Filter a list of my own type - Tuples?

How can I filter a list of this type by the third item in the tuple:

type Car = (S开发者_JAVA技巧tring, [String], Int [String])

I saw the snd and fst methods but here i dont think this will work and im not sure how to map without using the '_' wildcard.

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There aren't any pre-defined functions like fst or snd for tuples with more than two elements. As you said, you can use pattern matching and the wild card _ to do the job.

 cars = [ ("Foo", ["x", "y"], 2009, ["ab", "cd"]
        , ("Bar", ["z"],      1997, [])
        ]

 newCars = filter condition cars
     where condition (_, _, n, _) = n > 2005

However, this is usually a sign that you should change from using tuples to a record type.

 data Car = Car { model :: String
                , foo   :: [String]
                , year  :: Int
                , bar   :: [String] 
                }

 cars = [ Car "Foo" ["x", "y"] 2009 ["ab", "cd"]
        , Car "Bar" ["z"]      1997 []
        ]

Now, you can use model, foo, year and bar like you would use fst and snd on tuples.

 newCars = filter ((> 2005) . year) cars

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Or you could just use Data.Tuple.Utils?

MissingH is full of other good stuff too; almost all of my projects use it somewhere or other.

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Here is my solution for a similar problem:

  --construct a list of stock records with the number in stock less than the reorder level.

  getstock (name, stock, reorder) = stock
  getreorder (name, stock, reorder) = reorder

  check l = filter (\x ->(getstock x < getreorder x)) l

  main = print(check [("RAM",9,10),("ROM",12,10),("PROM",20,21)]); --[("RAM",9,10),("PROM",20,21)] 

The key was to understand that the filter function takes a predicate, not a boolean value.

So, simply saying

filter(getstock < getreorder)

would not work,

while

`filter (getstock < 10)

would