It might not be possible to reference types in other types

[MichaelXavier]

Given the minimal example file:

data Sum = Foo | Bar

data Baz = Baz
  { sum :: Sum
  }

main = do
  Text.putStrLn "hello world"

You'll get an error along the lines of Unknown type: TyCon "Sum". You get a similar thing if you reference Sum by Main.Sum. If you instead define some other record Quux and add a field to Baz of that type, you get the same kind of error. Because this affects both records (desugars to a Record type) and sums (desugars to a Variant type), maybe the syntactic sugar allows you to handle the values directly but isn't available to the type system?

[chrisdone]

That's right. Presently you can't reference user-defined types in your user-defined types. I plan to lift that limitation. 👌 Just needs a simple dependency analysis and then referencing Sum in Baz would be like a type-alias expanded at desugaring time to the tagged variant Tagged "Sum" ....

But recursively defined types present a challenge, so those would in the near-term not be supported.

[chrisdone]

It's probably rare that someone would want to define a recursive type for a shell script, so not sure it's worth the effort. There's also Data.Tree that could serve a lot of cases for scripting, which I was thinking to add to support the "tree-like structures" use-case.

[MichaelXavier]

@chrisdone Just saw that you added referencing custom types within records and snatched it up into my config to test it out. Nesting record types within records seems to work as expected, but sums seem to have a quirk. If you do this:

data MyRecord = MyRecord {sum :: Main.MySum}

data MySum = MySumL | MySumR

main = do
  let myRecord = Main.MyRecord {sum = Main.MySumR}
  Text.putStrLn "hello world"

You'll get an error:

hell: Unification error: Couldn't match type

  "Main.MySum"

against type

  "MySum"

I tried a few different permutations but it seems like the sum must be constructed with the Main module qualifier.

[chrisdone]

I fixed this in Ensure user-defined sum/record type are qualified by chrisdone. It was as suspected, lack of qualification. I should use a more disciplined type for names, but was thus far getting away with dodging it. ;-)

[chrisdone]

Oh, thanks for trying it! Looks like I’ve been inconsistent in a key area in terms of generating the name of the type. It should be qualified, and probably it was not qualified previously in the constructor. I’ll fix this when I’m back at keyboard.

On Wed, Jan 1, 2025, at 8:06 PM, Michael Xavier wrote: @chrisdone <https://github.com/chrisdone> Just saw that you added referencing custom types within records and snatched it up into my config to test it out. Nesting record types within records seems to work as expected, but sums seem to have a quirk. If you do this: data MyRecord = MyRecord {sum :: Main.MySum} data MySum = MySumL | MySumR main = do let myRecord = Main.MyRecord {sum = Main.MySumR} Text.putStrLn "hello world" You'll get an error: `hell: Unification error: Couldn't match type "Main.MySum" against type "MySum"

`

…

I tried a few different permutations but it seems like the sum must be constructed with the `Main` module qualifier. — Reply to this email directly, view it on GitHub <#64 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAACWC3XXX4CZ7DWS7BC26D2IRDEDAVCNFSM6AAAAABTVAE6N2VHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTCNZRGIYDKOA>. You are receiving this because you were mentioned.Message ID: ***@***.***>

[MichaelXavier]

That fixed it! Thank you!