• Matthías Páll Gissurarson's avatar
    Show valid substitutions for typed holes · 26c95f46
    Matthías Páll Gissurarson authored
    The idea is to implement a mechanism similar to PureScript, where they
    suggest which identifiers in scope would fit the given hole. In
    PureScript, they use subsumption (which is what we would like here as
    well). For subsumption, we would have to check each type in scope
    whether the hole is a subtype of the given type, but that would require
    `tcSubType` and constraint satisfiability checking. Currently,
    `TcSimplify` uses a lot of functions from `TcErrors`, so that would
    require more of a rewrite, I will hold on with that for now, and submit
    the more simpler type equality version.
    As an example, consider
    ps :: String -> IO ()
    ps = putStrLn
    ps2 :: a -> IO ()
    ps2 _ = putStrLn "hello, world"
    main :: IO ()
    main = _ "hello, world"
    The results would be something like
        • Found hole: _ :: [Char] -> IO ()
        • In the expression: _
          In a stmt of a 'do' block: _ "hello, world"
          In the expression:
            do _ "hello, world"
        • Relevant bindings include
            main :: IO () (bound at test.hs:13:1)
            ps :: String -> IO () (bound at test.hs:7:1)
            ps2 :: forall a. a  -> IO () (bound at test.hs:10:1)
          Valid substitutions include
            putStrLn :: String
                        -> IO () (imported from ‘Prelude’ at
                                  (and originally defined in
            putStr :: String
                      -> IO () (imported from ‘Prelude’ at
                                (and originally defined in ‘System.IO’))
    We'd like here for ps2 to be suggested as well, but for that we require
    Reviewers: austin, bgamari, dfeuer, mpickering
    Reviewed By: dfeuer, mpickering
    Subscribers: mpickering, Wizek, dfeuer, rwbarton, thomie
    Differential Revision: https://phabricator.haskell.org/D3361
TcErrors.hs 121 KB