Convert String to Number in Haskell?

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)

Even though this question already has an answer, I strongly suggest using reads for string conversion, because it's much safer, as it does not fail with an unrecoverable exception.

reads :: (Read a) => String -> [(a, String)]

Prelude> reads "5" :: [(Double, String)]
[(5.0,"")]
Prelude> reads "5ds" :: [(Double, String)]
[(5.0,"ds")]
Prelude> reads "dffd" :: [(Double, String)]
[]

On success, reads returns a list with exactly one element: A tuple consisting of the converted value and maybe unconvertable extra characters. On failure, reads returns an empty list.

It's easy to pattern-match on success and failure, and it will not blow up in your face!

Two things:

createGroceryItem [a, b, c] = makeGroceryItem a (parse b) (parse c)
-- pattern match error if not exactly 3 items in list

or alternatively

createGroceryItem (a : b : c : _) = makeGroceryItem a (parse b) (parse c)
-- pattern match error if fewer than 3 items in list, ignore excess items

because : is not the same as ++.

Meanwhile on the right hand side --- the side that's giving you the error message you see --- you have to group expressions using brackets. Otherwise parse is interpreted as being a value you want to pass to makeGroceryItem, so the compiler complains when you try to pass 5 arguments to a function that only takes 3 parameters.