refactoring some convoluted error handling code

error-handling-refactoring.purs

initial code with some really convoluted error handling. 
The problem on this is that it must check for 2 different error cases and also return a different result in those error cases

    dot :: Matrix -> Matrix -> Either MatrixError Number
    dot (Matrix a sa) (Matrix b sb) = if areSimilarVectors sa sb then Right $ dot' a b else failed sa sb
        where
            areSimilarVectors s1 s2 = isVector s1 s2 && isSameSize s1 s2
            dot' a b = _dot (join a) (join b)
            isVector sa sb = fst sa == 1 && fst sb == 1
            isSameSize sa sb = snd sa == snd sb
            failed sa@(Tuple ax ay) sb@(Tuple bx by) = 
                if isVector sa sb then 
                    if isSameSize sa sb then
                            Left $ UnexpectedError
                        else
                            Left $ InvalidVectorSize ay by
                    else
                        Left $ VectorsExpected
                                
made better by aligning the `then` and the `else` branch with `Right` and `Left` and also removing the shadowing of the values 
in the `where` clause

    dot :: Matrix -> Matrix -> Either MatrixError Number
    dot (Matrix a sa@(Tuple ax ay)) (Matrix b sb@(Tuple bx by)) = if isVector && isSameSize then Right (dot' a b) else Left $ failed
        where
            dot' a b = _dot (join a) (join b)
            isVector = ax == 1 && bx == 1
            isSameSize = ay == by
            failed = 
                if isVector then 
                    if isSameSize then
                        UnexpectedError
                    else
                        InvalidVectorSize ay by
                else
                    VectorsExpected

nicer already

And then this

    dot :: Matrix -> Matrix -> Either MatrixError Number
    dot (Matrix a (Tuple ax ay)) (Matrix b (Tuple bx by)) 
        | ax /= 1 && bx /= 1    = Left  $ VectorsExpected
        | ay /= by              = Left  $ InvalidVectorSize ay by
        | otherwise             = Right $ _dot (join a) (join b)