#9622: GHCi command to solve a constraint -------------------------------------+------------------------------------- Reporter: monoidal | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.3 checker) | Operating System: Keywords: | Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Difficulty: Unknown | None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- It would be nice to have a `:solve` command for ghci that took a class constraint and returned either its derivation or an error message. A possible interface could look like so: {{{ ghci> :solve Eq (Maybe Type1, Bool) 1) instance Eq Type1 -- Defined in ‘MyModule’ 2) instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Classes’ 3) instance Eq (Maybe Type1) -- Put a ~ Int in 2) and use 1) 4) instance Eq Bool -- Defined in ‘GHC.Classes’ 5) instance (Eq a, Eq b) => Eq (a,b) -- Defined in ‘GHC.Classes’ 6) instance Eq (Maybe Type1, Bool) -- Put a ~ Maybe Type1, b ~ Bool in 5) and use 3), 4) ghci> :solve Eq (Int -> Int) No instance for Eq (Int -> Int) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9622 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler