以下は、私の前の質問に対する Petr Pudlak の解決策に基づいた、MonadState のレンズ (Edward Kmett による) に関する一連の例/演習です。
これらの例は、いくつかの用途とレンズの能力を示すだけでなく、GHCi によって生成された型シグネチャを理解することがいかに難しいかを示しています。今後改善される見込みはありますか？
{-# LANGUAGE TemplateHaskell, RankNTypes #-}
import Control.Lens
import Control.Monad.State
---------- Example by Petr Pudlak ----------
-- | An example of a universal function that modifies any lens.
-- It reads a string and appends it to the existing value.
modif :: Lens' a String -> StateT a IO ()
modif l = do
s <- lift getLine
l %= (++ s)
-----------------------------------------------
以下のコメントタイプのシグネチャは、GHCi によって生成されたものです。もう 1 つは、Peter のものからの翻案です。個人的には、GHCi によって生成されたものよりも理解するのに苦労しています。なぜ GHCi は簡略化されたものを生成しないのでしょうか?
-------------------------------------------
-- modif2
-- :: (Profunctor p, MonadTrans t, MonadState s (t IO)) =>
-- (Int -> p a b) -> Setting p s s a b -> t IO ()
modif2 :: (Int -> Int -> Int) -> Lens' a Int -> StateT a IO ()
modif2 f l = do
s<- lift getLine
l %= f (read s :: Int)
---------------------------------------
-- modif3
-- :: (Profunctor p, MonadTrans t, MonadState s (t IO)) =>
-- (String -> p a b) -> Setting p s s a b -> t IO ()
modif3 :: (String -> Int -> Int) -> Lens' a Int -> StateT a IO ()
modif3 f l = do
s <- lift getLine
l %= f s
-- :t modif3 (\n -> (+) (read n :: Int)) == Lens' a Int -> StateT a IO ()
---------------------------------------
-- modif4
-- :: (Profunctor p, MonadTrans t, MonadState s (t IO)) =>
-- (t1 -> p a b) -> (String -> t1) -> Setting p s s a b -> t IO ()
modif4 :: (Bool -> Bool -> Bool) -> (String -> Bool) -> Lens' a Bool -> StateT a IO ()
modif4 f f2 l = do
s <- lift getLine
l %= f (f2 s)
-- :t modif4 (&&) (\s -> read s :: Bool) == Lens' a Bool -> StateT a IO ()
---------------------------------------
-- modif5
-- :: (Profunctor p, MonadTrans t, MonadState s (t IO)) =>
-- (t1 -> p a b) -> (String -> t1) -> Setting p s s a b -> t IO ()
modif5 :: (b -> b -> b) -> (String -> b) -> Lens' a b -> StateT a IO ()
modif5 f f2 l = do
s<- lift getLine
l %= f (f2 s)
-- :t modif5 (&&) (\s -> read s :: Bool) == Lens' a Bool -> StateT a IO ()
---------------------------------------
-- modif6
-- :: (Profunctor p, MonadState s m) =>
-- (t -> p a b) -> (t1 -> t) -> t1 -> Setting p s s a b -> m ()
modif6 :: (b -> b -> b) -> (c -> b) -> c -> Lens' a b -> StateT a IO ()
modif6 f f2 x l = do
l %= f (f2 x)
-- :t modif6 (&&) (\s -> read s :: Bool) "True" == MonadState s m => Setting (->) s s Bool Bool -> m ()
-- :t modif6 (&&) (\s -> read s :: Bool) "True"
---------------------------------------
-- modif7
-- :: (Profunctor p, MonadState s IO) =>
-- (t -> p a b) -> (String -> t) -> Setting p s s a b -> IO ()
modif7 :: (b -> b -> b) -> (String -> b) -> Lens' a b -> StateT a IO ()
modif7 f f2 l = do
s <- lift getLine
l %= f (f2 s)
-- :t modif7 (&&) (\s -> read s :: Bool) ==
-- :t modif7 (+) (\s -> read s :: Int) ==
---------------------------------------
p7a :: StateT Int IO ()
p7a = do
get
modif7 (+) (\s -> read s :: Int) id
test7a = execStateT p7a 10 -- if input 30 then result 40
---------------------------------------
p7b :: StateT Bool IO ()
p7b = do
get
modif7 (||) (\s -> read s :: Bool) id
test7b = execStateT p7b False -- if input "True" then result "True"
---------------------------------------
data Test = Test { _first :: Int
, _second :: Bool
}
deriving Show
$(makeLenses ''Test)
dataTest :: Test
dataTest = Test { _first = 1, _second = False }
monadTest :: StateT Test IO String
monadTest = do
get
lift . putStrLn $ "1) modify \"first\" (Int requested)"
lift . putStrLn $ "2) modify \"second\" (Bool requested)"
answ <- lift getLine
case answ of
"1" -> do lift . putStr $ "> Write an Int: "
modif7 (+) (\s -> read s :: Int) first
"2" -> do lift . putStr $ "> Write a Bool: "
modif7 (||) (\s -> read s :: Bool) second
_ -> error "Wrong choice!"
return answ
testMonadTest :: IO Test
testMonadTest = execStateT monadTest dataTest