I'm trying to write an evaluation function for a language that I am working on in which non-determinism can be permitted within an if-block, called a selection block. What I'm trying to achieve is the ability to pick an if/selection statement from the block whose guard is true and evaluate it but it doesn't matter which one I pick.
From searching, I found an example that performs in a similar way to what I would like to achieve through modelling coinflips. Below is my adapation of it but I'm having issue in applying this logic to my problem.
import Control.Monad
data BranchType = Valid | Invalid deriving (Show)
data Branch = If (Bool, Integer) deriving (Show, Eq)
f Valid = [If (True, 1)]
f Invalid = [If (False, 0)]
pick = [Invalid, Invalid, Valid, Invalid, Valid]
experiment = do
b <- pick
r <- f b
guard $ fstB r
return r
s = take 1 experiment
fstB :: Branch -> Bool
fstB (If (cond, int)) = cond
main :: IO ()
main = putStrLn $ show $ s -- shows first branch which could be taken.
Below is my ADT and what I have been trying to make work:
data HStatement
= Eval HVal
| Print HVal
| Skip String
| Do HVal [HStatement]
| If (HVal, [HStatement])
| IfBlock [HStatement] -- made up of many If
| Select [HStatement] -- made up of many If
deriving (Eq, Read)
fstIf :: HStatement -> Bool
fstIf (If (cond, body)) = if hval2bool cond == True
then True
else False
h :: Env -> HStatement -> IOThrowsError ()
h env sb = do
x <- g env sb
guard $ fstIf x -- Couldn't match expected type ‘HStatement’ with actual type ‘[HStatement]’
-- after guard, take 1 x then evaluate
g :: Env -> HStatement -> IOThrowsError [HStatement]
g env (Select sb) = mapM (\x -> f env x) sb
f :: Env -> HStatement -> IOThrowsError HStatement
f env (If (cond, body)) = evalHVal env cond >>= \x -> case x of
Bool True -> return $ If (Bool True, body)
Bool False -> return $ If (Bool False, body)
The error I receive is the following : Couldn't match expected type ‘HStatement’ with actual type ‘[HStatement]’ at the guard line. I believe the reason as to why the first section of code was successful was because the values were being drawn from List but in the second case although they're being drawn from a list, they're being drawn from a [HStatement], not something that just represents a list...if that makes any sort of sense, I feel like I'm missing the vocabulary.
In essence then what should occur is given a selection block of n statement, a subset of these are produced whose guards are true and only one statement is taken from it.
The error message is pretty clear now that you have some types written down. g returns IOThrowsError [HStatement], so when you bind its result to x in h, you have an [HStatement]. You then call fstIf, which expects a single HStatement, not a list. You need to decide how to handle the multiple results from g.