haskell,binding,bind,chain , How do I reuse an intermediate value in chain of Haskell Either binds?

How do I reuse an intermediate value in chain of Haskell Either binds?


Tag: haskell,binding,bind,chain

I'm sure this has been answered but apparently I don't know what to search for.

I am working on my first non-trivial Haskell program -- a simulation of a card game I enjoy. I have the program working but there is a situation that I am certain can be handled better.

If I have a series of binds which are Either String TypeChangesBasedOnTheFunction.

playerInput <- getLine
case playerInput of
   _ -> do let handValue = inputHasAllValidChars playerInput >>= makeDeckFromString >>= deckSubset (getCurrentPlayersDeck gameState) >>= whatHandRepresents >>= handBeatsCurrentTopTrick

Very soon after this part of the code runs, I need the value for whatHandRepresents. I can just run the same chain of binds again but I am certain there must be a better way by storing the value when it is determined in the code above.

My question is, is it possible to store this value. If so, how?


You have to thread it all the way back up to where you need it.

repr <- case playerInput of
    -- now you have to return something of the appropriate type,
    -- say, `Left "didn't get it"`, from these other cases.

    _ -> do let repr = ... >>= whatHandRepresents
                handValue = repr >>= handBeatsCurrentTopTrick
            -- use handValue
            return repr
-- now use repr :: Either String HandRepresentationOrWhatever

I sure hope you actually are using handValue within the default case and not trying to assign it for use later on. It will only be in scope within that case; if you want to use it afterward you have to return it also, just like repr (e.g. in a tuple).


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