Parse error in a function for maxheap

Question

I'm new to Haskell and am currently working on trees. In this, I wanted to create a function, that swaps the elements in a Tree until it becomes a maxheap. This, I would then use for a Heapsort algorithm.

data Bintree a = E | Tree (Bintree a) a (Bintree a) deriving( Eq, Ord, Show )

swaptree:: Bintree Int -> Bintree Int
swaptree (Tree E w E) = (Tree E w E)
swaptree (Tree E w (Tree lr x rr))|((max w x)==w)=(Tree E w (Tree lr x rr))
                              |((max w x)==x)=(Tree E x(swaptree(Tree lr w rr)))
                              |otherwise error "something went wrong or program is bad"
swaptree (Tree (Tree ll v rl) w E)|((max v w)==w)=(Tree (Tree ll v rl) w E)
                              |((max v w)==v)=(Tree (swaptree(Tree ll w rl)) v E)
                              |otherwise error "something went wrong or program is bad"
swaptree (Tree (Tree ll v rl) w (Tree lr x rr) )|((max3 v w x)==w) = (Tree (Tree ll v rl) w (Tree lr x rr)) 
                                            |((max v x)==x)=(Tree (Tree ll v rl) x (swaptree(Tree lr w rr)))
                                            |((max v x)==v)=(Tree(swaptree(Tree ll w rl)) v (Tree lr x rr) )
                                            |otherwise error "something went wrong or program is bad"
swaptree':: Bintree Int -> Bintree Int
swaptree' (Tree E w E) = (Tree E w E)
swaptree' (Tree lub w rub)  = (swaptree(Tree(swaptree lub) w (swaptree rub)))

This yields me this error:

   8:1: error:parse error (possibly incorrect indentation or mismatched brackets)

Now, I looked it up and read, that this usually happens, leSt is used without in, but since I didn't use it I have no Idea how to fix this. I would greatly appreciate any help or tips on how to improve my code!

PS: Please have mercy on me. As I said, I'm currently very bad at Haskell.

chi · Accepted Answer · 2017-02-19 16:04:14Z

1

The error was already spotted by Li-yao Xia, and a fix was suggested.

Note that you can also remove the offending line using this style:

swaptree (Tree E w (Tree lr x rr)) | x <= w    = Tree E w (Tree lr x rr)
                                   | otherwise = Tree E x (swaptree (Tree lr w rr))
-- etc.

Indeed, it is weird to check three cases max x w == x, max x w == w, and otherwise. If we do reach the "otherwise" something is really wrong with the definition of max. We can safely ignore that case.

answered Feb 19, 2017 at 16:04

chi

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1 Comment

Li-yao Xia Over a year ago

Your solution is much better!

Li-yao Xia · Accepted Answer · 2017-02-19 14:33:09Z

1

|otherwise error "something went wrong or program is bad"

should be

|otherwise = error "something went wrong or program is bad"

answered Feb 19, 2017 at 14:33

Li-yao Xia

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