Specialized type hints

Simon Peyton Jones simonpj at microsoft.com
Fri Mar 4 16:36:11 UTC 2016

indication that a more general type error improvement sweep would be welcome

Absolutely yes.  But it depends on someone saying “I want to do that”; and of course on how hard or easy it turns out to be.

general UX improvements I would like to undertake.

Great!  Make Trac tickets proposing changes; debate them with others to form a consensus about what would be good; implement them… that would all be amazing.


From: Christopher Allen [mailto:cma at bitemyapp.com]
Sent: 04 March 2016 16:24
To: Simon Peyton Jones <simonpj at microsoft.com>
Cc: ghc-devs at haskell.org
Subject: Re: Specialized type hints

I was looking for an indication that a more general type error improvement sweep would be welcome. I could pinpoint a particular type error if you liked (such as the one that prompted this email), but there are general UX improvements I would like to undertake.

On Fri, Mar 4, 2016 at 4:25 AM, Simon Peyton Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>> wrote:

Improving error message is an excellent goal, and one that I have spent more hours on than I care to tell you.

If you have a particular one in mind, could you open a Trac ticket with a particular reproducible test case, the error message you get, and the error message you’d like to get.



From: ghc-devs [mailto:ghc-devs-bounces at haskell.org<mailto:ghc-devs-bounces at haskell.org>] On Behalf Of Christopher Allen
Sent: 03 March 2016 07:55
To: ghc-devs at haskell.org<mailto:ghc-devs at haskell.org>
Subject: Specialized type hints

I'd like to see how warm people would be to catching GHC's type error quality up a bit.

I did a write-up on a confusion a reader of our book had:


This is not new. A lot of people complain about this particular type error in particular when they say GHC has bad type errors. I don't think GHC's type errors are bad, but I do think they could be improved and this particular issue has an unfortunate source to sink distance.

I would rather type error improvements not be buried behind a "silly beginners only" flag and that they just be part of improving the UX for everyone. With that proviso, how likely would specialized type error hints and some general error message fix ups be regarded?

By specialized I mean, "detect that they tried to find an instance of Num for (-> something something) and suggest that they did the wrong thing, with possible fixes: X Y Z". Ideally before the "hey do you want FlexibleContexts?!" thing fires.

I do not think I am capable of doing this, but being able to zoom in, clang style, to the expression where they are (probably accidentally) using a function like a Num or a Num like a function would be pretty valuable so they don't have to guess-n-check parenthesize their code.

Chris Allen

Chris Allen
Currently working on http://haskellbook.com<https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fhaskellbook.com&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c757f72d364664cd81b8508d344497063%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=EaRmV%2bS3%2f1vgr6klNmr0gM89ofO7CP2%2b7h12bJ%2bJwi8%3d>
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