[Haskell-cafe] How to define a common return and bind?
Bas van Dijk
v.dijk.bas at gmail.com
Thu Apr 9 06:40:53 EDT 2009
Hello,
Suppose you have defined a monad transformer such as:
> newtype T1 m a = T1 { unT1 :: A1 m a }
Where 'A1 m' is an arbitrary monad of your choosing.
For this discussion we just take the identity:
> type A1 m a = m a -- (can be any monad)
If you want to define a Monad instance for 'T1 m' you generally do this:
instance Monad m => Monad (T1 m) where
return = T1 . return
m >>= f = T1 $ unT1 m >>= unT1 . f
(I know I can use the 'GeneralizedNewtypeDeriving' language extension
to automatically derive a Monad but suppose that isn't available)
Now when I define a new monad transformer:
> newtype T2 m a = T2 { unT2 :: A2 m a }
Where 'A2 m' is again an arbitrary monad of your choosing but for now
just the identity:
> type A2 m a = m a -- (can be any monad)
The Monad instance for it is almost completely identical to the former:
instance Monad m => Monad (T2 m) where
return = T2 . return
m >>= f = T2 $ unT2 m >>= unT2 . f
Note that the only differences are:
* a function to convert
from the outer monad _to_ the inner monad:
'unT1' and 'unT2'
* a function to convert
_from_ the inner monad to the outer monad:
'T1' and 'T2'
The common parts seem to be:
liftReturn from = from . return
liftBind from to m f = from $ to m >>= to . f
My question is: can these be given suitable and general enough types
so that they can be used to define Monad instances for monad
transformers?
In other words can I use them to write:
instance Monad m => Monad (T1 m) where
return = liftReturn T1
(>>=) = liftBind T1 unT1
and:
instance Monad m => Monad (T2 m) where
return = liftReturn T2
(>>=) = liftBind T2 unT2
Thanks,
Bas
More information about the Haskell-Cafe
mailing list