forked from orion/obsidian
39 lines
1.0 KiB
Markdown
39 lines
1.0 KiB
Markdown
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Extends [[Alt]] with an empty value:
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```haskell
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class Alt f <= Plus f where
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empty :: forall a. f a
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```
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> [!info]
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> Conceptually, this lives next to [[Applicative|pure]] in my mind; in [[Maybe]] for example `pure` and `empty` are aliases for `Just` and `Nothing`.
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In [[Array]], `empty` is empty array `[]`. In [[Maybe]], `empty` is `Nothing`.
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The most common place you'll see `empty` is when using [[MaybeT]] as an early return:
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```haskell
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logDebug :: String -> Effect Unit
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logDebug msg = void $ runMaybeT do
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env <- MaybeT $ Process.lookupEnv "NODE_ENV"
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when (env /= "DEVELOPMENT") empty
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lift $ Console.log msg
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```
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this is equivalent to:
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```haskell
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logDebug :: String -> Effect Unit
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logDebug msg = do
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menv <- Process.lookupEnv "NODE_ENV"
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case menv of
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Just env ->
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if env == "DEVELOPMENT" then
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Console.log msg
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else
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pure unit
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Nothing -> pure unit
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```
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> [!info]
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> Note that [guard](https://pursuit.purescript.org/packages/purescript-control/docs/Control.Alternative#v:guard) is a shorthand available for `when (..) empty`
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