How to use an Integer when computing a Float

Question

totalPrice :: Product -> Integer -> Float    
totalPrice product x = case product of  

    Ramen    
    | x <= 10 -> 2 * x    
    | x <= 30 -> 1.8 * x   
    | x <= 100 -> 1.5 * x   
    | x <= 200 -> 1.4 * x   
    | x > 200 -> 1.35 * x   
    | otherwise -> 0   

Chips
    | x <= 2 -> 3 * x
    | x <= 5 -> 2.95 * x
    | x <= 10 -> 2.7 * x
    | x <= 20 -> 2.5 * x
    | x > 20 -> 2.35 * x
    | otherwise -> 0

When I compile this code it get error "Couldn't match expected type ‘Double’ with actual type ‘Integer’"

Any suggestions?

As a rule of thumb to help you decide what to look at when debugging problems in future: if you get a type error ("cannot match expected type ... with actual type ..."), it usually means that there is no issue with your syntax. In this example the problem had noting to do with guards or cases, you just used an operation that wasn't supported by your type. If you had a problem with the guard or case syntax Haskell wouldn't be able to figure out enough of what you were trying to do to even begin checking types. — Ben

Li-yao Xia Li-yao Xia · Accepted Answer · 2017-09-08T02:04:59

x is an Integer, and you are multiplying it with decimal values to produce a Float. The operands and result of arithmetic operators like (*) must be of the same type, and you must convert between types explicitly.

totalPrice product y = case product of
  Ramen
    | x <= 10 -> 2 * x
    ...
  where
    x = fromInteger y

How to use an Integer when computing a Float

1 Answers