Getting element out of IO in Haskell

Question

I am working through the amazing Write Yourself a Scheme in 48 Hours and have completed the core tasks and wanted to extend it but ran into problem. What I wanted to do is make eval function available to runtime, but have issue storing it into the global environment.

The runtime environment is of type:

type Env = IORef [(String, IORef LispVal)]

The Haskell eval implementation is of type:

eval :: Env -> LispVal -> IOThrowsError LispVal

The global environment is a mapping of type:

primitiveBindings :: IO Env

as it contain functions performing IO mixed with pure functions. My attempt was to set the runtime eval to the host eval partially applied with global environment like this:

baseFun :: [(String, [LispVal] -> IOThrowsError LispVal)]
baseFun = [("eval", unaryOp (eval (liftIO $ readIORef primitiveBindings)))]

Where unaryOp is:

unaryOp :: (LispVal -> ThrowsError LispVal) -> [LispVal] -> ThrowsError LispVal
unaryOp f [v] = f v

I wanted to then add the elements into the global environment but I get a compile error of:

Couldn't match expected type `IORef a'
       against inferred type `IO Env'
In the first argument of `readIORef', namely `primitiveBindings'
In the second argument of `($)', namely `readIORef primitiveBindings'
In the first argument of `eval', namely
    `(liftIO $ readIORef primitiveBindings)'

It appear that this pattern of readIORef env happen often in the code, so it unclear why it's not working here. I would greatly appreciate enlightenment in my errors. For reference my code is almost exactly like the final code for the original tutorial as a reference.

Thank you

Answer 1

As far as I can tell, primitiveBindings is not a global environment, but rather an action that, when performed, generates a fresh mapping with the primitive bindings already set up. Perhaps a better name for it would have been createPrimitiveBindings. There is no way to have proper globals in Haskell without unsafePerformIO hacks.

Therefore, the way you're trying to add eval would make it evaluate everything in a new environment, since you're re-running the primitiveBindings action. We can easily take care of the type errors, if that is indeed what you intended:

baseFun :: [(String, [LispVal] -> IOThrowsError LispVal)]
baseFun = [("eval", evalInNewEnv)]
    where evalInNewEnv args = do
              env <- liftIO primitiveBindings
              unaryOp (eval env) args

As you see, there is no need for any readIORef here, since eval already expects Env which just a type synonym for an IORef.

However, it sounds to me like you'd like eval to work in a "global" environment, in which case, you would need to have this environment passed to you somehow, since as I mentioned, there are no globals. For example, we can define something like this which would create a new environment with eval in it.

primitiveBindingsWithEval :: IO Env
primitiveBindingsWithEval = do
    env <- primitiveBindings    
    bindVars env [("eval", unaryOp (eval env))]

You can then replace existing uses of primitiveBindings with primitiveBindingsWithEval when you want a fresh environment with eval in it.