Determining if an instance is provided by the environment
David Fox
dsf at seereason.com
Sat May 7 00:06:41 UTC 2016
Is there a way to connect this to template-haskell or haskell-src-exts code?
On Fri, May 6, 2016 at 4:40 PM, Edward Z. Yang <ezyang at mit.edu> wrote:
> Thanks Simon, that has all the ingredients I need.
>
> I wrote some more docs for the function:
> https://phabricator.haskell.org/D2180
>
> Edward
>
> Excerpts from Simon Peyton Jones's message of 2016-05-06 02:05:31 -0700:
> > You probably want a variant on TcDeriv.simplifyDeriv, shorn of its
> complex error reporting.
> >
> > Simon
> >
> > | -----Original Message-----
> > | From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of
> > | Edward Z. Yang
> > | Sent: 06 May 2016 00:21
> > | To: ghc-devs <ghc-devs at haskell.org>
> > | Subject: Determining if an instance is provided by the environment
> > |
> > | Hello all,
> > |
> > | Suppose I have a ClsInst from typechecking the following instance
> > | declaration:
> > |
> > | instance Show [Char] -- i.e. String
> > |
> > | I'd now like to answer the question: "Is this instance 'provided'
> > | by the instance environment?" For example, this instance is provided
> > | given that I have these two instances in the environment:
> > |
> > | instance Show a => Show [a] -- (1)
> > | instance Show Char -- (2)
> > |
> > | However, if I have just instance (1) in the environment, it's not
> > | provided (and if you tried to use show "foo", you'd get the error that
> > | Char is not an instance of Show.)
> > |
> > | Is there are convenient way to do this from TcM? With 'tcMatchTys'
> > | and I can easily test if there is some instance in the environment
> > | which *matches* my instance head (e.g., Show [a] matches Show [Char])
> > | but this doesn't tell me if all the resulting constraints are
> solvable.
> > |
> > | Thanks,
> > | Edward
> > | _______________________________________________
> > | ghc-devs mailing list
> > | ghc-devs at haskell.org
> > |
> https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.ha
> > | skell.org%2fcgi-bin%2fmailman%2flistinfo%2fghc-
> > | devs&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com
> %7c8c304d9b355244c6
> > |
> ee7208d3753be740%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=iWdrnb6hC
> > | 8pexyVkWNG22G%2fgdO10tCBy8nuCxhnO0M8%3d
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20160506/c97e9459/attachment.html>
More information about the ghc-devs
mailing list