Determining if an instance is provided by the environment

[Edward Z. Yang]

Well, I have to write the new variant of this function :)

Simon, I ran into a minor complication: if my instance is
something like:

    instance Show a => Show [a]

I think I need to pass in a 'CtGiven' for 'Show a'.  However,
I don't know what to pass as the evidence to the 'CtGiven'
constraints.  My guess is that it doesn't matter?

Edward

Excerpts from David Fox's message of 2016-05-06 17:06:41 -0700:
> 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
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