[Haskell-beginners] How do I use Guards in record syntax?

[Gesh]

On April 14, 2014 9:03:20 AM GMT+03:00, Dimitri DeFigueiredo <defigueiredo at ucdavis.edu> wrote:
>
>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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Note that this has nothing to do with record syntax specifically. Rather, what you're asking is how to write a multi-way if expression. Your way is to introduce a local binding using a let statement, which allows you to use pattern guards as you did.
Usually, bowever, you'd use a case statement to avoid the binding. However, you could use the MultiWayIf LANGUAGE pragma, as suggested elsewhere in this thread. Or you could float out the binding to a where clause, except that doesn't seem to be what you're looking for.
Hoping to help,
Gesh