Determining if an instance is provided by the environment

[Simon Peyton Jones]

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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