18 Monad

Write bind using fmap and join

link:ch18_18.2_1.hs[role=include]

link:ch18_18.4_1.hs[role=include]

link:ch18_18.4_2.hs[role=include]

link:ch18_18.5_1.hs[role=include]

interesting
```
link:ch18_18.7_1.hs[role=include]
```
I took the Either example and just search/replace, a and b flipped, therefore Left and Right flipped too. Had to hide Left and Right not to collide with those from Prelude.
```
link:ch18_18.7_2.hs[role=include]
```
this went ok
```
link:ch18_18.7_3.hs[role=include]
```
pretty cool, I had to fix the Arbitrary instance for List. I had it broken before.
```
link:ch18_18.7_4.hs[role=include]
```
let j :: Monad m ⇒ m (m a) → m a; j = join
let l1 :: Monad m ⇒ (a → b) → m a → m b; l1 = liftM
let l2 :: Monad m ⇒ (a → b → c) → m a → m b → m c; l2 = liftM2
let a :: Monad m ⇒ m a → m (a → b) → m b; a = flip ap
Kinda clunky and there is already forM that does that. Anyway, I cracked it. Good feeling.
```
link:ch18_18.7_5.hs[role=include]
```
Still not totally clear about join, need to play with it more. And again sequence does just the same.
```
link:ch18_18.7_6.hs[role=include]
```

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