String in Haskell

In Haskell, you can check if a string is a number by using the reads function. The reads function takes a string and returns a list of pairs where each pair contains a parsed value and the remaining unparsed string. If the string can be parsed as a number, then the list contains a single pair with the parsed number and an empty string.

Here's an example code snippet:

isNumber :: String -> Bool
isNumber str =
  case reads str :: [(Double, String)] of
    [(_, "")] -> True
    _ -> False

In this code, we define a function isNumber which takes a string str and returns a boolean value indicating whether str can be parsed as a number.

The reads function is used with a type annotation specifying that we want to parse a Double, and the result is a list of pairs [(Double, String)].

The case expression checks whether the reads function returned a list with a single pair where the second element is an empty string. If so, then str can be parsed as a number and the function returns True. Otherwise, the function returns False.

Here are some example inputs and their expected outputs:

isNumber "123"         -- True
isNumber "0.123"       -- True
isNumber "-42"         -- True
isNumber "not a num"   -- False
isNumber ""            -- False