## Description

• Files you should submit: Calc.hs, containing a module of the same name.

As we saw in class, Haskell’s type classes provide ad-hoc polymorphism, that is, the ability to decide what to do based on the type of an input. This homework explores one interesting use of type classes in constructing domain-specific languages.

Expressions

On day one of your new job as a software engineer, you’ve been asked to program the brains of the company’s new blockbuster product: a calculator. But this isn’t just any calculator! Extensive focus group analysis has revealed that what people really want out of their calculator is something that can add and multiply integers. Anything more just clutters the interface.

Your boss has already started by modeling the domain with the following data type of arithmetic expressions:

data ExprT = Lit Integer

| Add ExprT ExprT

| Mul ExprT ExprT deriving (Show, Eq)

This type is capable of representing expressions involving integer constants, addition, and multiplication. For example, the expression (2 + 3)× 4 would be represented by the value

Mul (Add (Lit 2) (Lit 3)) (Lit 4).

Your boss has already provided the definition of ExprT in ExprT.hs, so as usual you just need to add import ExprT to the top of your file. However, this is where your boss got stuck.

Exercise 1

Write Version 1 of the calculator: an evaluator for ExprT, with the signature

eval :: ExprT -> Integer

For example, eval (Mul (Add (Lit 2) (Lit 3)) (Lit 4)) == 20.

Exercise 2

The UI department has internalized the focus group data and is ready to synergize with you. They have developed the front-facing user-interface: a parser that handles the textual representation of the selected language. They have sent you the module Parser.hs, which exports parseExp, a parser for arithmetic expressions. If you pass the constructors of ExprT to it as arguments, it will convert Strings representing arithmetic expressions into values of type ExprT. For example:

*Calc> parseExp Lit Add Mul “(2+3)*4”

Just (Mul (Add (Lit 2) (Lit 3)) (Lit 4))

*Calc> parseExp Lit Add Mul “2+3*4”

Just (Add (Lit 2) (Mul (Lit 3) (Lit 4)))

*Calc> parseExp Lit Add Mul “2+3*” Nothing

Leverage the assets of the UI team to implement the value-added function

evalStr :: String -> Maybe Integer

which evaluates arithmetic expressions given as a String, producing Nothing for inputs which are not well-formed expressions, and Just n for well-formed inputs that evaluate to n.

Exercise 3

Good news! Early customer feedback indicates that people really do love the interface! Unfortunately, there seems to be some disagreement over exactly how the calculator should go about its calculating business. The problem the software department (i.e. you) has is that while ExprT is nice, it is also rather inflexible, which makes catering to diverse demographics a bit clumsy. You decide to abstract away the properties of ExprT with a type class.

Create a type class called Expr with three methods called lit, add, and mul which parallel the constructors of ExprT. Make an instance of

Expr for the ExprT type, in such a way that

mul (add (lit 2) (lit 3)) (lit 4) :: ExprT

== Mul (Add (Lit 2) (Lit 3)) (Lit 4)

*Calc> :t Lit at the ghci prompt.

Remark. Take a look at the type of the foregoing example expression:

*Calc> :t mul (add (lit 2) (lit 3)) (lit 4)

Expr a => a

What does this mean? The expression mul (add (lit 2) (lit 3)) (lit 4) has any type which is an instance of the Expr type class. So writing it by itself is ambiguous: GHC doesn’t know what concrete type you want to use, so it doesn’t know which implementations of mul, add, and lit to pick.

reify :: ExprT -> ExprT reify = id

But its real purpose is to constrain the type of its argument to ExprT.

Now we can write things like reify $ mul (add (lit 2) (lit 3)) (lit 4)

at the ghci prompt.

Exercise 4

The marketing department has gotten wind of just how flexible the calculator project is and has promised custom calculators to some big clients. As you noticed after the initial roll-out, everyone loves the interface, but everyone seems to have their own opinion on what the semantics should be. Remember when we wrote ExprT and thought that addition and multiplication of integers was pretty cut and dried? Well, it turns out that some big clients want customized calculators with behaviors that they have decided are right for them.

The point of our Expr type class is that we can now write down arithmetic expressions once and have them interpreted in various ways just by using them at various types.

Make instances of Expr for each of the following types:

• Integer — works like the original calculator

• Bool — every literal value less than or equal to 0 is in-

terpreted as False, and all positive Integers are interpreted as True; “addition” is logical or,

“multiplication” is logical and

• MinMax — “addition” is taken to be the max function, while

“multiplication” is the min function

• Mod7 — all values should be in the ranage 0 . . . 6, and

all arithmetic is done modulo 7; for example,

5 + 3 = 1.

The last two variants work with Integers internally, but in order to provide different instances, we wrap those Integers in newtype wrappers. These are used just like the data constructors we’ve seen before.

newtype MinMax = MinMax Integer deriving (Eq, Show) newtype Mod7 = Mod7 Integer deriving (Eq, Show)

Once done, the following code should demonstrate our family of calculators:

testExp :: Expr a => Maybe a testExp = parseExp lit add mul “(3 * -4) + 5”

testInteger = testExp :: Maybe Integer

testBool = testExp :: Maybe Bool

testMM = testExp :: Maybe MinMax

testSat = testExp :: Maybe Mod7

Try printing out each of those tests in ghci to see if things are working. It’s great how easy it is for us to swap in new semantics for the same syntactic expression!

You must complete at least one of the following two exercises:

Exercise 5 (do this OR exercise 6)

The folks down in hardware have finished our new custom CPU, so we’d like to target that from now on. The catch is that a stackbased architecture was chosen to save money. You need to write a version of your calculator that will emit assembly language for the new processor.

The hardware group has provided you with StackVM.hs, which is a software simulation of the custom CPU. The CPU supports six operations, as embodied in the StackExp data type:

data StackExp = PushI Integer

| PushB Bool

| Add

| Mul

| And

| Or

deriving Show

type Program = [StackExp]

PushI and PushB push values onto the top of the stack, which can store both Integer and Bool values. Add, Mul, And, and Or each pop the top two items off the top of the stack, perform the appropriate operation, and push the result back onto the top of the stack. For example, executing the program

[PushB True, PushI 3, PushI 6, Mul]

will result in a stack holding True on the bottom, and 18 on top of that.

If there are not enough operands on top of the stack, or if an operation is performed on operands of the wrong type, the processor will melt into a puddle of silicon goo. For a more precise specification of the capabilities and behavior of the custom CPU, consult the reference implementation provided in StackVM.hs.

Your task is to implement a compiler for arithmetic expressions. Simply create an instance of the Expr type class for Program, so that arithmetic expressions can be interpreted as compiled programs. For any arithmetic expression exp :: Expr a => a it should be the case that

stackVM exp == Right [IVal exp]

Note that in order to make an instance for Program (which is a type synonym) you will need to enable the TypeSynonymInstances language extension, which you can do by adding

{-# LANGUAGE TypeSynonymInstances #-}

as the first line in your file.

Finally, put together the pieces you have to create a function

compile :: String -> Maybe Program

which takes Strings representing arithmetic expressions and compiles them into programs that can be run on the custom CPU.

Exercise 6 (do this OR exercise 5)

Some users of your calculator have requested the ability to give names to intermediate values and then reuse these stored values later.

To enable this, you first need to give arithmetic expressions the ability to contain variables. Create a new type class HasVars a which contains a single method var :: String -> a. Thus, types which are instances of HasVars have some notion of named variables.

Start out by creating a new data type VarExprT which is the same as ExprT but with an extra constructor for variables. Make VarExprT an instance of both Expr and HasVars. You should now be able to write things like

*Calc> add (lit 3) (var “x”) :: VarExprT

But we can’t stop there: we want to be able to interpret expressions containing variables, given a suitable mapping from variables to values. For storing mappings from variables to values, you should use the Data.Map module. Add import qualified Data.Map as M

at the top of your file. The qualified import means that you must prefix M. whenever you refer to things from Data.Map. This is standard practice, since Data.Map exports quite a few functions with names that overlap with names from the Prelude. Consult the Data.Map documentation to read about the operations that are sup-

ported on Maps. http://hackage.haskell.org/

Implement the following instances: packages/archive/containers/latest/ doc/html/Data-Map.html

instance HasVars (M.Map String Integer -> Maybe Integer)

instance Expr (M.Map String Integer -> Maybe Integer)

The first instance says that variables can be interpreted as functions from a mapping of variables to Integer values to (possibly) Integer values. It should work by looking up the variable in the mapping.

The second instance says that these same functions can be interpreted as expressions (by passing along the mapping to subexpressions and combining results appropriately).

Note: to write these instances you will need to enable the FlexibleInstances language extension by putting

{-# LANGUAGE FlexibleInstances #-}

as the first line in your file.

Once you have created these instances, you should be able to test them as follows:

withVars :: [(String, Integer)]

-> (M.Map String Integer -> Maybe Integer)

-> Maybe Integer withVars vs exp = exp $ M.fromList vs

*Calc> :t add (lit 3) (var “x”) add (lit 3) (var “x”) :: (Expr a, HasVars a) => a *Calc> withVars [(“x”, 6)] $ add (lit 3) (var “x”)

Just 9

*Expr> withVars [(“x”, 6)] $ add (lit 3) (var “y”)

Nothing

*Calc> withVars [(“x”, 6), (“y”, 3)]

$ mul (var “x”) (add (var “y”) (var “x”))

Just 54

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