# WHILE with Types

```
{-# LANGUAGE FlexibleInstances #-}
{-# OPTIONS -fwarn-incomplete-patterns #-}
module WhileTypes where
```

```
import Data.Map (Map)
import qualified Data.Map as Map
import Data.List (nub)
import Control.Monad.Error
import Control.Monad.State
import Test.QuickCheck
```

How would we go about writing a type checker for the simple WHILE programming language? As we go through today's discussion, think about the following questions:

How do type checkers work?

What makes a good type system?

## Syntax

The definition of the WHILE language, including a parser, pretty printer and instances of the `Arbitrary`

type class are in the auxiliary file WhileSyntax.lhs that can be downloaded here.

`import WhileSyntax `

Semantics of WHILE programming language |
---|

This semantics is the similar to that found in Homeworks 3 and 5

As before, we'll use the State monad to keep track of the values of variables.

`data Store = Store (Map Variable Value) deriving (Eq, Show)`

And an error monad to enable runtime errors.

`type EvMonad a = StateT Store (Either String) a`

We'll package these up as an *Evaluation Monad*

```
-- Empty store
empty :: Store
empty = Store Map.empty
```

```
-- lookup variables in the store
slookup :: Variable -> EvMonad Value
slookup x = do
Store m <- get
case (Map.lookup x m) of
Just v -> return v
Nothing -> throwError "Uninitialized variable"
```

```
-- update the value of a variable in the store
update :: Variable -> Value -> EvMonad ()
update x v = do
(Store m) <- get
put (Store (Map.insert x v m))
```

```
evalE :: Expression -> EvMonad Value
evalE (Var x) = slookup x
evalE (Val v) = return v
evalE (Op o e1 e2) = do
v1 <- evalE e1
v2 <- evalE e2
evalB o v1 v2
```

```
evalB :: Bop -> Value -> Value -> EvMonad Value
evalB Plus (IntVal i1) (IntVal i2) = return $ IntVal (i1 + i2)
evalB Minus (IntVal i1) (IntVal i2) = return $ IntVal (i1 - i2)
evalB Times (IntVal i1) (IntVal i2) = return $ IntVal (i1 * i2)
evalB Gt (IntVal i1) (IntVal i2) = return $ BoolVal (i1 > i2)
evalB Ge (IntVal i1) (IntVal i2) = return $ BoolVal (i1 >= i2)
evalB Lt (IntVal i1) (IntVal i2) = return $ BoolVal (i1 < i2)
evalB Le (IntVal i1) (IntVal i2) = return $ BoolVal (i1 <= i2)
evalB _ _ _ = throwError "Incorrect arguments to boolean operator"
```

```
evalS :: Statement -> EvMonad ()
evalS w@(While e s) = do
v <- evalE e
case v of
BoolVal True -> evalS (Sequence s w)
BoolVal False -> return ()
_ -> throwError "While expects boolean expression"
evalS Skip = return ()
evalS (Sequence s1 s2) = evalS s1 >> evalS s2
evalS (Assign x e) = do
v <- evalE e
update x v
evalS (If e s1 s2) = do
v <- evalE e
case v of
BoolVal True -> evalS s1
BoolVal False -> evalS s2
_ -> throwError "If expects boolean expression"
```

## The top-level loop

```
repl :: IO ()
repl = repl' empty where
repl' store = do
putStr "%> "
line <- getLine
case parseCmd line of
Statement stmt ->
case (execStateT (evalS stmt) store) of
Left err -> putStrLn err >> repl' store
Right store' -> repl' store'
Expression exp -> do
case (evalStateT (evalE exp) store) of
Left err -> putStrLn err
Right v -> putStrLn $ display v
repl' store
_ -> putStrLn "what?!?" >> repl' store
```

## Why types?

What is bad about this language?

For one thing, it's hard to reason about programs written in this language. Recall these properties from the quickcheck lecture. They seemed very reasonable, yet sadly they are false.

```
instance Arbitrary Store where
arbitrary = liftM (Store . Map.fromList) arbitrary
shrink (Store m) = map (Store . Map.fromList) (shrink (Map.toList m))
```

```
(===) :: Expression -> Expression -> Property
e1 === e2 = forAll arbitrary $ \st -> evalStateT (evalE e1) st == evalStateT (evalE e2) st
```

```
prop_add_zero_elim :: Property
prop_add_zero_elim = forAll arbitrary $ \ e ->
(Op Plus e $ Val (IntVal 0)) === e
```

```
prop_sub_zero_elim :: Property
prop_sub_zero_elim = forAll arbitrary $ \ e ->
(Op Minus e $ Val (IntVal 0)) === e
```

## Type-checking WHILE

What if we wanted to create a version of the WHILE language that does type checking at compile time?

We can view type checking as an *abstraction* of the evaluator. I.e. instead of calculating a precise value for expressions, we'll approximate those values with types.

We have two sorts of values in this language:

`data Value = IntVal Int | BoolVal Bool deriving (Eq, Show)`

So we'll need exactly two types of values:

```
data Type =
IntTy
| BoolTy
deriving (Eq, Show)
```

Wherever we use `Value`

in the evaluator, we'll replace that usage with a `Type`

instead.

For example, we'll define a monad for the type checker, and then analogous functions for `evalE`

, `evalB`

, and `evalS`

.

typeE :: Expression -> TcMonad Type typeB :: Bop -> Type -> Type -> TcMonad Type typeS :: Statement -> TcMonad ()

However, we'll diverge a bit in the treatment of variables. Like most programming languages, we'll expect the types of variables to be invariant throughout the execution.

That means we'll reject some perfectly fine programs, such as:

X := true; X := 1

Instead, we'll brutally require that variables have a single type.

But how do we determine that type? The WHILE language doesn't include variable declarations!

For simplicity, we'll use an old hack. If the name of the variable starts with a letter at the beginning of the alphabet (like 'A', 'B', etc.) we'll assume that it is a boolean variable. If it starts with a letter at the end, we'll decree that it must be an integer variable.

```
typeVar :: Variable -> TcMonad Type
typeVar (V x) = case x of
(h : _) | h >= 'A' && h <= 'L' -> return BoolTy
(h : _) | h >= 'M' && h <= 'Z' -> return IntTy
_ -> throwError "invalid variable name"
```

That means that our type checking monad only needs to keep track of typing errors, it doesn't need to remember any information about the types of variables.

`type TcMonad a = Either String a`

```
typeE :: Expression -> TcMonad Type
typeE = undefined
```

```
typeB :: Bop -> Type -> Type -> TcMonad Type
typeB = undefined
```

```
typeS :: Statement -> TcMonad ()
typeS = undefined
```

## Why do variables have types?

Why do we need to require that variables have constant types throughout their execution? It's tempting to define the type monad by analogy with the expression monad---if we did so, we could track the types of variables at any point in the program. Thus,

` type TcMonad a = StateT (Map Variable Type) (Either String) a`

However, this doesn't work. Should the following WHILE program type check?

```
if false
then X := false
else X := 3
endif;
Y := X + 4
```

We know that this program will produce the value 7, but the type checker can't know that. Recall that while type checking `if`

statements, the type checker only knows that the type of the expression is a boolean. It doesn't know whether that boolean will be true or false, it's computing an approximate value. That means that the type checker has to check both branches of an `if`

statement, in sequence. But in doing so, we would be passing the type store from one branch to another. That doesn't make sense!

## The REVISED top-level loop

Now we can revise our top-level loop to do type checking FIRST, before evaluation.

```
tyrepl :: IO ()
tyrepl = repl' empty where
repl' store = do
putStr "%> "
line <- getLine
case parseCmd line of
Statement stmt ->
case typeS stmt of
Left err -> putStrLn ("TypeError: " ++ err) >> repl' store
Right () ->
case (execStateT (evalS stmt) store) of
Left err -> putStrLn err >> repl' store
Right store' -> repl' store'
Expression exp ->
case typeE exp of
Left err -> putStrLn ("TypeError: " ++ err) >> repl' store
Right _ -> do
case (evalStateT (evalE exp) store) of
Left err -> putStrLn err
Right v -> putStrLn $ display v
repl' store
_ -> putStrLn "what?!?" >> repl' store
```

## Type Soundness

How do we know that our type system is right? If a program type checks, what does that mean?

A strong static type system is one that has the type soundness property.

*Type Soundness: If a program p type checks, then evaluating p will not produce a runtime error.*

For now, we can think about trying to test this property.

```
prop_no_errorE :: Expression -> Property
prop_no_errorE e = forAll arbitrary $ \s -> typeChecks e ==>
case runStateT (evalE e) s of
Left err -> False
Right v -> True
```

```
typeChecks :: Expression -> Bool
typeChecks e = case typeE e of
Left err -> False
Right ty -> True
```

Unfortunately, we can't test the analogous property for statements. Why not?

## News :

Welcome to CIS 552!

See the home page for basic
information about the course, the schedule for the lecture notes
and assignments, the resources for links to the required software
and online references, and the syllabus for detailed information about
the course policies.