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Add examples for sequence and traverse #115

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Oct 15, 2020
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Add examples for sequence and traverse #115

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a6b00ec
Add examples for sequence and traverse
milesfrain May 22, 2020
4d703fb
Review feedback, unnecessary parens and logShow
milesfrain May 25, 2020
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20 changes: 20 additions & 0 deletions src/Data/Traversable.purs
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Expand Up @@ -32,6 +32,26 @@ import Data.Traversable.Accum.Internal (StateL(..), StateR(..), stateL, stateR)
-- | - `sequence` runs the actions _contained_ in a data structure,
-- | and accumulates the results.
-- |
-- | ```purescript
-- | import Data.Traversable
-- | import Data.Maybe
-- | import Data.Int (fromNumber)
-- |
-- | sequence [Just 1, Just 2, Just 3] == Just [1,2,3]
-- | sequence [Nothing, Just 2, Just 3] == Nothing
-- |
-- | traverse fromNumber [1.0, 2.0, 3.0] == Just [1,2,3]
-- | traverse fromNumber [1.5, 2.0, 3.0] == Nothing
-- |
-- | traverse logShow [1,2,3]
-- | -- prints:
-- | 1
-- | 2
-- | 3
-- |
-- | traverse (\x -> [x, 0]) [1,2,3] == [[1,2,3],[1,2,0],[1,0,3],[1,0,0],[0,2,3],[0,2,0],[0,0,3],[0,0,0]]
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I'm not sure this example will be particularly illuminating; it doesn't seem very useful at first glance. I think probably most of the time when I'm using traverse, it's with Effect, Aff, or a monad transformer stack with one of these at the bottom, so I think an effectful example would probably be better. How about something like:

traverse logShow [1,2,3]
-- prints:
1
2
3

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Added the logShow example.

I think it's also nice to show how traverse can be used for array expansion. Reading code that does this often trips me up because I never expect traverse to do something like this. For example in filterA https://github.com/purescript/purescript-arrays/blob/463dcacb99455de5e28ac4a3312bb795f293d2d9/src/Data/Array.purs#L609-L612

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Fair enough. I guess it’s good to show a few different examples to illustrate that it can do a whole lot of very different things based on the Applicative instance in use.

-- | ```
-- |
-- | The `traverse` and `sequence` functions should be compatible in the
-- | following sense:
-- |
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