I have the following pattern of a Reader with a Semigroup Element:
runFunction :: Reader Env Element
runFunction = do
a <- getA
b <- getB
c <- getC
return $ a <> b <> c
Where getA :: Reader Env Element.
Is there a way to:
runFunction = do
getA
getB
getC
I feel like I see this pattern alot, where I imperatively chain monadic calls, and they get turned into a single element at the end.
Note: I don't want to do getA >>= getB >>= getC since getB isn't :: Element -> Reader Env Element
It feels like a State Monad, that automatically modifies state with <>, but I don't know.
Working with monadic code is still quite fresh to me.
The WriterT monad transformer can be used to build up a monoidal value with each action.
You can use lift to wrap your Reader actions into the WriterT monad transformer, then move the result of each action into the monad's environment by using tell, sequence the actions, and then unwrap the result back to a single Reader action using execWriterT.
Here is what I mean:
import Control.Monad.Writer
wrap :: (Monad m, Monoid w) => m w -> WriterT w m ()
wrap m = lift m >>= tell
unwrap :: (Monad m) => WriterT w m a -> m w
unwrap = execWriterT
runFunction :: Reader Env Element
runFunction = unwrap $ do
wrap getA
wrap getB
wrap getC
As you can see, this generalizes to any monad, not just Reader.
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