gistfile1.hs

makeTrade :: [(String, Maybe String)] -> Maybe Trade

-- using applicative
makeTrade alist =
    Trade <$> lookup1 "account"                      alist
             <*> lookup1 "instrument"                   alist
             <*> (read <$> (lookup1 "market"        alist))
             <*> lookup1 "ref_no"                       alist
             <*> (read <$> (lookup1 "unit_price"    alist))
             <*> (read <$> (lookup1 "quantity"      alist))

-- using monadic lift and ap
makeTrade alist =
    Trade `liftM` lookup1 "account"                      alist
             `ap` lookup1 "instrument"                   alist
             `ap` (read `liftM` (lookup1 "market"        alist))
             `ap` lookup1 "ref_no"                       alist
             `ap` (read `liftM` (lookup1 "unit_price"    alist))
             `ap` (read `liftM` (lookup1 "quantity"      alist))

lookup1 key alist = case lookup key alist of
                      Just (Just s@(_:_)) -> Just s
                      _ -> Nothing