I have a function using the following 2 types:

pub type BalanceOf<T> = <<T as pallet::Config>::Currency as Currency<<T as frame_system::Config>::AccountId>>::Balance;
pub type AccountIdOf<T> = <T as frame_system::Config>::AccountId;

and I use them to implement a method in a struct using generics:

#[derive(Debug, PartialEq, Encode, Decode)]
pub struct Investor<T,U>{
    pub account_id:T,
    pub nft:U,

impl<T:pallet::Config,U> Investor<T,U>{
    pub fn contribute(origin:OriginFor<T>,acc:AccountIdOf<T> ,value:BalanceOf<T>) -> DispatchResult{
        let c1=Contribution::new(&acc,&value);
        let _who = ensure_signed(origin)?;
        let _now = <frame_system::Pallet<T>>::block_number();
        if ContributionsLog::<T>::contains_key(c1.account){
            ContributionsLog::<T>::mutate(c1.account, |val|{
                *val += *c1.amount;
        } else {
        //function taking contribution storage and amount as inputs here

the problem is when I try to call the contribute method as below


I get the following error:

Cannot infer type for type parameter 'T'

how/where should I define an explicit type for T?

1 Answer 1


Rather than call Investor::contribute(who,dev.account_id,rent);

Seems you should call:

Investor::<T::AccountId, u32>::contribute(who,dev.account_id,rent);

And just be explicit with your type definitions of the generics when accessing the contribute function.

  • Thank you! I have a follow up question: let's say that I call the contribute method inside a function that has the following parameters : fn do_something(origin: OriginFor<T>, something: u32, acc:T::AccountId ,rent:BalanceOf<T>,cd:CID,prop:Properties,start:Option<BlockNumberOf<T>>,end:Option<BlockNumberOf<T>>) -> DispatchResult Rust considers that T is a frame_system::Config>::AccountId and therefore does not have the right T to correctly define for example let _who = ensure_signed(origin)?; Commented Mar 15, 2022 at 10:29
  • I'm not sure what you are asking. Can you open a new question with a bit more clarity?
    – Shawn Tabrizi
    Commented Mar 15, 2022 at 12:32
  • Sorry for the confusion, I think a look a the full code will make it clear: github.com/Fair-Squares/fair-squares/blob/structure_template/… On the branch "structure_template" the line 144 when uncommented prevents me from using OriginOf. This line is calling the method "contribute" mentioned earlier. Commented Mar 15, 2022 at 13:12
  • 1
    In that case, It should be Investor::<T::AccountId, u32>::contribute(who,dev.account_id,rent);, or pub account_id:T::AccountId,
    – Shawn Tabrizi
    Commented Mar 15, 2022 at 14:36

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