Executing a function randomly of a list of functions - haskell

Question

=== Evaluation of HStatement (bar if and selection) ===
evalStatement_ :: Env -> HStatement -> IOThrowsError ()
evalStatement_ env (Do cond expr) = evalVal env cond >>= \x -> case x of
HBool False -> return ()
HBool True -> do
traverse_ (evalStatement_ env) expr
evalStatement_ env (Do cond expr)
evalStatement_ env (Skip skip) = return ()
evalStatement_ env (Print (HString val)) = getVar env val >>= \x -> liftIO $ putStrLn $ show x
evalStatement_ env (Print val) = evalVal env val >>= \x -> liftIO $ putStrLn $ show x
evalStatement_ env (Eval val) = do
result <- evalVal env val
return ()
=== Representation of Selection & If ===
parseIf :: Parser HStatement
parseIf = do
string "("
cond <- parseArith
string ")->"
spaces
expr <- many1 $ parseStatements
spaces
return $ If (cond, expr)
parseSelection :: Parser HStatement
parseSelection = do
_ <- string "if"
spaces
selection <- many1 $ parseIf
spaces
_ <- string "fi"
spaces
return $ Selection selection
N.B : If evaluation of selection is changed to the below, then the program runs and terminates and does give output:
evalStatement_ env (Selection if_ selection fi_ n) = evalStatement_ env (selection !! randIdx n) >>= \res -> if res == ()
then return ()
else return ()
The output however gives varying amounts of the even integers between 1 and 10. For example one output would print all even integers and another prints on the number 6.
tldr; is there a way to execute a random function from a list of functions randomly and if the result is not ideal, reexecute the function to execute a random function until the result is idea?
I'm trying to write a function which executes a random entry in a list of functions. Each entry in the list is constructed in the following way: If (HVal, HStatement) -- If (Guard,Statement) where
HVal:
data HVal
= HInteger Integer
HBool Bool
HString String
HList [HVal]
Length HVal
Arith HVal Op HVal
Assign String HVal
deriving (Eq, Read)
HStatement:
data HStatement
= Eval HVal
| Print HVal
| Do HVal [HStatement]
| If (HVal, [HStatement])
| Selection [HStatement]
deriving (Eq, Read)
What I tried so far was using Asyncs race function as per my question yesterday. My thinking behind this was if there exists a list of n entries in a list that are constructed as If (HVal, HStatement), then running a race function over a list that only contain a list of HStatements whose guards were evaluated to true would return the function that executes the fastest of the true guards.
Trying to incorporate this raceAll behaviour into my code base proved to be too difficult to me due to the constraint of IO. I redid the approach by considering using a random number generator.
So now I'm generating a random index of the list of guard statements pairs. I execute the entry in this list and perform a case analysis. If the output is () then I call the function again otherwise I return the output. To do this I'm using two functions wherein selection represents a list of if's:
evalStatement_ env (If (cond, expr)) = evalVal env cond >>= \x -> case x of
HBool False -> return ()
HBool True -> traverse_ (evalStatement_ env) expr
evalStatement_ env (Selection selection) = evalStatement_ env (selection !! randIdx 1) >>= \res -> case res of -- randIdx produces an index between 0 and 1 representing the two branches in the selection block that could be taken
() -> evalStatement_ env (Selection selection)
_ -> return $ res
randIdx n = unsafePerformIO (getStdRandom (randomR (0, n - 1)))
Take the following program as example:
f := [1 2 3 4 5 6 7 8 9 10]
n := 0
N := len(f)
Do (n < N)->
a := f.n
if ((a % 2) = 0)-> print(a)
((a % 1) = 1)-> print(a)
fi
n := n + 1
Od
What occurs here is that the program gives no output at all and doesn't terminate. What I would have expected to happen was that a random index is generated between 0 and the number of possible branches minus one. Then this would have been evaluated and if it returned a value, this would have been taken otherwise if it was the unit type, a new random index would have been generated and that would have been used.
I can execute the program however if the function definition for selection is traverse_ (evalStatement_ env) selection but I'm just unsure on how to achieve this pseudo randomness. Any help would be appreciated!

Answer 1

You say,
If the output is () then I call the function again otherwise I return the output.
This is a strange thing to say, because there is no "otherwise" -- if your thing returns () sometimes, it can never return anything but (), because there is no other value with the same type! In particular, this means it is impossible to reach the _ branch of your case.
In your language as shown here, statements fundamentally do not compute data. Perhaps you should change your Selection constructor to take an [(HVal, HStatement)] instead of an [HStatement] (representing pairs of the computation that returns something interesting that you can case on together with the statement to execute in some appropriate branch of that case), or modify the type that statements compute to something richer than ().