#9948: Recommend class constraint instead of instance constraint -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by crockeea): Replying to [comment:7 rwbarton]:
I can see why you might want this behavior in some cases, but would you really want
{{{ isPalindrome :: [a] -> Bool isPalindrome xs = xs == reverse xs }}}
to suggest adding an `Eq [a]` constraint (that's not even valid Haskell 98!), or some function to dump a data structure to suggest adding a `Show [(Int, Maybe a)]` constraint?
I see your point on these examples. How's the feature request for GHC mind-reading coming along?
I'm also a bit confused by the workaround that you mentioned; if you know that `Foo` is the class containing `foo` (which you must to comment out its instances) then don't you already know what constraint to add? Is it very difficult in your use cases to work out what the parameter to the class should be?
Yes, it should be pretty obvious which constraint I need. The only minor point I could make is that it isn't always immediately clear if the function you are using is top-level or a method. The only way to resolve that is to look at some documentation. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9948#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler