[Haskell-cafe] ANN: To Kata Haskellen Evangelion

David Fox dsf at seereason.com
Sun Jul 28 15:51:18 UTC 2019

On page 39 you say

    A type can have Ord instance only when it has Eq instance, since if you
> want to compare items, you need a way to test if they are equal.

So from my reading of this you are saying that an Eq instance for a type
needs to be supplied before we are allowed to implement compare.  However
it is easy to write a compare function that makes no use of the underlying
(==) function.  Indeed, once you have done this you can turn around and
write a == b = compare a b == EQ.  So I was wondering if you could clarify
the role of constraints in the class declaration.

On Sun, Jul 28, 2019 at 2:38 AM Cosmia Fu <cosmiafu at gmail.com> wrote:

> Hi everyone,
> Though some of you might already knows, I'm pleased to announce a new
> Haskell book,
> *To Kata Haskellen Evangelion*.
> Link: https://cosmius.bitbucket.io/tkhe/
> I believe that it does not have to be hard to learn Haskell.
> I begun to write the book in 2017 December, and... to be honest,
> I don't know what to write now, though I still think it incomplete.
> It will probably not be updated for quite a while.
> So I decide to announce it earlier.
> I am not a native English speaker, so it might be not fluent or even with
> a lot of grammar mistakes.
> It would be very nice of you if you can tell me the mistakes in it,
> factual, technical or grammar.
> And also feel free to tell me if you want to read some topic in it.
> Thank you
> ----
> Cosmia Fu
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