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55 lines
1.5 KiB
55 lines
1.5 KiB
# 23 State |
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## 23.5 Exercises: Roll Your Own |
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```haskell |
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repeat :: a -> [a] |
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repeat a = a : repeat a |
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instance Functor (State s) where |
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fmap f st = state $ \s -> let (a, s') = runState st s |
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in (f a, s') |
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-- Applying this to infiniteDie means that we advance the StdGen once and |
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-- repeat the rolled die. I.e. we only call randomR once and then repeat |
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-- the result. |
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replicateM :: Applicative f => Int -> f a -> f [a] |
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replicateM n fa |
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| n <= 0 = pure [] |
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| otherwise = (:) <$> fa <*> replicateM (n - 1) fa |
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instance Applicative (State s) where |
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pure a = state $ \s -> (a, s) |
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stf <*> sta = state $ \s -> let (f, s') = runState stf s |
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(a, s'') = runState sta s' |
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in (f a, s'') |
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-- randomR will be called for every fmap statement in replicateM. This is |
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-- because we now from previous statement that fmap will call randomR and the |
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-- resulting State object will no longer call randomR. The second argument |
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-- in <*> will eventually reduce to pure [] which won't change the state and |
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-- call a randomR once for each fmap. |
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-- Hence we get an array with a call to randomR for each element, propagating |
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-- the StdGen forward along it. |
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``` |
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[src/RandomExample2.hs](./src/RandomExample2.hs) |
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To run in ghci: |
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```shell |
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Prelude> :l src/RandomExample2.hs src/RandomExample.hs |
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``` |
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## 23.6 Write `State` for yourself |
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[src/Moi.hs](./src/Moi.hs) |
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## 23.7 Fizzbuzz differently |
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[src/fizzbuzz.hs](./src/fizzbuzz.hs) |
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## 23.8 Chapter Exercises |
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[src/chapter.hs](./src/chapter.hs) |