(please note that the below is just a rough design - feel free to adapt if needed)

• module_validate_xyz() - a shiny module executing validation:
  • srv_validate_xyz(x) - it does the following:

     check_1 <- module_check_foo(x, ...)
     check_2 <- module_check_bar(x, ...)
     check_3 <- module_check_baz(x, ...)
     ...
     checks <- list(check_1, check_2, check_3, ...)
     r_validate <- reactive({
       for (check in checks) {
         validate(need(check()$isEmpty(), check$getMessages()))
       }
       x()
     })
     output$errors <- renderUI({
       r_validate()
       NULL
     })
     x
    

  • ui_validate_xyz() - display error message:

     tagList(
       uiOutput(NS(id, "errors"))
     )
    

    • we might assign a dedicated CSS class and apply custom styling of that element.
  • The goal is to allow nesting calls of validate module from another validate module
• module_check_xyz() - a shiny module executing checks:
  • srv_module_check_xyz() - returns reactive with the AssertCollection object (see: https://mllg.github.io/checkmate/reference/AssertCollection.html)
  • ui_module_check_xyz() - empty fun (we can even don't create it at all)
• check_xyz() - for custom checks - only if needed, function returning TRUE if ok else error message (similar to checkmate)
• test_xyz() - for custom checks - only if needed, function returning boolean (similar to checkmate)

Extra:
I do expect quite a few module_check_xyz() functions, e.g. module_check_reactive(), module_check_tryerror(), module_check_shinysilenterror(), module_check_qenverror. Instead of exporting a lot of objects, we might create a factory to create them dynamically if needed. (or both?)

The goals it to have a functionality that is:

• testable
• extendable - e.g. one validate module calling another validate module
• returning consistent validation errors

The use cases:

• teal:::srv_validate_reactive_teal_data
• teal modules (I assume this is going to be exported)