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Most of the example FA2 implementations I've found, as well as the deployed contracts I've investigated have the required FA2 entrypoints (transfer, balance_of, update_operators) buried in the second level of the parameter.

Simplifying the linked example from minter-sdk:

type fa2_entry_points =
  | Transfer of transfer list
  | Balance_of of balance_of_param
  | Update_operators of update_operator list

type nft_asset_entrypoints =
  | Assets of fa2_entry_points
  | Admin of admin_entrypoints

let nft_asset_main (param, storage : nft_asset_entrypoints * nft_asset_storage)
    : operation list * nft_asset_storage =
  match param with
  | Assets fa2 ->
    let u = fail_if_paused(storage.admin) in
    let ops, new_assets = fa2_main (fa2, storage.assets) in
    let new_storage = { storage with assets = new_assets; } in
    ops, new_storage
...

Here, the FA2 entrypoints are keyed under "Assets," which is what I've seen in most deployed contracts as well, presumably because they're derived from the minter-sdk contracts.

This organization is more convenient from a code modularity perspective, but if this were compliant with the spec it seems like it would make it impossible to call otherwise compliant FA2 contracts in a general way because you'd need to know at what level their entrypoints live. I don't know Michelson so apologies if my terminology isn't right. Are contracts like this still considered FA2 compliant due to ambiguity of the spec, or are they actually not quite compliant?

2 Answers 2

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You don't need to know the position of an entrypoint to call it. For this reason, the position does not matter much so it is not imposed by the standard.

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  • I figured this might be the case, but since the parameters encode to michelson differently it didn't seem right. Are entrypoint "names" unique then?
    – Nox
    Dec 16, 2021 at 23:18
  • Yes, entrypoints are uniquely identified by their names. Jan 3 at 15:25
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I found a further explanation of how entrypoints work in Michelson here. The names of entrypoints are unique, so specifying the name of an entrypoint when passing the parameter will automatically wrap it.

[A] contract cannot define two entrypoints with the same name.

An external transaction can include an entrypoint name alongside the parameter value. In that case, if there is a constructor annotation with this name at any position in the nesting of or types, the value is automatically wrapped into the according constructors.

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