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Update tutorial (NT changes)

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This commit is contained in:
Oleg Grenrus 2016-11-16 08:29:36 +02:00
parent b1ac9dc450
commit 34166ae749
1 changed files with 6 additions and 6 deletions
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12
doc/tutorial/Server.lhs
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@ -1044,17 +1044,17 @@ If we have a function that gets us from an `m a` to an `n a`, for any `a`, what
do we have?
``` haskell ignore
newtype m :~> n = Nat { unNat :: forall a. m a -> n a}
newtype m :~> n = NT { ($$) :: forall a. m a -> n a}
```
For example:
``` haskell
listToMaybeNat :: [] :~> Maybe
listToMaybeNat = Nat listToMaybe -- from Data.Maybe
listToMaybeNT :: [] :~> Maybe
listToMaybeNT = NT listToMaybe -- from Data.Maybe
```
(`Nat` comes from "natural transformation", in case you're wondering.)
(`NT` comes from "natural transformation", in case you're wondering.)
So if you want to write handlers using another monad/type than `Handler`, say the `Reader String` monad, the first thing you have to
prepare is a function:
@ -1066,14 +1066,14 @@ readerToHandler :: Reader String :~> Handler
Let's start with `readerToHandler'`. We obviously have to run the `Reader`
computation by supplying it with a `String`, like `"hi"`. We get an `a` out
from that and can then just `return` it into `ExceptT`. We can then just wrap
that function with the `Nat` constructor to make it have the fancier type.
that function with the `NT` constructor to make it have the fancier type.
``` haskell
readerToHandler' :: forall a. Reader String a -> Handler a
readerToHandler' r = return (runReader r "hi")
readerToHandler :: Reader String :~> Handler
readerToHandler = Nat readerToHandler'
readerToHandler = NT readerToHandler'
```
We can write some simple webservice with the handlers running in `Reader String`.