Add `mergeFlipped`

[sigma-andex]

Add mergeFlipped and an infix operator //

As a user I'm typically more thinking in terms of mergeFlipped than merge, as I want to overwrite a record with some new fields. E.g. in Typescript I can do

const newRecord = { ...otherRecord, more: 23 }

and in dhall // is avaiable as a mergeFlipped operator:

{ home       = "/home/bill"
, privateKey = "/home/bill/.ssh/id_ed25519"
, publicKey  = "/home/blil/.ssh/id_ed25519.pub"
} // { home = "/home/john" }

So in Purescript I would also like to do the following and I think it makes for a nice addition to purescript-record:

person :: { age :: Int
, firstName :: Maybe String
, professional :: String
}
person = 
  { firstName: "John", professional: true } //
  { professional: "Software Engineer", age: 58 } // 
  { firstName: Just "Mike" }

I have created a PR to illustrate this #83

[JordanMartinez]

I think I'm fine with mergeFlipped (though flip merge is shorter...) because we have other similar functions throughout other core libs (e.g. composeKleisliFlipped). I'm more hesitant about the // infix operator. I get that Dhall uses it for record merging, but I'm not sure whether we should follow in that regard.
As a separate thought, a weak argument against adding it is that // indicates a comment in JavaScript.

[sigma-andex]

I think there is a need for an operator to make working with records easier and it has come up in the discord before. E.g. @i-am-the-slime and @mikesol also proposed operators for frequently used record operations (though they were taking about disjointUnion and disjoint).
Personally I think // is my favourite for mergeFlipped and aligns nicely with dhall which we already use in spago. I think that // is a comment in JS is not really something I consider relevant.
An alternative could be ... as in Typescript:

{ x: 1, y: 2 } ... { y: "y" } ... { z: true })

but tbh I think if I wouldn't know it I think // as in dhall is more obvious what its intention is.

[natefaubion]

I think I'm fine with mergeFlipped (though flip merge is shorter...)

I don't think there's any point to having mergeFlipped if you aren't going to have the operator, for the same reason that nobody ever references composeKleisliFlipped without the operator.

The non operator left-to-right alternative is:

  { firstName: "John", professional: true }
    # merge { professional: "Software Engineer", age: 58 }
    # merge { firstName: Just "Mike" }

I'm personally ambivalent towards this. I don't think the operator is more clear than # merge by any means, and I think it's a little weird to say that mergeFlipped deserves an operator but merge doesn't, but I understand it satisfies the behavior for how people usually think of record merging working.

[JordanMartinez]

I don't think there's any point to having mergeFlipped if you aren't going to have the operator, for the same reason that nobody ever references composeKleisliFlipped without the operator.

Good point.

The non operator left-to-right alternative is:

  { firstName: "John", professional: true }
   # merge { professional: "Software Engineer", age: 58 }
   # merge { firstName: Just "Mike" }

Just wanted to say that I really like this style of syntax.

[sigma-andex]

I'm personally ambivalent towards this. I don't think the operator is more clear than # merge by any means, and I think it's a little weird to say that mergeFlipped deserves an operator but merge doesn't, but I understand it satisfies the behavior for how people usually think of record merging working.

I would argue that // is a bit more beginner-friendly, but of course this is maybe a subjective opinion/impression.