Question

i’m trying to select some lists from a list when a condition is true, something like this:

I’ve made a data structure -> data File = File {name :: String, size :: Integer, comment :: String} deriving Show

And i have already made a library with all the files following this structure:

files = [[“name1″,size1,”coment1”],[“name2″,size2,”coment2”],[“name3″,size3,”coment3”],…]

Now what i need is a function that selects me all the lists which the sizes are for example >= 500, something like

list = select ((>=500.size) files)

So if i had:

files = [["asd",345,"coment1"],["fgh",678,"coment2"],["hjk",123,"coment3"],...]

I would get:

list = [["fgh",678,"coment2"]]

Any help would be gladly appreciated.

Thanks in advance.

Answer 1

The Prelude contains the useful

filter :: (a -> Bool) -> [a] -> [a]

which does what you intend your select to do.

Answering your comment to Jon Purdy’s answer:

filter ((>= 500) . size) files

Aside:

files = [["asd",345,"coment1"],["fgh",678,"coment2"],["hjk",123,"coment3"],...]

won’t work, lists are homogeneous. It should in the context of the question be

files = [File "asd" 345 "coment1", File "fgh" 678 "coment2", ... ]

File having been defined with record-syntax, you can use it with record syntax or vanilla positional syntax, whatever is better in a given situation. Record syntax would be more typing than the above, but if you used it, files = [File{ name = "asd", size = 345, comment = "coment1" }, ... ] would continue to work if you added fields to the type – the added fields would then be instantiated with undefined, which may or may not be better than the code not compiling without changes.