I'm having some trouble understanding where I can or cannot use guards inside record syntax. I'm writing a simple conversion routine, but I am not able to write it without inserting an extra let. Do I need a let expression here? Am I missing something?

--------------
data OldTrade = OldTrade {
    oldprice   :: Double   ,
    oldamount  :: Double   ,
    oldbuysell :: String  -- "buy", "sell" or ""
    } deriving( Eq, Show)


data BuyOrSell = Buy | Sell | Unknown deriving(Eq, Show)

data Trade = Trade {
    price   :: Double   ,
    amount  :: Double   ,
    buysell :: BuyOrSell
    } deriving( Eq, Show)

convert :: OldTrade -> Trade

convert ot  = Trade { price   = oldprice  ot,
                      amount  = oldamount ot,
                      buysell = let x | oldbuysell ot == "buy"  = Buy
                                      | oldbuysell ot == "sell" = Sell
                                      | otherwise               = Unknown
                                in x
                    }

-- how do I eliminate the 'let' expression here?
-- I wanted to write something like:
--
--                      buysell | oldbuysell ot == "buy"  = Buy
--                              | oldbuysell ot == "sell" = Sell
--                              | otherwise               = Unknown

--------------

Thanks!

Dimitri




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