Haskell列表-计算元组的fst并与列表长度进行比较

Question

I have the following list:

[(Libri,50.0),(Proxis,20.0),(Proxis,45.0),(Amazon,45.0)]

And I have a certain List called articles . I want to transform the list above, with tuples so that if I have (length articles) times the first element of a tuple showing up in the list, get only that!

So, for example, if I have a list articles=["HP","Haskell"] , the list should show:

[(Proxis,20.0),(Proxis,45.0)]

Because Proxis shows up two times!

Edit:

Data Types:

data Magasin = Proxis | Amazon | Libri deriving (Eq, Show)
type Article = String
type Prix = Float
data Entree = E Magasin Article Prix deriving (Eq, Show)
type Stock = [Entree]

This is my current code:

disponible::[Article]->Stock->[(Magasin,Float)]
disponible [] stk = []
disponible (art:reste) stk = (foldl(\acc (E m a p)->if a==art then (m,p):acc else acc) [] stk)++(disponible reste stk)

Any ideas?

Edit:

disponible::[Article]->Stock->[(Magasin,Float)]
disponible articles stock = map(\(m,ls)->(m,sum $ map (\(E _ _ p)->p) ls)) $ filter ((==length articles).length.snd) magasinsArticles
     where contientArticles = (filter (\(E _ a _)->a`elem`articles) stock)
           magasins = foldl (\acc e-> if e`elem`acc then acc else (e:acc)) [] $ map (\(E m _ _)->m) contientArticles
           magasinsArticles = map (\m->(m,filter(\(E m2 _ _)->m2==m) contientArticles)) magasins

Found the answer I was looking for.

Answer 1

disponible::[Article]->Stock->[(Magasin,Float)]
disponible articles stock = map(\(m,ls)->(m,sum $ map (\(E _ _ p)->p) ls)) $ filter ((==length articles).length.snd) magasinsArticles
     where contientArticles = (filter (\(E _ a _)->a`elem`articles) stock)
           magasins = foldl (\acc e-> if e`elem`acc then acc else (e:acc)) [] $ map (\(E m _ _)->m) contientArticles
           magasinsArticles = map (\m->(m,filter(\(E m2 _ _)->m2==m) contientArticles)) magasins