#9622: GHCi command to solve a constraint -------------------------------------+------------------------------------- Reporter: monoidal | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.8.3 Component: Compiler | Keywords: (Type checker) | Architecture: Unknown/Multiple Resolution: | Difficulty: Unknown Operating System: | Blocked By: Unknown/Multiple | Related Tickets: Type of failure: | None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Description changed by monoidal: Old description:

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) }}}

New description: 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) Eq Type1 -- Defined in ‘MyModule’ 2) Eq a => Eq (Maybe a) -- Defined in ‘GHC.Classes’ 3) Eq (Maybe Type1) -- Put a ~ Int in 2) and use 1) 4) Eq Bool -- Defined in ‘GHC.Classes’ 5) (Eq a, Eq b) => Eq (a,b) -- Defined in ‘GHC.Classes’ 6) 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#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler