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?

Question:

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
   {- CASE STUFF NOT IMPORTANT TO THIS QUESTION -}
   _ -> 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?


Answer:

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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