Question

Here is a small program showing a problem:

import Data.Fixed

main = do
    print x
  where
    x :: Pico
    x = read "12" -- error: no instance for 'Read Pico'

I see library GHC source code in Fixed.hs – there is instance for Read (copying some code):

type Pico = Fixed E12

data E12 = E12

instance HasResolution E12 where
    resolution _ = 1000000000000

instance (HasResolution a) => Read (Fixed a) where
    readsPrec _ = readsFixed

What’s wrong with my reasoning and why compiler doesn’t see that Pico is instance of Read?

Answer 1

Fixed has a Read instance since base 4.4.0.0, which is GHC 7.2. You probably have an older version.