Thanks everyone :-)

I think the "case of" is what I was looking for. I keep thinking of using "case of" as in pattern matching to find the "Shape" of the result of an expression and of using guards to evaluate predicates. Forgetting that True and False are constructors themselves. I just have to change that mindset.

Cheers,

Dimitri


Em 14/04/14 09:04, Gesh escreveu:

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