Why do I get a parse error for my binary tree definition?

Question

When I type this code in Haskell:

data BT a = Empty | Fork a (BT a) (BT a) 
  Empty :: BT a
  Fork  :: a -> BT a -> BT a -> BT a

I get

error: parse error on input `::'

I can't see what the issue is though, if I add deriving (show) to the top line, it changes to

error: parse error on input `Empty'

What happens if you remove the last two lines? (In other words, why did you write them in the first place?) — mkrieger1
– mkrieger1, Commented Oct 25, 2022 at 0:06
The last two lines are already implicitly defined by Haskell since you gave the data definition. You must not include them. — chi
– chi, Commented Oct 25, 2022 at 0:19

Iceland_jack · Accepted Answer · 2022-10-25 00:58:06Z

6

Either write it using the "legacy" uniform datatype syntax:

data BT a = Empty | Fork a (BT a) (BT a)

or use equivalent GADT syntax

{-# Language GADTs #-}

data BT a where 
  Empty :: BT a
  Fork  :: a -> BT a -> BT a -> BT a

But not both :)

answered Oct 25, 2022 at 0:58

Iceland_jack

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