Pattern Guards example does not work out of the box

[maueroats]

This refers to PR #59.

1. The standard prelude does not include readMaybe. Text.Read needs to be imported.
2. The type annotation was insufficient. I needed to add Read a to the Nothing case.
3. These two issues make it seem too complicated for inclusion in a beginner's text.
   It should be revised or deleted ("feature creep" is the problem).

This code compiles:

densityTell :: String -> String  
densityTell input  
    | Just density <- readMaybe input, density < 1.2 = "Wow! You're going for a ride in the sky!"  
    | Just density <- readMaybe input, density <= 1000.0 = "Have fun swimming, but watch out for sharks!"  
    | Nothing <- readMaybe input :: ((Read a, RealFloat a) => Maybe a) = "You know I need a density, right?"  
    | otherwise   = "If it's sink or swim, you're going to sink."

Suggested revision (to be completed by me someday). I actually think this is ugly (and bad style), and the topic should be removed, but this (relatively cleanly) fixes the issue I noted.

densityRead:: String -> Maybe Double
densityRead input = readMaybe input

densityTell' :: String -> String
densityTell' input
   | Just density <- result, density < 1.2 = "Wow! You're going for a ride in the sky!"  
   | Just density <- result, density < 1000.0 = "Have fun swimming, but watch out for sharks!"  
   | Nothing <- result = "You know I need a density, right?"  
   | otherwise = "If it's sink or swim, you're going to sink."
   where result = densityRead input

[Stef9998]

As someone who just started to learn Haskell a few days ago I have to agree to 3. While including pattern guards at this point could be fine, the way put in by PR #59 seems to difficult for me. When I was reading it on the LYAH, it already seemed out of place by having the readMaybe function and not explaining it. The function is by it's name pretty self-explanitory, but still inlcuding something and not explaining how it works (Just/Nothing) should be addressed.
Secondly not beeing able to compile it and not even finding a solution from the error from google was really annoying and I only found the solution, because I wanted to open a Issue here myself about it not working.

From my opinion as a new learner I think the code example needs to be completely overhauled and the explanation be more thorough.