[Haskell-beginners] Replacing equals with equals not working

[martin]

Am 12/09/2015 um 01:03 AM schrieb Francesco Ariis:
> On Tue, Dec 08, 2015 at 11:29:22PM +0100, martin wrote:
>> Hello all,
>>
>> with this code
>>
>> [...]
>>
>> The commented line -- < -- does not work, though I am just replacing equals
>> with equals. I get
>>
>>     No instance for (Ord b0) arising from a use of ‘compare’
> 
> Try to erase the `build` function and see what happens; You will get
> the same error. GHC complains that there is no good `Ord` instance
> and this can be solved by adding an appropriate type signature:
> 
>     tcmp :: (Ord a) => Tree a -> Tree a -> Ordering
> 
> Now, with the `build` function present (the one with
> `let xs = sortBy tcmp t`), GHC will infer `tcmp` signature, as t is
> an Int
> 
>     λ> :t tcmp
>     tcmp :: Tree Int -> Tree Int -> Ordering
> 
> as since Int is an instance of Ord everything is fine.
> Does that help?

Oh, I see. I was assuming the message referred to the build function (because I made chages there) and not to tcmp.
Didn't check the line number. But really tcmp has lost its roots by not being used in build anymore.