How do we achieve a variable type of struct in smartPy

Question

I am translating following code in solidity to smartpy. How do we achieve a variable type of struct in smartPy.

        Types.AssetTransferDetail[]
            memory _assetDetails = new Types.AssetTransferDetail[](
                _coinNames.length
            );
        for (uint256 i = 0; i < _coinNames.length; i++) {
            _assets[i] = Types.Asset(_coinNames[i], _values[i]);
            _assetDetails[i] = Types.AssetTransferDetail(
                _coinNames[i],
                _values[i],
                _fees[i]
            );
        }

These structs are defined in Types.sol library.

struct Asset {
        string coinName;
        uint256 value;
    }

   struct AssetTransferDetail {
        string coinName;
        uint256 value;
        uint256 fee;
    }

Answer 1

In SmartPy it corresponds to records.

With the new syntax:

import smartpy as sp

@sp.module
def main():
    Asset: type = sp.record(
        coinName=sp.string,
        value=sp.nat,
    )
    AssetTransferDetail: type = sp.record(
        coinName=sp.string,
        value=sp.nat,
        fee=sp.mutez,
    )
    
    class MyContract(sp.Contract):
        def __init__(self, coinNames, values, fees):
            self.data.assets = sp.cast(sp.big_map(), sp.big_map[sp.nat, Asset])
            self.data.assetDetails = sp.cast(sp.big_map(), sp.big_map[sp.nat, AssetTransferDetail])
            for i in coinNames.keys():
                self.data.assets[i] = sp.record(
                    coinName=coinNames[i],
                    value=values[i],
                )
                self.data.assetDetails[i] = sp.record(
                    coinName=coinNames[i],
                    value=values[i],
                    fee=fees[i]
                )

        @sp.entrypoint
        def ep(self):
            pass


if "templates" not in __name__:
    @sp.add_test(name="MyContract")
    def test():
        sc = sp.test_scenario(main)
        c1 = main.MyContract(
            {0: "MyCoin"}, {0: 0}, {0: sp.mutez(5)}
        )
        sc += c1
        c1.ep()

With the legacy syntax:

import smartpy as sp

Asset = sp.TRecord(
    coinName=sp.TString,
    value=sp.TNat,
)
AssetTransferDetail = sp.TRecord(
    coinName=sp.TString,
    value=sp.TNat,
    fee=sp.TMutez,
)

class MyContract(sp.Contract):
    def __init__(self, coinNames, values, fees):
        assets = {}
        assetDetails = {}
        for i in range(len(coinNames)):
            assets[i] = sp.record(
                coinName=coinNames[i],
                value=values[i],
            )
            assetDetails[i] = sp.record(
                coinName=coinNames[i],
                value=values[i],
                fee=fees[i]
            )

        self.init(
            assets=sp.big_map(assets, tkey=sp.TNat, tvalue=Asset),
            assetDetails=sp.big_map(assetDetails, tkey=sp.TNat, tvalue=AssetTransferDetail),
        )

    @sp.entry_point
    def ep(self):
        pass


if "templates" not in __name__:
    @sp.add_test(name="MyContract")
    def test():
        sc = sp.test_scenario()
        c1 = MyContract(
            ["MyCoin"], [0], [sp.mutez(5)]
        )
        sc += c1
        c1.ep()

Something to notice: on Tezos the constructor is never pushed onchain. With the legacy syntax what you can and cannot do inside the init is different from what you cna do in an entrypoint. If you want the same logic inside an entrypoint you have to make to changes: instead of giving three lists you may prefer give one, you'll have to use sp.if instead of if.

Mind you, this translation is an example. It's not necessarily the best way to create this contract or something that I would approve for production.