haskell: parse error using higher order functions

Question

I'm trying to create an insertion sort method that allows me to do an insertion sort based on a function of an item, eg function applied to 1 could be greater than function applied to 2 or vice versa depending on the function.

I have this so far to work out if I can insert an element into a part of a list.

insertBy :: Ord b => (a -> b) -> a -> [a] -> [a] 
insertBy f a [] = [a]
insertBy f a (x:xs) = if ( (f a)< (f x )) then a:x:xs else insertBy f a xs

However, I'm getting a parse error on the second line. Sorry if it's really obvious but I can't see it.

Once I have that part I will call an insertion sort function that uses that to sort a list but first I need help with this :(

edit: exact error "Parse error in Pattern insertBy"

In this kind of question, it would be good if you would give us the error message you're seeing. — Dale Hagglund
– Dale Hagglund, Commented Nov 2, 2013 at 4:11
When I insert exactly this text into a module on its own, ghc compiles it fine, and does what it's supposed to, including the bug mentioned below. Same appears to be true for hugs. — Dale Hagglund
– Dale Hagglund, Commented Nov 2, 2013 at 4:28
There is nothing wrong with the formatting of the code you have on here. I would guess you have made a typo or some other error in your local copy of the code. — kqr
– kqr, Commented Nov 2, 2013 at 13:11

Dale Hagglund · Accepted Answer · 2013-11-02 04:15:54Z

1

I think you want the final expression to be

x : insertBy f a xs

Otherwise you're discarding the first element of the list in that case.

answered Nov 2, 2013 at 4:15

Dale Hagglund

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2 Comments

Alex Over a year ago

yeah you're right but I still get the same error. There is something wrong with line 2

Alex Over a year ago

insertBy is already defined in the haskell library. I fixed it by changing any reference to insertBy to insertBy1. Thanks for that bug also!

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