Ashley Davis (Jul 16 2024 at 22:32): Ashley Davis (Jul 16 2024 at 22:32):I have the following function that I'm using to create fake gallery items for testing:
# Makes a gallery item for testing.
makeDefaultItem : _ -> GalleryItem
makeDefaultItem = \_ -> {
width: 10,
height: 20,
headings: []
}
I'd like to extend it to include optional parameters to configure it.
In TypeScript I'd do something like this:
function makeDefaultItem({ width?: number, height?: number, headings?: string[] }): GalleryItem {
return {
width: width !== undefined ? width : 10,
height: height !== undefined ? height : 20,
headings: headings !== undefined ? headings : [],
};
}
This way I can create gallery items for testing specifying the parameters I care about and defaulting the rest. Say for example my next test really only cares about the width of a gallery item, so in TypeScript I could create it like this:
const galleryItem = makeDefaultItem({ width: 32 });
Is it possible to have optional parameters like this for a Roc function?
Luke Boswell (Jul 16 2024 at 22:40):Here's a demo of the syntax
app [main] {
cli: platform "https://github.com/roc-lang/basic-cli/releases/download/0.10.0/vNe6s9hWzoTZtFmNkvEICPErI9ptji_ySjicO6CkucY.tar.br",
}
import cli.Stdout
import cli.Task exposing [Task]
main : Task {} _
main =
one : GalleryItem
one = makeDefaultItem {}
two : GalleryItem
two = makeDefaultItem {
width: 100,
height: 200,
headings: ["a", "b", "c"],
}
Stdout.line! (Inspect.toStr one)
Stdout.line! (Inspect.toStr two)
GalleryItem : {
width: U64,
height: U64,
headings: List Str,
}
makeDefaultItem : { width ? U64, height ? U64, headings ? List Str } -> GalleryItem
makeDefaultItem = \{ width ? 10, height ? 20, headings ? [] } -> {
width,
height,
headings,
}
Unfortunately there's a strange compiler bug :ladybug: if we have both calls, either one is happy but together it crashes.
$ roc demo.roc
thread 'main' panicked at crates/compiler/gen_llvm/src/llvm/build.rs:5759:19:
Error in alias analysis: error in module ModName("UserApp"), function definition FuncName("\x10\x00\x00\x00\x00\x00\x00\x00\'\xae\xa4\xef\x8b\xd4\xa4\x87"), definition of value binding ValueId(3): expected type '(((heap_cell, bag<(heap_cell,)>), (), ()),)', found type '((),)'
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
I can't get the optional syntax to work... definitely something strange going on.
app [main] {
cli: platform "https://github.com/roc-lang/basic-cli/releases/download/0.10.0/vNe6s9hWzoTZtFmNkvEICPErI9ptji_ySjicO6CkucY.tar.br",
}
import cli.Stdout
import cli.Task exposing [Task]
main : Task {} _
main =
items = [
makeDefaultItem {
height: 200u64,
headings: ["A"],
},
makeDefaultItem {
width: 100u64,
height: 200u64,
headings: ["a", "b", "c"],
},
]
items
|> List.map toStr
|> Str.joinWith "\n\n"
|> Stdout.line!
GalleryItem : {
width : U64,
height : U64,
headings : List Str,
}
makeDefaultItem : { width ? U64, height : U64, headings : List Str } -> GalleryItem
makeDefaultItem = \{ width ? 10, height, headings } -> {
width,
height,
headings,
}
toStr : GalleryItem -> Str
toStr = \{ width, height, headings } ->
headingsStr = Str.joinWith headings ", "
"""
Gallery Item:
width: $(Num.toStr width)
height: $(Num.toStr height)
headings: $(headingsStr)
"""
$ roc demo.roc
thread 'main' panicked at crates/compiler/gen_llvm/src/llvm/build.rs:5759:19:
Error in alias analysis: error in module ModName("UserApp"), function definition FuncName("\x10\x00\x00\x00\x00\x00\x00\x00F\x93\xd4\x88\xfa\xad\x9b\x90"), definition of value binding ValueId(12): expected type '(((heap_cell, bag<(heap_cell,)>), (), ()),)', found type '(((heap_cell, bag<(heap_cell,)>), ()),)'
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Ok I might steer clear of optional parameters for the moment. Thanks for your help @Luke Boswell
Brendan Hansknecht (Jul 16 2024 at 23:29):Yeah, compiler bug that seems to only allow for one specialization of the function.
Brendan Hansknecht (Jul 16 2024 at 23:29):Cause it should create a different version of the function for each optional configuration used.
Brendan Hansknecht (Jul 16 2024 at 23:30):Probably a case of missing some wiring to tell it to create the function many times instead of specializing it once
Brendan Hansknecht (Jul 16 2024 at 23:30):That said, I am not very familiar with that part of the code, but I would guess it is a relatively small fix.
Luke Boswell (Jul 17 2024 at 00:01):https://github.com/roc-lang/roc/issues/6909
^^^ Issue logged
Brendan Hansknecht (Jul 17 2024 at 00:37):I think we already have https://github.com/roc-lang/roc/issues/6423
Last updated: Jul 26 2025 at 12:14 UTC