RE: Determining if an instance is provided by the environment

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@haskell.org] On Behalf Of | Edward Z. Yang | Sent: 06 May 2016 00:21 | To: ghc-devs | 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@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