CW2.hs

type Name = String
type Customer = (Name, Int, Int, Int)
type VStore = [Customer]

-- BEGIN TEST DATA --

cust1 :: Customer
cust1 = ("Neil", 311, 271, 345)

cust2 :: Customer
cust2 = ("Fred", 0, 271, 0)

vstore :: VStore
vstore = [cust1, cust2]

-- END TEST DATA --

myloans :: VStore -> Name -> [Int]
myloans vs n = loans $ head $ filter (\(name,_,_,_) -> name == n) vs
    where loans (_,a,b,c) = [l | l <- [a,b,c], l /= 0]

members :: VStore -> [Name]
members vs = [n | (n,_,_,_) <- vs]

owners :: VStore -> Int -> [Name]
owners vs id = [n | (n,a,b,c) <- vs, a == id || b == id || c == id]

borrow :: VStore -> Name -> Int -> VStore
borrow vs n id = map borrow' vs
    where
        borrow' (name,a,b,c)
            | name == n = loan (name,a,b,c)
            | otherwise = (name,a,b,c)
        loan (name,a,b,c)
            | a == 0    = (name,id,b,c)
            | b == 0    = (name,a,id,c)
            | c == 0    = (name,a,b,id)
            | otherwise = (name,a,b,c)

return :: VStore -> Name -> Int -> VStore
return vs n id = map return' vs
    where
        return' (name,a,b,c)
            | name == n = loan (name,a,b,c)
            | otherwise = (name,a,b,c)
        loan (name,a,b,c)
            | a == id   = (name,0,b,c)
            | b == id   = (name,a,0,c)
            | c == id   = (name,a,b,0)
            | otherwise = (name,a,b,c)