Convert String to Integer/Float in Haskell?

Question

data GroceryItem = CartItem ItemName Price Quantity | StockItem ItemName Price Quantity  makeGroceryItem :: String -> Float -> Int -> GroceryItem makeGroceryItem name price quantity = CartItem name price quantity  I want to create a `GroceryItem` when using a `String` or `[String]`  createGroceryItem :: [String] -> GroceryItem createGroceryItem (a:b:c) = makeGroceryItem a b c 

The input will be in the format ["Apple","15.00","5"] which I broke up using Haskell's words function.

I get the following error which I think is because makeGroceryItem accepts a Float and an Int.

*Type error in application *** Expression     : makeGroceryItem a read b read c *** Term           : makeGroceryItem *** Type           : String -> Float -> Int -> GroceryItem *** Does not match : a -> b -> c -> d -> e -> f* 

But how do I make b and c of type Float and Int, respectively?

Answer 1

read can parse a string into float and int:

Prelude> :set +t Prelude> read "123.456" :: Float 123.456 it :: Float Prelude> read "123456" :: Int 123456 it :: Int 

But the problem (1) is in your pattern:

createGroceryItem (a:b:c) = ... 

Here : is a (right-associative) binary operator which prepends an element to a list. The RHS of an element must be a list. Therefore, given the expression a:b:c, Haskell will infer the following types:

a :: String b :: String c :: [String] 

i.e. c will be thought as a list of strings. Obviously it can't be read or passed into any functions expecting a String.

Instead you should use

createGroceryItem [a, b, c] = ... 

if the list must have exactly 3 items, or

createGroceryItem (a:b:c:xs) = ... 

if ≥3 items is acceptable.

Also (2), the expression

makeGroceryItem a read b read c 

will be interpreted as makeGroceryItem taking 5 arguments, 2 of which are the read function. You need to use parenthesis:

makeGroceryItem a (read b) (read c)