"Parse error in pattern" when implementing tree

Question

Here is my code

quadtreeToPic QNode x y w avg Q4(q1 q2 q3 q4) = array ((0,0) (w-1,w-1)) (concat (map quadtreeToPic [q1, q2, q3, q4]))

Basically, a Quadtree is x y w avg QChildren where QChildren is qither Q0 or Q4 Quadtree Quadtree Quadtree Quadtree.

When I do what I tried above, I get this error

Quadtree.hs:13:34: error: Parse error in pattern: q1
   |
13 | quadtreeToPic QNode x y w avg Q4(q1 q2 q3 q4) = array ((0,0) (w-1,w-1)) (concat (map quadtreeToPic [q1, q2, q3, q4])) 
   |                                  ^^^^^^^^^^^

Why is this?

You need to add brackets around the Q4 data constructor, so quadtreeToPic QNode x y w avg (Q4 q1 q2 q3 q4) = ... — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jul 30, 2019 at 17:41

willeM_ Van Onsem · Accepted Answer · 2019-07-30 17:42:41Z

3

The Q4 dataconstructor and the parameters are a single parameter, here, so you need to add brackets including the Q4 data constructor. For example:

quadtreeToPic QNode x y w avg (Q4 q1 q2 q3 q4) = ...

Note that you can use concatMap :: (a -> [b]) -> [a] -> [b] here instead of concat (map ...):

... concatMap quadtreeToPic [q1, q2, q3, q4]

answered Jul 30, 2019 at 17:42

willeM_ Van Onsem

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4 Comments

SilentDev Over a year ago

I get the error

Quadtree.hs:13:85: error:     • Couldn't match type ‘Array (Int, Int) Word8’                      with ‘[((Int, Int), Word8)]’       Expected type: [[((Int, Int), Word8)]]         Actual type: [Array (Int, Int) Word8]     • In the first argument of ‘concat’, namely         ‘(map quadtreeToPic [q1, q2, q3, q4])’       In the second argument of ‘array’, namely         ‘(concat (map quadtreeToPic [q1, q2, q3, q4]))’       In the expression:         array           ((0, 0) (w - 1, w - 1))           (concat (map quadtreeToPic [q1, q2, q3, q4]))

SilentDev Over a year ago

after I make this fix. Any idea why?

willeM_ Van Onsem Over a year ago

@user2719875: that's a type error: your quadtreeToPic is supposed to return [((Int, Int), Word8)]]s, but it returns [Array (Int, Int) Word8]?

SilentDev Over a year ago

it looks like it's a different question altogether. I asked it here, if you can help: stackoverflow.com/questions/57277522/…

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