[Haskell-cafe] New Eq instance

[Bayley, Alistair]

> From: haskell-cafe-bounces at haskell.org 
> [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of 
> rodrigo.bonifacio
> 
> data Step = Step Id Scenario Action State Response
> 
> How can I define Step as an "Eq Instance", in such way that 
> two steps are equals if they have the same Id (Id is defined 
> as a synonimous for the String type).
> 
> I tried the following code, but something is wrong....
> 
> instance Eq Step where 
>   Step id1 scenario1 action1 state1 response1 == Step id2 
> scenario2 action2 state2 response2 = id == id
>   _ == _ = False


What error messages are you getting?

Anyway, you have the right idea, you just need a bit of cleaning up and
correct syntax. As I don't have your definitions for Scenario, Action,
etc, I'll just ignore them (we don't need them for your Eq anyway):

> data Step = Step String String
> instance Eq Step where 
>   Step id1 _ == Step id2 _ = id1 == id2

Note:
 - the _ == _ case will never be reached (AFAICT)
 - I assume " = id == id" is a typo, and that you intended "id1 == id2"

Alistair
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