Translate possible ErrBalanceTx constructors to ErrCreatePayment

[david-a-clark]

The problem that you wish to solve

The current error handling in createPayment wraps the entire ErrBalanceTx type, including error cases that are impossible in the deposit wallet context. This makes the code less maintainable and creates confusion about which errors users might actually encounter. We need to streamline the error types to only include relevant, actionable errors specific to the payment creation process.

Description

Change createPayment’s ErrCreatePayment from wrapping ErrBalanceTx to inlining the subset of cases that are possible in the context of the deposit wallet. Impossible errors can be thrown with error (if we later realize we want to throw in IO, we can handle that later).

ACs

• ErrCreatePayment must not wrap ErrBalanceTx
• ErrCreatePayment must not contain cases which are impossible from the context of createPayment
• createPayment must never hit an error from translateBalanceTxError
• Each error in ErrCreatePayment should have a clear and documented cause and be actionable by the user

Implementation suggestions

Sketch

data ErrCreatePayment
    = NotEnoughAda { available :: Coin, shortfall :: Coin, required :: Coin }
    | EmptyUTxO -- probably merge into NotEnoughAda
    | UnableToConstructChange -- probably merge with the above

    | TooManyPayments -- partial tx too big
    | TxMaxSizeLimitExceeded { ? } -- balanced tx too big; selection_too_big
    -- Potential solutions:
    -- 1. Fewer outputs
    -- 2. Less total ada
    -- 3. Fund wallet with more ada
    -- 4. Send ada to self to create bigger UTxOs
    -- 5. Just retry for different random result
    --
    -- Long term: we want larger utxos if possible

    | TxOutMinAdaInsufficient { out :: TxOut, recommendedAda :: Coin }

-- Translate possible ErrBalanceTx cases to ErrCreatePayment.
-- Impossible cases should be thrown with 'error'
translateBalanceTxError :: ErrBalanceTx -> ErrCreatePayment
translateBalanceTxError = error "todo"

-- | Create a payment to a list of destinations.
createPayment
    :: Read.EraValue Read.PParams
    -> Write.TimeTranslation
    -> [(Address, Write.Value)]
    -> WalletState
    -> Either ErrCreatePayment Write.Tx