48df410 · servant · git.iko.soy

599

600## The `Handler` monad

601

602−At the heart of the handlers is the monad they run in, namely a newtype `Handler` around `ExceptT ServerErroror IO`

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

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

604One might wonder: why this monad? The answer is that it is the

605simplest monad with the following properties:

622

623The module [`Control.Monad.Except`](https://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Except.html#t:ExceptT)

624from which `ExceptT` comes is worth looking at.

625−Perhaps most importantly, `ExceptT` and `Handler` are an instances of `MonadError`, so

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

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

627 is enough to return a success).

628

632

633### Performing IO

634

635−Another important instances from the list above are `MonadIO m => MonadIO

635+Other important instances from the list above are `MonadIO m => MonadIO

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

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

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

638is a class from the **transformers** package defined as:

639