#9819: Create typesafe method of obtaining dictionary types from class definitions, and constraint objects from dictionary types -------------------------------------+------------------------------------- Reporter: | Owner: spacekitteh | Status: new Type: feature | Milestone: request | Version: Priority: normal | Keywords: Component: Compiler | Architecture: Unknown/Multiple Resolution: | Difficulty: Moderate (less Operating System: | than a day) Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by goldfire): I'm not sure what you mean by "grant access"... but perhaps this will help: When you pattern match on the `Dict` constructor, you get the `c` constraint in the context. So, the following is conceivable: {{{ -- this type is actually from ekmett's "constraints" package data Dict c where Dict :: c => Dict c class IntLike a where fromInt :: Int -> a toInt :: a -> Int dictForIntLikeString :: Dict (IntLike String) dictForIntLikeString = ... stringToInt :: String -> Int stringToInt s = case dictForIntLikeString of Dict -> toInt s }}} Note that there is no `instance IntLike String` around -- the reason `toInt` can be called in `stringToInt` is because of the pattern-match on `Dict`. Now, this example falls apart because the implementation of `dictForIntLikeString` seems impossible without a `instance IntLike String` dictionary available. But perhaps this answers your question about what `Dict` can do. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9819#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler