quicksort orderby id asc

How can I use quicksort开发者_运维知识库 to orderby ID ascending on the list and then show elements ? I have error: No instance for (Ord FigureType). My code is:

showRectangles [] = "No rectangles"
showRectangles x = concat (map showRectangle (qsort x))

showRectangle :: FigureType -> String
showRectangle (Figure id width height) = "id: " ++ show id ++ " width: " ++ show width ++ " height: " ++ show height ++ "\n";

data FigureType = Figure Int Int Int deriving(Show, Read)

qsort [] = []
qsort (x:xs) = qsort (filter (< x) xs) ++ [x] ++ qsort (filter (>= x) xs)

You need to make FigureType comparable. The simplest thing to do is to make it derive Eq and Ord.

data FigureType = Figure Int Int Int deriving(Show, Read, Ord, Eq)

BTW, there is already a sort function in Data.List.

The short answer is that it don't know what < and >= means. You haven't told haskell that in that program.

To solve this, you must make an instance Ord FigureTye. I assume you've seen the syntax before. If not, you can look for haskell type classes. Since Ord is a subclass of Eq, you must also create the instance Eq for FigureType, however, this can just be derived by using deriving(Show, Read, Eq). You could also write deriving(Show, Read, Eq, Ord). But the auto-generated instance for Ord is probably not the implementation you have in mind.

Here is a link with the Ord typeclass, it says what minimal amount of functions you have to implement. http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.html#t:Ord

Here is Real world haskells chapter on type-classes. It doesn't cover any instance of Ord however. http://book.realworldhaskell.org/read/using-typeclasses.html