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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 test.hs:1:1-14 (and originally defined in ‘System.IO’)) putStr :: String -> IO () (imported from ‘Prelude’ at test.hs:1:1-14 (and originally defined in ‘System.IO’)) ``` We'd like here for ps2 to be suggested as well, but for that we require subsumption. Reviewers: austin, bgamari, dfeuer, mpickering Reviewed By: dfeuer, mpickering Subscribers: mpickering, Wizek, dfeuer, rwbarton, thomie Differential Revision: https://phabricator.haskell.org/D3361
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