Question

I’m doing some simple regexp work in Haskell using this excellent tutorial as a basis, and I’m following the author’s advice of using explicit type signatures to get the =~ regexp operator to return a String. My problem is that I want to then manipulate this further (basically toUpper the result) and I can’t quite see how to do this.

I’ve tried each of the below and with each I get a Couldn't match expected type 'String' against inferred type 'Char' error or similar:

getSKU :: FilePath -> String
getSKU path = 
    toUpper $ path =~ "^sku[0-9]{5}" :: String

-- or...
getSKU path = 
    let key = (path =~ "^sku[0-9]{5}")
    in toUpper key

-- or ...
getSKU = 
    toUpper . sub
    where
        sub p = (p =~ "^sku[0-9]{5}") :: String

I’m stumped – how do I express a type signature part way through an expression without resorting to a whole separate function?

Answer 1

The function toUpper has the signature Char -> Char. Use map to map toUpper over all chars in a String:

getSKU :: FilePath -> String
getSKU path = 
    map toUpper $ path =~ "^sku[0-9]{5}" :: String