TypeInferenceProblems.hs

data Foo  = Foo { firstItem :: Int
                , secondItem :: Float}




processFoos incomingFoos =  do
   let foos = fmap firstItem incomingFoost
   return $ toJSON $ foos



-- without specification the inferred signature looks like
processFoos :: (Functor f) => f a -> IO ()


-- But there are a lot of Functors out there.
-- one might serialize as a list as "[1,2,3,4,5]"
-- but a tree as "{'leaf':[{'leaf':3},{'leaf':4}]}"

runProcessFoos :: IO ()
runProcessFoos do
  foosFromDB   <- getFoosFromDB
  processFoos foosFromDB  


-- So the DB foos can be changed from a Tree to a list w/o
-- any warning from the compiler


-- however, if the type signature is specified, then the warning
-- comes in nice and clear!

processFoos :: [Foo] -> IO ()


-- Now that code can't be changed w/o a warning from the compiler