On Thu, Jan 24, 2013 at 9:56 PM, Simon Peter Nicholls <simon@mintsource.org> wrote:

I'd like to convert this Maybe Document to a Maybe MyType without unpacking and repacking the Maybe. Is this possible?

That would be exactly 

fmap :: (Document -> MyType) -> (Maybe Document -> Maybe MyType)

where I've instantiated the type variables for your use case.

 

The fromBson function, that converts from a Document to an end user type, confuses me due to it's type:

    fromBson :: Monad m => Document -> m a

All the small, single letters are type variables. Because they are implicitly universally quantified; you, the caller, gets to specify what you want them to be. Written out in full, it's actually

fromBson :: forall (m :: * -> *), a. Monad m => Document -> m a

Here, again specializing for your use case, fromBson probably needs to be of type

fromBson :: Document -> Maybe MyType

So if you have a rightEres :: Maybe Document

then

fmap fromBson rightEres :: Maybe (Maybe MyType)

which you could then

join $ fmap fromBson rightEres :: Maybe MyType

What you're really after is the more idiomatic

rightEres >>= fromBson :: Maybe MyType

HTH,

-- Kim-Ee

 

Thanks,

Si

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