如何在Yesod / Persistent中正确使用runDB

Question

I have the following persistent model:

User
    ident Text
    password Text Maybe
     UniqueUser ident
    deriving Typeable

Payment
    timestamp UTCTime
    from UserId
    to UserId
    amount Int

I am trying to build a page that displays a list of all payments in the database, with for each payment links to the users that paid / received the payment. Currently my code for this is:

getPaymentsR :: Handler Html
getPaymentsR = do
    let payments = map addFromTo $ runDB $ selectList [] [Desc PaymentTimestamp]
    defaultLayout
        [whamlet|
            <h3> Payments
            <ul>
                $forall (key, value, from, to) <- payments
                    <li>
                        <a href=@{UserR $ paymentFrom value}> #{from} </a> paid #{paymentAmount value} to <a href=@{UserR $ paymentTo value}> #{to} </a> on #{printDay $ paymentTimestamp value}
            $if null payments
                <ul>
                    <li> No payments
        |]
    where 
        addFromTo :: Entity Payment -> (Key Payment, Payment, Text, Text)
        addFromTo (Entity key val) = do
            let from = runDB $ get404 $ paymentFrom val
            let to = runDB $ get404 $ paymentTo val
            (key, val, userIdent from, userIdent to)

However I am getting the following errors:

Handler/Payment.hs:9:36:
    Couldn't match expected type `[Entity Payment]'
                with actual type `HandlerT site0 IO [Entity Payment]'
    In the second argument of `($)', namely
      `runDB $ selectList [] [Desc PaymentTimestamp]'
    In the expression:
      map addFromTo $ runDB $ selectList [] [Desc PaymentTimestamp]
    In an equation for `payments':
        payments
          = map addFromTo $ runDB $ selectList [] [Desc PaymentTimestamp]

Handler/Payment.hs:26:34:
    Couldn't match expected type `User'
                with actual type `HandlerT site0 IO User'
    In the first argument of `userIdent', namely `from'
    In the expression: userIdent from
    In a stmt of a 'do' block: (key, val, userIdent from, userIdent to)

Handler/Payment.hs:26:50:
    Couldn't match expected type `User'
                with actual type `HandlerT site1 IO User'
    In the first argument of `userIdent', namely `to'
    In the expression: userIdent to
    In a stmt of a 'do' block: (key, val, userIdent from, userIdent to)

Does anybody know what I am doing wrong, or how to solve these errors? I believe it has a lot to do with Extracting database field values inside a Handler . However I was still unable to resolve the problem with the information in above link.

Update

After editing my code using Michael Snoyman's answer, my new addFromTo function is still giving type errors:

addFromTo :: Entity Payment -> (Key Payment, Payment, Text, Text)
addFromTo (Entity key val) = do
    from' <- runDB $ get404 $ paymentFrom val
    to' <- runDB $ get404 $ paymentTo val
    let from = userIdent from'
    let to = userIdent to'
    (key, val, from, to)

Answer 1

The issue here is that you're treating a monadic action as a pure value. The easier way to resolve this is to use do-notation and the slurp operator:

payments' <- runDB $ selectList [] [Desc PaymentTimestamp]
let payments = map addFromTo payments'

Or if you want to get fancier (not necessarily recommended):

payments <- fmap (map addFromTo) $ runDB $ selectList [] [Desc PaymentTimestamp]