In Haskell, strings are represented as lists of characters. To compare strings, we can compare each element of the list using the equality operator (==) or the comparison operators (<, >, <=, >=).
Here is an example of comparing two strings using the equality operator:
compareStrings :: String -> String -> Bool
compareStrings s1 s2 = s1 == s2
We can also compare two strings character by character using pattern matching:
compareStrings :: String -> String -> Bool
compareStrings [] [] = True
compareStrings (x:xs) (y:ys) = x == y && compareStrings xs ys
compareStrings _ _ = False
The first pattern matches on empty strings and returns True. The second pattern matches the heads of the two strings (x and y) and recursively checks the tails of the strings (xs and ys). If the heads are equal and the tails are equal, it returns True. The third pattern matches any other cases and returns False.
Example of how to compare two strings using the above function:
ghci> compareStrings "hello" "hello" -- returns True
ghci> compareStrings "hello" "world" -- returns False
Note that there is also a built-in function compare which returns an Ordering value (LT, EQ, or GT) and can be used to sort strings. Here is an example:
ghci> compare "hello" "world" -- returns LT