haskell - How to search for matching items in a list? -

i'm doing exercise practice exam tomorrow. text tells me implement database library, define items can books or magazines. each book save name + author. each magazine save name:

 data item = book string string       | magazine string       deriving(show)  data requisition = req string item  type database = [requisition]  exdb :: database exdb = [req "john" (book "pf" "hs"),req "jay"(book "apple" "steve jobs"),req "francis"(magazine "forbes")]  books :: database -> string -> [item] books db name = {-- what's next?-} 

now, need create function named books :: database -> string -> [item], searches name on database , gives me books person requested.

how do this?

this straightforward using filter function:

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

it takes condition apply each element of list, returns list of elements meet condition. in case, want books have been requested given person. can break down 2 problems:

1. find requests given person
2. find books in requisition list

to solve first, say

requestsfromperson :: database -> string -> database requestsfromperson db name = filter matchesname db             matchesname (req name' item) = name == name' 

the second step can done filter, i'm going let fill in implementation

onlybooks :: database -> database onlybooks db = filter ??? db 

then can combine these two:

books db name = onlybooks (requestsfromperson db name) 

so have fill in ??? onlybooks , should work! hint: pattern matching friend.