I am a beginner at learning Rust/substrate/Polkadot. I am trying to build my custom substrate pallet

My goal is, Create a custom Substrate pallet that allows only the Root origin account to control the list of group members to a set of accounts for at least two different groups, where

  • Only the Root origin account can add an account as a member of a group in on-chain storage
  • Only the Root origin account can remove an account from being a member of a group in on-chain storage

There are many Rust errors. Anyone who can teach me. I attached my working source code.

// The pallet's runtime storage items.
// https://substrate.dev/docs/en/knowledgebase/runtime/storage
decl_storage! {
    trait Store for Module<T: Config> as GroupModule {
        pub Group get(fn get_group): map hasher(blake2_128_concat) T::AccountId => Vec<u8>;
// Errors inform users that something went wrong.
decl_error! {
    pub enum Error for Module<T: Config> {
decl_module! {
    pub struct Module<T: Config> for enum Call where origin: T::Origin {
        // Errors must be initialized if they are used by the pallet.
        type Error = Error<T>;

        // Events must be initialized if they are used by the pallet.
        fn deposit_event() = default;

        #[weight = 10_000 + T::DbWeight::get().reads_writes(1, 1)]
        pub fn add_group_member(
            new_account: T::AccountId,          
            group_no: u8,
        ) -> dispatch::DispatchResult {

            // Can only be called by the root.

            // Read a vector value from storage.
            let mut groups = <Group<T>>::get(&new_account);
            //match <Group<T>>::get(&new_account) {  //error: mismatched types
                //None => Err(Error::<T>::AccountNotFound)?, //error
            //  groups => {
                    if groups.contains(&group_no) {   //error[E0308]: mismatched types
                    } else {  //error: expected enum `Result`, found `()`
                        // Update storage.
                        <Group<T>>::insert(&new_account, &groups);
                        // Emit an event.
                        Self::deposit_event(RawEvent::GroupMemberAdded(new_account, groups));
                        // Return a successful DispatchResult

Thank you.

  • 1
    Run the tutorials first and try to understand what is going on. 2 focus on the errors from the compiler, edit this post and include the errors.
    – Simson
    May 10, 2022 at 13:12
  • 1
    I recommend NOT to use the old decl_*! syntax, and instead use the newest attribute macros in FRAME v2. Otherwise, yes, you need to post information about the errors you are facing.
    – Shawn Tabrizi
    May 10, 2022 at 18:15

1 Answer 1


This is using the decl_* macro syntax. It is out-dated and got recently removed from the Substrate code base after 6 months grace period.

We added a bunch of example pallets that show of the new syntax here and can help you get started with it.

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