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)