I'm learning Monad Transformers and decided to write an interpreter for a simple language(with loop constructs) similar to Brainfuck using Monad Transformers. I would like to terminate the interpreter after certain number of statements.
This simple language is made of single memory cell capable of holding an Int and 5 instructions Input, Output, Increment, Decrement and Loop. A loop terminates when value in the memory is zero. Input is read from a list and similarly output is written to another list. Increment and Decrement does +1 and -1 to memory correspondingly.
I'm using World
type to keep track of input, output (streams) and memory, Sum Int
to count number of instructions evaluated. Except World
to terminate evaluation after certain statements.
module Transformers where
import qualified Data.Map as Map
import Data.Maybe
import Control.Monad.State.Lazy
import Control.Monad.Writer.Lazy
import Control.Monad.Except
data Term = Input
| Output
| Increment
| Decrement
| Loop [Term]
deriving (Show)
data World = World {
inp :: [Int],
out :: [Int],
mem :: Int
} deriving Show
op_limit = 5
loop
:: [Term]
-> StateT World (WriterT (Sum Int) (Except World)) ()
-> StateT World (WriterT (Sum Int) (Except World)) ()
loop terms sp = sp >> do
s <- get
if mem s == 0 then put s else loop terms (foldM (\_ t -> eval t) () terms)
limit :: StateT World (WriterT (Sum Int) (Except World)) ()
limit = do
(s, count) <- listen get
when (count >= op_limit) $ throwError s
tick :: StateT World (WriterT (Sum Int) (Except World)) ()
tick = tell 1
eval :: Term -> StateT World (WriterT (Sum Int) (Except World)) ()
eval Input =
limit >> tick >> modify (\s -> s { inp = tail (inp s), mem = head (inp s) })
eval Output = limit >> tick >> modify (\s -> s { out = mem s : out s })
eval Increment = limit >> tick >> modify (\s -> s { mem = mem s + 1 })
eval Decrement = limit >> tick >> modify (\s -> s { mem = mem s - 1 })
eval (Loop terms) = loop terms (void get)
type Instructions = [Term]
interp :: Instructions -> World -> Either World (World, Sum Int)
interp insts w =
let sp = foldM (\_ inst -> eval inst) () insts
in runExcept (runWriterT (execStateT sp w))
Example run in ghci:
*Transformers> interp [Loop [Output, Decrement]] $ World [] [] 5
Right (World {inp = [], out = [1,2,3,4,5], mem = 0},Sum {getSum = 10})
The monad limit
based on count and should decide to either Fail with current state or do nothing. But I noticed that count
in (s, count) <- listen get
is always zero. I don't understand why is this happening. Please help me understand where I went wrong.
- Is my ordering of transformers in the stack correct? Are there any rules (informal) to decide the layering?