State 单子 - 在 Haskell

Question

I'm trying to create a calculator with an integer state which allows the user to update this state using commands. This is what I've got so far.

{-# LANGUAGE FlexibleContexts #-}
import Control.Monad.Except
import Control.Monad.State


data CalcCmd = EnterC
         | StoreC Int CalcCmd
         | AddC Int CalcCmd
         | MultC Int CalcCmd
         | DivC Int CalcCmd
         | SubC Int CalcCmd


run :: (MonadState Int m, MonadError String m) => CalcCmd -> m ()
run (EnterC) = return()

run (StoreC a b) = do
                  put (a)
                  run b

run (AddC a b) = do
                modify((+) a)
                run b

run (MultC a b) = do
                modify((*) a)
                run b

run (DivC a b) = do
                modify ((div) a)
                run b

run (SubC a b) = do
                modify((-) a)
                run b

No matter what the commands are, the state always ends up as 0. For example, Both "StoreC 7 (EnterC)" and "StoreC 7 (AddC 14 (DivC 3 EnterC))" give the state back as 0. What am I doing wrong?

The textbook says The StoreC command should manually update the state And the EnterC command should terminate the calculation, returning the unit type.

Also When using state monads, is there a few things which I should be doing no matter what the function is about?

Here's the output.

Answer 1

Could not reproduce. runStateT (run cmd3:: CS ()) (0:: Int) returns Right ((), 7) here. (The others do return 0 , because div 3 ((+) 14 7) = div 3 21 = 0 and div 0 ((*) 2 10) = div 0 20 = 0 . In other words, your State monad is doing the calculation you asked it to do, so your misunderstanding, whatever it is, probably is not related to state monads.)

Are you double-plus sure you are loading the file you saved? Is it possible you're saving to a different directory than you are loading from, eg? Or perhaps forgot to press save (I've seen it happen even to professionals...)?