I would like to play with tuples and lens this way :
myfct :: ReaderT (a,b,c,d,e) m a -> ReaderT (a,c,d) m a
myfct = zoom ...
be able to modify the input tuple to a subset of it...
pseudo code would be something like this :
zoom (_1,_3,_4)
As @dfeuer notes, you probably meant to write:
myfct' :: Monad m => ReaderT (a,c,d) m a -> ReaderT (a,b,c,d,e) m a
This takes an action that only requires access to the context (a,c,d) and lifts it to an action that can run in a larger context that supplies (a,b,c,d,e). This can be written using magnify like so:
myfct' = magnify . to $ \(a,_,c,d,_) -> (a,c,d)
On the other hand, if you actually meant what you wrote:
myfct :: Monad m => ReaderT (a,b,c,d,e) m a -> ReaderT (a,c,d) m a
then you're going to have to explain what this is supposed to do. In particular, if you have an action that accesses the b :: String component:
action :: Reader (Int,String,Int,Int,Int) Int
action = asks $ \(_,b,_,_,_) -> length (b :: String)
How do you want to run it in a context with no b :: String?
test' :: Int
test' = runReader (myfct action) (1,2,3)