Question

I need to take the list of tuples from a file and multiple 2nd a 3rd values of each tuple. For example: [(1,”A”, 100,2),(2,”B”, 50,3)] . I need to find 100*2=200 and 50* 3=150. I only want to display the final total. That is 350. I am taking the list of tuples from a file. I am getting an error lik this:

- Type error in generator
*** Term           : generator c
*** Type           : Int
*** Does not match : IO a

Code is given below.

type Code=Int
type Price=Int
type Quantity=Int
type Name=String
type ProductDatabase=(Code,Name,Price,Quantity)  
 bill=do
           b<-cart_list_returner     
           let c :: [ProductDatabase]
               c = b    
           w<-generator c
           let r :: String
               r = w
           putStrLn r

    generator::[ProductDatabase]->Int
    generator c=foldl (\a (id,x, y, z) -> a + y*z) 0 c

I just want the program to take the list of tuples in file and produce the total amount. Some one plz help me. thanks in advance

Answer 1

Use the <- binding operator only when you want to run an IO function.

For giving names to results of pure functions, use let.

generator is a pure function — its result is of type Int, not of type IO Int.

So replace the line

       w<-generator c

with

       let w = generator c