Determining if an instance is provided by the environment

[Simon Peyton Jones]

|  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?

Correct, since you just want a yes/no answer.  Just make up some fresh Ids with newEvVar.

Simon

|  -----Original Message-----
|  From: Edward Z. Yang [mailto:ezyang at mit.edu]
|  Sent: 07 May 2016 01:34
|  To: David Fox <dsf at seereason.com>
|  Cc: Simon Peyton Jones <simonpj at microsoft.com>; ghc-devs <ghc-
|  devs at haskell.org>
|  Subject: Re: Determining if an instance is provided by the environment
|  
|  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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