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

14 Apr 2014


      On April 14, 2014 9:03:20 AM GMT+03:00, Dimitri DeFigueiredo  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