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|
-- The UndecidableInstances here is benign, just for the "advanced"
-- example at the end.
{-# LANGUAGE FlexibleContexts, ScopedTypeVariables, UndecidableInstances #-}
-- I don't demonstrate the advantages over implicit parameters here,
-- like multiple reifications of the same thing coexisting in
-- different, statically-checked contexts, etc.; this is intended as a
-- comparison to Reader(T).
import Data.Proxy
import Data.Reflection
import System.IO
data MyConfig = MyConfig
{ magic :: Bool
, volume :: Integer
}
data Report p = Report
{ magicality :: String
, loud :: Bool
} deriving (Show)
-- some arbitrary thing we do calculations with
newtype Datum p = Datum Integer deriving (Read, Show)
report :: forall p. (Reifies p MyConfig) => Report p
report = Report
{ magicality = if magic conf then "Magical." else "Not so magical..."
, loud = volume conf >= 11
} where
conf = reflect (Proxy :: Proxy p)
calculate :: forall p. (Reifies p MyConfig) => Datum p -> Datum p -> Datum p
calculate (Datum m) (Datum n) = Datum ((m+n) * volume conf) where
conf = reflect (Proxy :: Proxy p)
run :: forall p. (Reifies p MyConfig) => Proxy p -> IO ()
run _ = do
d1 <- ask "Datum 1:" :: IO (Datum p)
d2 <- ask "Datum 2:" :: IO (Datum p)
-- look ma, no plumbing
print $ calculate d1 (calculate d2 d1)
print (report :: Report p)
ask :: Read s => String -> IO s
ask prompt = do
putStr prompt
putChar ' '
hFlush stdout
readLn
main :: IO ()
main = do
conf <- MyConfig <$> ask "Magic?" <*> ask "Volume:"
reify conf run
-- If you're feeling adventurous, here is something we could not do
-- with ReaderT:
instance Reifies p MyConfig => Num (Datum p) where
(+) = calculate
m * n
| magic conf = m + n
| otherwise = Datum 0 -- sorry, no magic for you.
where conf = reflect (Proxy :: Proxy p)
abs = undefined
signum = undefined
(-) = undefined
negate = undefined
fromInteger = Datum
|
