wren ng thornton wrote:
Daryoush Mehrtash wrote:
The MaybeT transformer is defined as:
newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)}
....
Question: What does "runMaybeT x" mean?
As for "what does it do", I think everyone else has handled that pretty
well. As far as "what does it mean", it may help to think categorically.
For every monad |m| we have another monad |MaybeT m|. If we ignore some
details, we can think of the transformed monad as |Maybe| composed with
|m|, sic: |Maybe . m|. With this perspective, runMaybeT is inverting
|MaybeT m| into |m . Maybe| by pushing the Maybe down over/through the
monad m. Hence we can envision the function as:
| runMaybeT :: (MaybeT m) a -> (m . Maybe) a
| runMaybeT NothingT = return Nothing
| runMaybeT (JustT ma) = fmap Just ma
Erh, whoops, I said that backwards. |MaybeT m| is like |m . Maybe| and
runMaybeT breaks apart the implicit composition into an explicit one.
The MaybeT monad just gives the additional possibility of a Nothing
value at *each* object in |m a|.
To see why this is different than the above, consider where |m| is a
list or Logic. |MaybeT []| says that each element of the list
independently could be Nothing, whereas |ListT Maybe| says that we have
either Nothing or Just xs. The pseudocode above gives the latter case
since it assumes a top level Nothing means Nothing everywhere, which is
wrong for |MaybeT m|.
--
Live well,
~wren
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