List comprehension and converting string to object

Question

I have a string, like 2.7+i*3.4. I want to parse this string and get Complex number object. I try to do this:

newtype MyComplexNumber = MyComplexNumber (Complex Float)

myReadsCmplx s = [(MyComplexNumber (a :+ b)) |
                               (a, '+':r1) <- reads s :: [(Float, String)],
                               (i, '*':r2) <- reads r1 :: [(String, String)],
                               (b, r3) <- reads r2 :: [(Float, String)]]

But I have empty list:

*Main Data.Complex> myReadsCmplx "2.7+i*3.4"
[]
*Main Data.Complex>

Answer 1

You seem to be using reads as though it were a full monadic parser. It's not. It comes up with a match or none, and if the match it finds does not match your pattern, you get nada. You will be much better off using something like parsec, attoparsec, or even something super-simple like regex-applicative.