Quoting Ruben Astudillo (2019-02-09 22:24:52)
Dear list
Playing on ghci I encountered the following type
GHCi, version 8.6.3: http://www.haskell.org/ghc/ :? for help Prelude> :m +Control.Concurrent.STM Prelude Control.Concurrent.STM> var1 <- atomically $ newEmptyTMVar Prelude Control.Concurrent.STM> :t var1 var1 :: TMVar GHC.Types.Any Prelude Control.Concurrent.STM>
I would think `var1 :: TMVar a` as the documentation says it should. Reading on `GHC.Types` `Any` seems to be a type family related to some laws on `unsafeCoerce`. Can anybody explain me why is this sensible defaulting and how to obtain the expected result?
There's a fine distinction here: The docs don't specify: `var1 :: forall a. TMVar a` but rather: `newEmptyTMVar :: forall a. STM (TMVar a)`. I've inserted the explicit foralls for clarity. Suppose the behavior was as you expected (I'll use regular MVars just for brevity): var <- newEmptyMVar ghci> :t var var :: forall a. MVar a Then, because the variable is polymorphic, we should be able to do: putMVar var "Hello" and, we should also be able to do: putMVar var 43 But this is clearly unsafe, because we've put values of two different types in the same MVar! In a full program, what would most likely happen is, the first time GHC sees a use of putMVar, it constrains the type variable to match that type. In something like this: do var <- newEmptyMVar putMVar var "hello" ... On the second line, GHC constrains the `a` type variable to be `String`, and so marks that particular use of `newEmptyMVar` as having been used at type `IO (MVar String)`. If we don't actually do anything with the MVar, like: someAction = do var <- newEmptyMVar ... return var then `someAction` is inferred to be of type `forall a. IO (MVar a)`, just like `newEmptyMVar` itself. The constraining will happen wherever the MVar actually ends up getting used. But, in the REPL, we're in this weird state where we have access to the variable and can as what it's type is, but then things can happen *later* that will force GHC to change the answer. -Ian