Add `Either` ↔ `Maybe` conversion functions by futpib · Pull Request #644 · sanctuary-js/sanctuary

futpib · 2019-07-31T12:43:35Z

This pull request adds the following functions:

justToLeft  :: b -> Maybe a -> Either a b
justToRight :: a -> Maybe b -> Either a b

leftToJust  :: Either a b -> Maybe a
rightToJust :: Either a b -> Maybe b

davidchambers · 2019-07-31T13:16:31Z

index.js

+  //. Right ('No negative numbers')
+  //.
+  //. > S.maybeToLeft ('No negative numbers') (S.find (S.lt (0)) ([-1, 0, 1]))
+  //. Left (-1)


These are nice examples. :)

davidchambers · 2019-07-31T13:18:48Z

index.js

+  _.fromEither = {
+    consts: {},
+    types: [$.Either (a) (a), a],
    impl: fromEither


Could we use either (I) (I) here directly?

davidchambers · 2019-07-31T14:52:59Z

index.js

+  _.fromLeft = {
+    consts: {},
+    types: [a, $.Either (a) (b), a],
+    impl: fromLeft


What do you think of using B (either (I)) (K) here?

I don't see a difference, except perhaps stylistic, so you decide.

Let's stick with your implementation. :)

futpib · 2019-07-31T15:17:28Z

index.js

    consts: {},
    types: [b, $.Either (a) (b), b],
-    impl: fromEither
+    impl: fromRight


@davidchambers Should use B (C (either) (I)) (K) here to match fromLeft?

@davidchambers 👋

I find the existing implementation of fromRight easy to read. Let's keep it. Let's also keep your implementation of fromLeft, as there is value in similar things looking similar. :)

davidchambers · 2019-08-01T13:22:08Z

What do others think of these (breaking) changes? @Avaq? @Bradcomp? @masaeedu?

masaeedu · 2019-08-01T14:38:34Z

@davidchambers do we have a swapEither :: Either a b -> Either b a function? This sort of thing might be better handled by a lens library, but absent a good story for those, functions like the one in this PR can be pretty handy.

davidchambers · 2019-08-01T14:48:32Z

We have swap :: Pair a b -> Pair b a but not swapEither :: Either a b -> Either b a. Could you explain the relationship you see between swapEither, a lens library, and the functions in this pull request? You seem to be suggesting that there are multiple ways to express these ideas, and that a different approach could render these changes unnecessary.

masaeedu · 2019-08-01T15:13:01Z

@davidchambers Right, there's a number of different alternatives here. FWIW I think going down the road of exhaustively comparing all these alternatives would swamp the PR, and I think the functions in the PR are simple and handy enough to justify addition as is.

To explain the relationship between the functions here and swapEither: if I understand the changes here correctly, they're implementing a "swapped" version of each xyzEither function.

So for example instead of only having maybeToEither :: a -> Maybe b -> Either a b, it adds maybeToLeft :: a -> Maybe b -> Either b a, which could also be seen as a composition of maybeToEither and swapEither.

Similarly, instead of only having fromEither :: b -> Either a b -> b, you additionally have fromLeft :: b -> Either b a -> b, which again can be seen as b => compose(fromEither(b))(swapEither).

There's also wacky stuff you can do with lenses (have an iso between Either a b and Either b a, and compose it with optics relating maybes and eithers), but there's no good story for this at the moment, and it'd be a ways off.

davidchambers · 2019-08-01T15:20:55Z

Thanks for the clarification, Asad. I understand your point now. :)

Avaq · 2019-08-05T15:38:33Z

What do others think of these (breaking) changes?

I'm happy with them. My only concern is that in implementing the entire matrix of conversions between a, Maybe a, Either a b, and Either b a, we might be increasing API surface drastically for functions that can be expressed as simple compositions. The current "matrix":

from ⬇️ \ to ➡️	`a`	`Maybe a`	`Either a b`	`Either b a`
`a`	`I`	`Just`	`Left`	`Right`
`Maybe a`	`fromMaybe`	`I`	`maybeToLeft`	`maybeToRight`
`Either a b`	`fromLeft`	`leftToMaybe`	`I`	~~`swapEither`~~
`Either b a`	`fromRight`	`rightToMaybe`	~~`swapEither`~~	`I`

Instead of leftToMaybe, one could use a composition of Just and fromLeft, for example. Or the other way around, use a composition of leftToMaybe and fromMaybe to derive fromLeft.

My concern exists because the composition model is easily scalable under inclusion of additional types, whereas the above matrix would grow exponentially with every new constructor added.

I've just spent a lot of words explaining this concern, which makes it seem as though I might feel strongly about this, but it's really very minor - just something you might not have considered.

Avaq · 2019-09-28T18:49:37Z

Going over these changes again, I think they represent an improvement over the status quo. My API surface scaling concern is small, because these functions will always have descriptive names / namespaces.

davidchambers · 2020-08-09T11:31:13Z

@futpib, are you interested in submitting a small pull request to change S.fromEither as you did here?

futpib · 2020-08-09T12:48:48Z

I also dropped the commit with new fromEither from this PR. This is not a major change now, but #690 is.

davidchambers · 2020-08-09T15:59:27Z

Lovely work, @futpib. Thank you for pointing out that this pull request no longer contains breaking changes.

Please update the commit messages and this pull request's title to reflect the fact that this pull request is purely additive now that #664 has been merged. Also, this pull request's description is no longer accurate.

futpib · 2020-08-11T02:21:50Z

@davidchambers Done.

davidchambers · 2020-08-11T06:42:44Z

Thank you, @futpib. This pull request is in a very good state. :)

I am not certain that these functions should be added to the library. Here they are in use:

> maybeToRight ('XXX') (Nothing)
Left ("XXX")

> maybeToRight ('XXX') (Just (42))
Right (42)

> rightToMaybe (Left ('XXX'))
Nothing

> rightToMaybe (Right (42))
Just (42)

The transformations above are possible with existing functions:

> maybe (Left ('XXX')) (Right) (Nothing)
Left ("XXX")

> maybe (Left ('XXX')) (Right) (Just (42))
Right (42)

> either (K (Nothing)) (Just) (Left ('XXX'))
Nothing

> either (K (Nothing)) (Just) (Right (42))
Just (42)

To me, the expressions involving maybe and either are at least as clear as those involving the specialized functions.

Note that maybeToRight can return a Left, which makes sense but reveals the trouble with naming things (see also NaN).

Do these functions offer enough value to justify their inclusion? I would like to hear from @masaeedu, @Avaq, and others. :)

Avaq · 2020-08-11T09:54:38Z

Note that maybeToRight can return a Left, which makes sense but reveals the trouble with naming things (see also NaN).

Should we consider justToRight?

This addresses the naming concern raised in PR sanctuary-js#644 review where Avaq noted that `maybeToRight` can return a Left, creating a naming paradox similar to NaN. The new names emphasize what happens to a Just value, making the API clearer and more intuitive.

This completes the naming consistency improvements started in the previous commit. All four conversion functions now use consistent naming that emphasizes the target constructor: - justToLeft / justToRight (Just → Left/Right) - leftToJust / rightToJust (Left/Right → Just)

davidchambers reviewed Jul 31, 2019

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futpib force-pushed the split-either-functions branch from f7a9689 to 7a28ebe Compare July 31, 2019 14:13

davidchambers reviewed Jul 31, 2019

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davidchambers added the major label Jul 31, 2019

futpib commented Jul 31, 2019

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davidchambers mentioned this pull request Jan 16, 2020

maybe, either: remove S.maybeToEither and S.eitherToMaybe #664

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davidchambers mentioned this pull request Jul 26, 2020

fromLeft and fromRight #683

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futpib force-pushed the split-either-functions branch from 7a28ebe to a71a210 Compare August 3, 2020 06:05

Rename maybeToEither to maybeToRight and add maybeToLeft

161a7e1

futpib force-pushed the split-either-functions branch from a71a210 to f3d6f71 Compare August 9, 2020 12:42

Rename eitherToMaybe to rightToMaybe and add leftToMaybe

c7e798c

futpib force-pushed the split-either-functions branch from f3d6f71 to c7e798c Compare August 9, 2020 12:47

futpib mentioned this pull request Aug 9, 2020

Change the meaning of fromEither #690

Merged

davidchambers added minor and removed major labels Aug 9, 2020

futpib changed the title ~~Split Either functions to Left and Right pairs~~ Add Either ↔ Maybe conversion functions Aug 11, 2020

futpib mentioned this pull request Aug 11, 2020

fromEither should be called fromRight to match Haskell and allow for fromLeft #642

Closed

futpib added 3 commits August 16, 2020 21:12

Merge branch 'master' into split-either-functions

ea8e1b4

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