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

14 Apr 2014

      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  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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