Determining if an instance is provided by the environment

[Edward Z. Yang]

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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