Haskell - Selectively Adding Lists

Question

I have the following type:

type Rating = (String, Int)
type Film = (String, String, Int, [Rating])

testDatabase :: [Film]
testDatabase = [(\"Director 1\",\"         


        

Answer 1

There are many useful functions in Data.List for data analysis.

In particular, groupBy is super useful:

> :t groupBy
groupBy :: (a -> a -> Bool) -> [a] -> [[a]]

Given an equality function, group a list into buckets.

A function to access the directory:

 > let fst4 (x,_,_,_) = x

Then, sort on the director name, and bucket (group) by that director.

 > let db0 = sortBy (comparing fst4) testDatabase
 > let db1 = groupBy ((==) `on` fst4) db0
 [ [("Director 1","Film 1",2012, [("TestRat",8)])]
 , [("Director 2","Film 2",2    ,[])]
 ]

groupBy is very useful...