense for the [[Data Structures|data]] type.
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```haskell
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class Functor f <= Alt f where
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alt :: forall a. f a -> f a -> f a
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```
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### Examples
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In [[Maybe]], Alt allows chainable defaults:
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```haskell
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Just 1 <|> Just 2
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-- Just 1
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Nothing <|> Just 1
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-- Just 1
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Nothing <|> Nothing
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-- Nothing
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Nothing <|> Nothing <|> Just 1
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-- Just 1
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```
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In [[Either]], Alt works as a try/catch:
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```haskell
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Left "error!" <|> Right "it's ok i recovered"
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-- Right "it's ok i recovered"
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Right "i succeeded!" <|> Right "backup"
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-- Right "i succeeded!"
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Right unit <|> Left "uh oh!"
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-- Right unit
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Left "a" <|> Left "b"
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-- Left "b"
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```
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> [!attention]
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> Note that Alt in Either takes the last `Left` when all are `Left`!
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In [[Array]], Alt is append:
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```haskell
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[] <|> [] -- []
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["a"] <|> [] -- ["a"]
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[] <|> ["a"] -- ["a"]
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["a"] <|> ["b"] -- ["a", "b"]
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["a"] <|> ["b", "c"] -- ["a", "b", "c"]
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```
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