As MonadIO was no longer being automatically derived without mtl, I added these
lines:
instance MonadIO MyMonad where
liftIO m = MyM (liftIO m)
and this seems to have fixed the problem. I no longer need mtl and I can use
Heist as needed!
--- On Mon, 6/14/10, Kevin Jardine <[email protected]> wrote:
> From: Kevin Jardine <[email protected]>
> Subject: [Snap Framework] Splice functions and a custom monad
> To: [email protected]
> Date: Monday, June 14, 2010, 10:52 PM
> I'd like to run Heist template splice
> functions in my own custom monad (not Snap).
>
> I can define a splice function as:
>
> mySplice :: Splice MyMonad
>
> However, when I try to call functions that return values in
> my monad in mySplice, I get a compile error:
>
> Couldn't match expected type TemplateMonad MyMonad a
> against inferred type MyMonad ()
>
> I've seen an example in the Heist documentation where the
> inner MyMonad is accessed using lift. But when I try that, I
> get another compile error:
>
> No instance for (MonadTrans TemplateMonad)
> arising from the use of 'lift'
>
> Gregory Collins advised replacing mtl with monads-fd and
> transformers.
>
> However, this breaks my custom monad (eg. MonadIO is no
> longer automatically derived).
>
> How do I get this to work?
>
> Kevin
>
>
>
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