haskell error on input else

Question

having a bit of trouble with the following code. I keep getting an error message "saying parse error on input 'else' ". I cant see what im doing wrong, i have the else statement indented from the if and the syntax inside the statements are correct. Bit of background, its part of an insertion method for 2-3-4 trees, specifically for inserting where a fournode(Root3) needs to be split into its parent. x y and z are the values in the fournode while c1 - c4 are its child nodes. have looked at other questions on here about these types of errors but couldn't find anything useful :( Anyone have any ideas?

insert t (Root1 a left right) | t <= a = case left of (Root3 x y z c1 c2 c3 c4) -> if t <= y then Root2 y a (insert t (Root1 x c1 c2) (Root1 z c3 c4) right else Root2 y a (Root1 x c1 c2) (insert t (Root1 z c3 c4)) right _ -> Root1 a (insert t(left)) (right)

jamshidh · Accepted Answer · 2014-12-11 23:17:37Z

1

There may be other errors, but immediately I can see that your parenthesis aren't matched.... Take a look at the parens in the then. The compiler doesn't expect the else until after the then clause is finished, but the missing end paren is keeping this from happening.

answered Dec 11, 2014 at 23:17

jamshidh

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4 Comments

mcgovec9 Over a year ago

could you please explain? i dont see anything wrong with the parenthesis in this code. I should mention that this is only part of my insert method, but its the part thats not working. The code compiles fine and runs without it, but obviously as a result the fournode doesn't split properly

jamshidh Over a year ago

Count your parens, there are more opening than closing. In particular, after the then, I see an opening paren before insert, and although there are some open-and-close parens after, this particular one doesn't end before the else.

mcgovec9 Over a year ago

ok so i added the missing paren and that did the trick, but now im getting a similar error on the next line " _ -> Root1 a (insert t(left)) (right) " and i really cant see anything wrong with it? pretty sure my logic is correct at least :p

jamshidh Over a year ago

You need to put the same whitespace before each item in a case-of. That means that the (Root3 x y z c1 c2 c3 c4) -> and _ -> need to be indented to the same place, and they are not.

amalloy · Accepted Answer · 2014-12-11 23:30:42Z

0

Your indentation has mixed tabs and spaces, which is a big no-no: indentation is very significant in Haskell, and you want to make sure that what you see is the same as what the compiler sees. Just because a tab looks about as wide as eight spaces doesn't mean you can put one in instead of the eight spaces.

I recommend you fix this to use spaces-only, make sure things are lined up correctly, and then if you're still having issues ask a new question.

answered Dec 11, 2014 at 23:30

amalloy

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