forked from orion/obsidian
45 lines
1.5 KiB
Markdown
45 lines
1.5 KiB
Markdown
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Most commonly used [[Infix Operators]] have flipped variants, e.g.
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- [[Functions#Composition|function composition]] has `g <<< f` or `f >>> g`
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- function application has `f $ a` or `a # f`
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- [[Functor|map]] has `f <$> a` or `a <#> f`
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- [[Bind|bind]] has `f =<< m` or `m >>= f`
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In general, right-to-left operators tend to be easier to refactor into & out of because they closely mirror the expressions they replace:
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```haskell
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map (add 1) maybeNum
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-- into
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add 1 <$> maybeNum
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foo (bar a)
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-- into
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foo <<< bar
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```
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Left-to-right, on the other hand, can read better to humans and plays better with pipelines containing both [[Bind|bind `>>=`]] and [[Functor|map `<#>`]].
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Consider an expression piping `Maybe String` to `split :: String -> Array String` then `Data.Array.NonEmpty.fromArray :: Array a -> Maybe (NonEmptyArray a)`, then `toLower` each element:
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```haskell
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String.toLower
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<$> (
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Array.NonEmpty.fromArray
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=<< String.split "."
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<$> mStr
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)
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```
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We need to wrap the right hand of the last `map` because the precedence of `=<<` is `1` (the lowest) and the precedence of `<$>` is `4`.
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Written RTL, though, gives:
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```haskell
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mStr
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<#> String.split "."
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>>= Array.NonEmpty.fromArray
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<#> String.toLower
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```
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This works because `<#>`'s [[Precedence|precedence]] (1) is the same as `>>=`. The lower precedence on flipped map means you'll often need more parentheses wrapping its arguments `(..) <#> (..) >>= (..)` as opposed to entire expressions `.. <$> (.. =<< ..)`.
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Personally, I try to stick to RTL except for expressions including bind.
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