What's the difference between calling T::Currency::transfer() and T::Currency::withdraw()

Question

In the crowdfund pallet linked here:

https://github.com/substrate-developer-hub/substrate-how-to-guides/blob/main/example-code/template-node/pallets/simple-crowdfund/src/lib.rs

the withdraw function works by calling the T::Currency::withdraw() fn in currency trait and passing that return value into the T::Currency::resolve_into_existing() fn instead why can i just use T::Currency::transfer like its done in the contribute() fn but swap the source and dest ?

Answer 1

A great resource for this is to just look at the definitions of transfer and withdraw for the Currency trait inside of the Substrate repo.

Transfer:

    /// Transfer some liquid free balance to another staker.
    ///
    /// This is a very high-level function. It will ensure all appropriate fees are paid
    /// and no imbalance in the system remains.
    fn transfer(
        source: &AccountId,
        dest: &AccountId,
        value: Self::Balance,
        existence_requirement: ExistenceRequirement,
    ) -> DispatchResult;

Withdraw

    /// Removes some free balance from `who` account for `reason` if possible. If `liveness` is
    /// `KeepAlive`, then no less than `ExistentialDeposit` must be left remaining.
    ///
    /// This checks any locks, vesting, and liquidity requirements. If the removal is not possible,
    /// then it returns `Err`.
    ///
    /// If the operation is successful, this will return `Ok` with a `NegativeImbalance` whose value
    /// is `value`.
    fn withdraw(
        who: &AccountId,
        value: Self::Balance,
        reasons: WithdrawReasons,
        liveness: ExistenceRequirement,
    ) -> Result<Self::NegativeImbalance, DispatchError>;

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Most runtimes use the pallet_balances inside the Substrate repo that implements the Currency trait so reading the implementation of both methods in this pallet can also help a lot to understand how they work and what they do.