(4) Discriminated union errors don’t tell you which variant was matched
First, simple case: field name is unique, you can guess more or less easily:
schema = Zoi.discriminated_union(:type, [
Zoi.map(%{type: Zoi.literal("credit_card"), cvv: Zoi.string() |> Zoi.min(3)}),
Zoi.map(%{type: Zoi.literal("wallet"), wallet_id: Zoi.string()})
])
# input: %{"type" => "credit_card", "cvv" => "1"}
# error: %Zoi.Error{path: [:cvv], code: :greater_than_or_equal_to}
# you can guess "credit_card" because only that variant has :cvv. Not ideal but it works.
Secondly, ambiguous case: same field name in multiple variants, you can’t guess where it is:
schema = Zoi.discriminated_union(:type, [
Zoi.map(%{type: Zoi.literal("retail"), amount: Zoi.integer() |> Zoi.min(1)}),
Zoi.map(%{type: Zoi.literal("refund"), amount: Zoi.integer() |> Zoi.max(0)})
])
# input: %{"type" => "retail", "amount" => 0}
# error: %Zoi.Error{path: [:amount], code: :greater_than_or_equal_to}
# which variant? Both have :amount. No way to tell programmatically.
Zoi resolved the variant internally (it validated amount against the retail map) but that information is discarded and not visible to a user of the library. In the ambiguous case, a consumer (or library) building error messages or routing errors to the right form section has no machine-readable way to determine which variant failed.
Can you include the matched variant somewhere? Either in the path f.ex. [:retail, :amount], or as an extra key in issue opts f.ex. variant: "retail"?
Unsure how would I address this but it’s a DX item and we could discuss it until we arrive at something we’re happy with.
