Returning updated records

[endeav0r]

The following code illustrates the bug (This will run in http://gluon-lang.org/try/)

let {List} = import! "std/list.glu"

type R = { a: String, b: Int }

let f r n : R -> Int -> R =
  {b=n, ..r}

let r = {a="hello", b=1}

let r2 = f r 2

r2.b

The order of members of R has been changed, so that where we had {a: String, b: Int} we now have {b: Int, a: String}, and these two types are, apparently, not equal.

<top>:Line: 6, Column: 3: Expected the following types to be equal
Expected: <top>.R
Found: { b : Int, a : String }
2 errors were found during unification:
Field names in record do not match.
	Expected: a
	Found: b
Field names in record do not match.
	Expected: b
	Found: a
  {b=n, ..r}
  ^~~~~~~~~~

This makes it difficult to create functions which update and return specific fields in complex structs/records.

[Marwes]

I think this case may be solvable by taking the type of ..r into account first, before appending and replacing the other fields. That way field order of r is preserved making the behavior less surprising.

More generally we might want to accept code like this.

type R = { a: String, b: Int }

let r: R = {b=1, a="hello"}
r

where the field order is "wrongly" specified. I think the only way to solve that is to have a deterministic field order for records (can be done by sorting the fields lexically).