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Briefly correct ExceptT mentions into Handler (#693)

29f94a6Oleg Grenrus10 years ago

doc/tutorial/Server.lhs+8−9

compare with master

unifiedsplit

@@ -603,8 +603,8 @@ $ curl -H 'Accept: text/html' http://localhost:8081/persons

603

604## The `Handler` monad

605

606−At the heart of the handlers is the monad they run in, namely `ExceptT ServantErr IO`

606+At the heart of the handlers is the monad they run in, namely a newtype `Handler` around `ExceptT ServantErr IO`

607−([haddock documentation for `ExceptT`](http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#t:ExceptT)), which is aliased as `Handler`.

607+([haddock documentation for `ExceptT`](http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#t:ExceptT)).

608One might wonder: why this monad? The answer is that it is the

609simplest monad with the following properties:

610

@@ -626,7 +626,7 @@ action that either returns an error or a result.

626

627The module [`Control.Monad.Except`](https://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#t:ExceptT)

628from which `ExceptT` comes is worth looking at.

629−Perhaps most importantly, `ExceptT` is an instance of `MonadError`, so

629+Perhaps most importantly, `ExceptT` and `Handler` are an instances of `MonadError`, so

630`throwError` can be used to return an error from your handler (whereas `return`

631 is enough to return a success).

632

@@ -636,8 +636,8 @@ kind and abort early. The next two sections cover how to do just that.

636

637### Performing IO

638

639−Another important instance from the list above is `MonadIO m => MonadIO

639+Another important instances from the list above are `MonadIO m => MonadIO

640−(ExceptT e m)`.

640+(ExceptT e m)`, and therefore also `MonadIO Handler` as there is `MonadIO IO` instance..

641[`MonadIO`](http://hackage.haskell.org/package/transformers-0.4.3.0/docs/Control-Monad-IO-Class.html)

642is a class from the **transformers** package defined as:

643

@@ -646,8 +646,7 @@ class Monad m => MonadIO m where

646 liftIO :: IO a -> m a

647```

648

649−The `IO` monad provides a `MonadIO` instance. Hence for any type

650−`e`, `ExceptT e IO` has a `MonadIO` instance. So if you want to run any kind of

649+So if you want to run any kind of

651IO computation in your handlers, just use `liftIO`:

650IO computation in your handlers, just use `liftIO`:

652

651

653``` haskell

652``` haskell

@@ -861,7 +860,7 @@ Server UserAPI4 = Int -> ( Handler User

861

860

862In the first case, each handler receives the *userid* argument. In the latter,

861In the first case, each handler receives the *userid* argument. In the latter,

863the whole `Server` takes the *userid* and has handlers that are just

862the whole `Server` takes the *userid* and has handlers that are just

864−computations in `ExceptT`, with no arguments. In other words:

863+computations in `Handler`, with no arguments. In other words:

865

864

866``` haskell

865``` haskell

867server8 :: Server UserAPI3

866server8 :: Server UserAPI3

@@ -1069,7 +1068,7 @@ readerToHandler :: Reader String :~> Handler

1069

1068

1070Let's start with `readerToHandler'`. We obviously have to run the `Reader`

1069Let's start with `readerToHandler'`. We obviously have to run the `Reader`

1071computation by supplying it with a `String`, like `"hi"`. We get an `a` out

1070computation by supplying it with a `String`, like `"hi"`. We get an `a` out

1072−from that and can then just `return` it into `ExceptT`. We can then just wrap

1071+from that and can then just `return` it into `Handler`. We can then just wrap

1073that function with the `NT` constructor to make it have the fancier type.

1072that function with the `NT` constructor to make it have the fancier type.

1074

1073

1075``` haskell

1074``` haskell