Typed holes and inline typing.

findtype.hs

main = do
    foo <- getLine
    return ()

-- the type of `getLine` is IO String, but how can we figure that out?
-- let's put the wrong type inline and let GHC yell at us
-- "Couldn't match expected type ‘()’ with actual type ‘IO String’"
main2 = do
    -- we're checking the type of getLine here
    foo <- getLine :: ()
    -- we are checking the type of bar
    (bar :: ()) <- getLine
    return ()

-- Typed holes don't really solve this problem, but they're very cool
-- they tell you what you need to make the types fit
-- here, I want to print out the 4. Why doesn't it work?
main3 = putStrLn $ 4

-- so let's put in a typed hole. As in. What do I need to fill in the blank to make my code work.
-- GHC says: Found hole ‘_’ with type: Integer -> String
-- so I need to replace _ with some function of type: Integer -> String
main4 = putStrLn $ _ 4

-- what converts an Integer to a String? `show`. Replace _ with show.
main5 = putStrLn $ show 4

Typed holes: http://dev.stephendiehl.com/hask/#typed-holes

Here's trace which lets you print out debugging stuff if you need. http://dev.stephendiehl.com/hask/#trace

Here's a cool trick to use typed holes to figure out the first question: http://stackoverflow.com/questions/30138726/what-is-the-nearest-equivalent-of-type-in-ghci-in-a-ghc-source-file

Whoa, this is awesome! Thanks for such a detailed answer! This is very understandable.