Type-level error messages

Ben Gamari ben at well-typed.com
Tue Oct 27 18:27:33 UTC 2015

Iavor Diatchki <iavor.diatchki at gmail.com> writes:

> Hello,

Very good timing on the message; we just finished discussing your work
not more than an hour ago. You can disregard my message; I didn't notice
yours before sending it.

> On Thu, Oct 22, 2015 at 9:47 AM, Simon Peyton Jones <simonpj at microsoft.com>
> wrote:
>> I’ve forgotten the state of your type-level error messages work.  How’s it
>> going?
>> I think we should try to add it to 8.0.1.  The current status is that the
>> idea is implemented on a branch.  Then, there were some comments and
>> suggestions that maybe we should do things in a different way,
>> implementation wise.  I haven't had a chance to look into these in detail,
>> or implement them, and as far as I know nobody else has stepped up to make
>> the changes.  So we could simply go with the current version, and if for
>> some reason we want to change the implementation we could do it later, as I
>> don't think the API will be affected in any way.    When do the changes
>> need to happen by, so that it makes it in 8.0?   I have been a bit busy,
>> but I could probably find some time to make whatever changes are required
>> for this to be merged.
>> OK good!  In that case
>> ·         Add it to the hoped-for features in the GHC 8.01. status page
> Where is the GHC 8.01 status page?


> ·         Write a wiki page with a specification
>> Here is the specification:
> https://ghc.haskell.org/trac/ghc/wiki/CustomTypeErros
Great! I have moved this to,


and left a redirect page in the old location.

Is there a Trac ticket for this?

>> ·         Announce the proposal and seek feedback
> We already had a discussion about it, there are notes on the wiki.  I don't
> think any of the comments were about the actual design, the comments seem
> to be more about the implementation.  On that front, if I am to make the
> necessary changes, maybe we could have a chat (or e-mail conversation) to
> make sure that I understand the changes and the motivation for them---the
> wiki is not a great media for question/answer type of discussions.
I'll let you and Simon work this out.

>> ·         Meanwhile make sure the branch reflects what you want to be in
>> it
>> I think that the branch should be in an OK state, except that it is not
> merged with the latest HEAD.
Where can this branch be found?


- Ben
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