Brian Teague (Feb 21 2024 at 21:42): Brian Teague (Feb 21 2024 at 21:42):How to properly destructure a function with two record types that both have optional arguments?
Point : { x: Frac a, y: Frac a, z? Frac a }
distanceBetweenPoints : Point, Point -> Frac a
distanceBetweenPoints = \point1, point2 ->
# How / where to default point1.z and point2.z since they have to defaulted when they are destructured?
{ x, y, z? 0.0 } = point1 #destructure point1
{ x1, y1, z1 } = { x, y, z }
{ x, y, z? 0.0 } = point2 #destructure point2 requires shadowing
{ x2, y2, z2 } = { x, y, z }
Num.sqrt (Num.pow (x2 - x1) 2.0 + Num.pow (y2 - y1) 2.0 + Num.pow (z2 - z1) 2.0)
Should the ROC compiler automatically have default values based on type? Should the default value be specified at the type definition, so destructuring is not required?
bool -> Bool.false
int -> 0
Frac -> 0.0
Str -> ""
Point : { x: Fac a, y: Frac a, z? 1.0 Frac a } # 0.0 if no default is specificed
I think this might be an unhandled edge case rn. I don't think the parser currently supports both renaming a variable and assigning a default value.
I would expect it to look something like this
distanceBetweenPoints : Point a, Point a -> Frac a
distanceBetweenPoints = \{x: x1, y: y1, z: z1 ? 2 }, {x: x2, y: y2, z: z2 ? 3 } ->
Num.sqrt (Num.pow (x2 - x1) 2.0 + Num.pow (y2 - y1) 2.0 + Num.pow (z2 - z1) 2.0)
This is the error I get
── UNFINISHED RECORD PATTERN in Test.roc ───────────────────────────────────────
I am partway through parsing a record pattern, but I got stuck here:
8│ distanceBetweenPoints = \{x: x1, y: y1, z: z1 ? 2 }, {x: x2, y: y2, z: z2 ? 3 } ->
^
Would this be a new requirement where we need a way to rename variables and specify defaults for records?
Here is how Javascript renames variables when they are being destructured.
const obj = {foo: 1, bar: 2};
const {foo: renamedFoo, bar: renamedBar} = obj;
Point a : { x: Frac a, y: Frac a, z ? Frac a }
distanceBetweenPoints : Point a, Point a -> Frac a
distanceBetweenPoints = \point1, point2 ->
{ x: x1, y: y1, z: z1 ? 0.0 } = point1
{ x: x2, y: y2, z: z2 ? 0.0 } = point2
Num.sqrt (
Num.pow (x2 - x1) 2.0 +
Num.pow (y2 - y1) 2.0 +
Num.pow (z2 - z1) 2.0
)
Last updated: Jul 26 2025 at 12:14 UTC