Oskar Hahn (Dec 26 2023 at 19:32): Oskar Hahn (Dec 26 2023 at 19:32):I am playing around with opaque types. I was able to find out how to implement the Equal ability. But I can not figure out, how to implement the inspect ability.
Here is an example type for a rational number:
Rational a := (Int a, Int a)
implements [
Eq { isEq: qEq },
# Inspect {
# toInspector: toInspectorRational,
# },
]
qEq = \@Rational (a, b), @Rational (c, d) ->
a * d == c * b
qToStr = \@Rational (a, b) ->
"\(a |> Num.toStr) / \(b |> Num.toStr)"
How do I implement the toInspectorRational function, that it calls qToStr?
Brendan Hansknecht (Dec 26 2023 at 19:58):It has to be defined based on the inspect primitives listed here: https://www.roc-lang.org/builtins/Inspect#InspectFormatter
Brendan Hansknecht (Dec 26 2023 at 20:00):This is an example implementation: https://github.com/roc-lang/roc/blob/2e7343e857f6ab73793cef2ad6c8e4a20dba30cb/examples/Community.roc#L93
Brendan Hansknecht (Dec 26 2023 at 20:01):I guess with autoderiving of inspect, we removed our only example of implementing this.
Oskar Hahn (Dec 26 2023 at 21:03):Thank you. That was helpful.
Rational a := (Int a, Int a)
implements [
Eq { isEq: qEq },
Inspect {
toInspector: toInspectorRational,
},
]
toInspectorRational = \@Rational (a, b) ->
f0 <- Inspect.custom
"\(a |> Num.toStr) / \(b |> Num.toStr)"
|> Inspect.str
|> Inspect.apply f0
This works. It prints the value as "5 / 1"
When I use the type in another type and inspect that, I get some not so nice quotation marks:
toInspectorOtherType = \@OtherType (x, y, z) ->
f0 <- Inspect.custom
"\(x |> Inspect.toStr), \(y |> Inspect.toStr), \(z |> Inspect.toStr)"
|> Inspect.str
|> Inspect.apply f0
v = ""1 / 1", "2 / 1", "3 / 1""
How would you do it?
Brendan Hansknecht (Dec 26 2023 at 21:54):Yeah, we may need to expand primitives for inspect. One for escaped string and one for arbitrary strings
Brendan Hansknecht (Dec 26 2023 at 21:55):An important note is that inspect is not guaranteed to output in a string format at all. So it is super flexible which makes defining the exact API a bit more nebulous
Brendan Hansknecht (Dec 26 2023 at 21:57):The reason for this is that inspect can then generate a tree structure that can be understood more thoroughly than. Be used in generating a GUI with collapsible lists, etc.
Brendan Hansknecht (Dec 26 2023 at 23:11):Aside, should other type render as a tuple or list?
Brendan Hansknecht (Dec 26 2023 at 23:12):Not everything needs to be strings
Oskar Hahn (Dec 27 2023 at 20:25):Could you give me an example with tuple?
when I write:
toInspectorOtherType= \@OtherType (x, y, z) ->
f0 <- Inspect.custom
[x, y, z]
|> Inspect.tuple
|> Inspect.apply f0
I get the error, that [x,y,z] is List (Rational a), and not List Inspector f where f implements InspectFormatter. But I don't know how to convert x, y and z to an Inspector. I tried Inspect.inspect an Inspect.apply f0, but this did not work.
Brendan Hansknecht (Dec 27 2023 at 20:33):Inspect.inspect x
Brendan Hansknecht (Dec 27 2023 at 20:33):Just do that to each value in the list
Oskar Hahn (Dec 27 2023 at 20:43):If I write:
toInspectorOtherType = \@OtherType (x, y, z) ->
f0 <- Inspect.custom
[x, y, z]
|> List.map \v -> Inspect.inspect v
|> Inspect.tuple
|> Inspect.apply f0
I get the error:
This expression has a type that does not implement the abilities it's expected to:
341│> [x, y, z]
342│> |> List.map \v -> Inspect.inspect v
The type Inspector does not fully implement the ability
InspectFormatter.
Does it work if you leave out the redundant lambda around Inspect.inspect?
|> List.map Inspect.inspect
No. Its the same error.
I also get the same error, if I write:
f0 <- Inspect.custom
Inspect.tuple [Inspect.inspect 5]
|> Inspect.apply f0
This expression has a type that does not implement the abilities it's expected to:
354│ Inspect.tuple [Inspect.inspect 5]
^^^^^^^^^^^^^^^^^^^
The type Inspector does not fully implement the ability
InspectFormatter.
I must have something mixed up...one second
Oskar Hahn (Dec 27 2023 at 21:00):If I use Inspect.inspect in the repl, it crashes:
roc repl
The rockin' roc repl
────────────────────────
Enter an expression, or :help, or :q to quit.
» Inspect.inspect 5
thread 'main' panicked at '[Abilities] not yet implemented. Tracking issue: https://github.com/roc-lang/roc/issues/2463.
Additional information: Not reachable yet', crates/compiler/mono/src/layout.rs:511:55
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
The error seems outdated. The issue is closed.
I have build roc from source at commit 613c6c4e29ac37f2adcf724fe31af3557eadcdc4 (two days ago)
Brendan Hansknecht (Dec 27 2023 at 21:01):Try Inspect.toInspector
Oskar Hahn (Dec 27 2023 at 21:05):That worked! Thanks!
Here is the working code, if other need it:
toInspectorOtherType = \@OtherType (x, y, z) ->
f0 <- Inspect.custom
[x, y, z]
|> List.map Inspect.toInspector
|> Inspect.tuple
|> Inspect.apply f0
Note, if x, y, and z are not the same type, you won't be able to generate the original list and will have to manually call toInspector on each arg.
Brendan Hansknecht (Dec 27 2023 at 21:12):Cause tuples can be heterogenous
Brendan Hansknecht (Dec 27 2023 at 21:15):Also, if you can autoderive the inspect implementation of opaque types. They will print out as @OpaqueName innerPrintout
So in this case, something like:
@OtherType ("1 / 1", "2 / 1", "3 / 1")
Brendan Hansknecht (Dec 27 2023 at 21:17):Autoderive is like:
OtherType a := (Rational a, Rational a, Rational a)
implements [Inspect]
Brendan Hansknecht said:
Also, if you can autoderive the inspect implementation of opaque types. They will print out as
@OpaqueName innerPrintoutSo in this case, something like:
@OtherType ("1 / 1", "2 / 1", "3 / 1")
Ahh. I did not know this. This is useful.
Brendan Hansknecht (Dec 28 2023 at 02:57):Yeah, can also autoderive other abilities (like Eq and Hash). Probably we need to put that in docs somewhere
Last updated: Jul 26 2025 at 12:14 UTC