Re: [Haskell-cafe] Help: Main: thread blocked in MVar operation

Hey, So, I couldn't really get a small code sample. But I have a new example of the same problem. Anyway, let me just give you the overall picture. I am building an interpreter and I want to evaluate the definition of a recursive function. This entails registering the function symbol in the symbol table while at the same time associating the function name to the expression that represents the function body. Naturally, the expression needs to be evaluated, thus the circularity and the DoRec extension as in the following code: evalM (DefnStx str body) = do rec addBindM str expr expr <- evalM body return expr I am using the state monad so I don't have to manually pass the symbol table around all the time. Now, there are two things: the first is that this works with the state monad without any problem: type InterpreterM a = StateT ExprEnv a The second is that, similarly to the previous email, when I use the state monad transformer the program no longer works. With the IO monad as the wrapped monad the program crashed with the "MVar..." error message. In the new code I am no longer using the IO monad. Instead, I am using the Error monad. So I have the following definition type InterpreterM a = StateT ExprEnv (Either String) a With this definition, the program enters an infinite loop. I am not even using the "throwError" and "catchError" functions yet! I just change the definition of InterpreterM, which is the evaluator monad. What can I do ? Best regards, José On 24-10-2012 01:01, Joey Adams wrote:

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Hey everyone,

I changed my code I now I get the following error message Main: thread blocked indefinitely in an MVar operation

Before the change, I was using the State monad with runState. Then, I changed the code to use the StateT monad transformer wrapped around IO monad and runStateT. And this change introduced the error message.

BTW I am using DoRec extension, maybe it is the source of the problem, but I don't know. See if you can reproduce the problem using a small code sample. The

On Tue, Oct 23, 2012 at 5:03 PM, "José A. Lopes" wrote: problem is likely that your program is trying to use a state value that hasn't been produced yet.

DoRec uses fixIO for the IO monad. fixIO passes a callback its own return value. It's not magic; it only works if the thunk is not forced within the callback.

Take a look at how fixIO is implemented:

http://hackage.haskell.org/packages/archive/base/latest/doc/html/src/System-...

-- José António Branquinho de Oliveira Lopes Instituto Superior Técnico Technical University of Lisbon