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?
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)