Haskell: check if string is valid number

Question

How do I check for a decimal point when checking a string is a valid number?

What I am thinking is that I use something like the following, but add code to check for

Answer 1

Here's a simple strategy:

1. Strip off all the digits at the beginning of the string.

2. The remaining string should now be either

   a) the empty string, or

   b) a decimal point followed by all digits.

Well, almost. This would also match the empty string "" and "." but we can treat those as special cases.

Translated to Haskell:

isNumber :: String -> Bool
isNumber ""  = False
isNumber "." = False
isNumber xs  =
  case dropWhile isDigit xs of
    ""       -> True
    ('.':ys) -> all isDigit ys
    _        -> False

Answer 2

A simple approach involves using readMaybe for converting a string into a number,

import Text.Read

and so for checking whether it is a Double,

readMaybe "123" :: Maybe Double
Just 123.0

readMaybe "12a3" :: Maybe Double
Nothing

The latter returns Nothing, the string is not a valid number. In a similar fashion, if we assume it is an Int,

readMaybe "12.3" :: Maybe Int
Nothing

Answer 3

Take a look at reads, then:

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

Maybe there is a better way, though.

Note that this will return True for numbers that are considered valid in Haskell, your particular use case is not fully covered by this. If you need custom parsing according to your specification you should use something like Parsec, as @CarstenKönig has suggested in his comment.