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We saw how the Maybe monad captures a computational feature that implements a kind of simple exception handling. The Reader monad similarly captures a computational feature that threads a read-only environment through a computation. Recall the signature of the current eval function for FBAE:

evalM :: Env -> FBAE -> (Maybe FBAE)


where Env is defined as a list of string, value pairs:

type Env = [ (string,FBAEVal) ]


When using evalM, an environment is explicitly passed as an argument. The environment is updated by bind and application, then passed to subsequent calls to evalM. Even when a term does not require an environment for evaluation, it still appears in the evalM signature. While this works fine, we typically don’t pass environments to interpreters to evaluation functions. For example, one does not pass an environment to the Haskell or Racket evaluator. Instead, the environment exists as an ephemeral data structure the interpreter is aware of implicitly. It is there in the background, ready to be used when needed.

The Reader monad gives us exactly this capability for maintaining an environment during evaluation. Our next interpreter will use the Reader to manage the environment during execution, making it available without explicitly passing it around as a parameter. It will be there when needed and disappear from most terms in the evalM definition.

The Reader is a datatype with a single constructor whose argument is a function from environment to value:

data Reader e a = Reader (e -> a)


Recall thatMaybe is also a data type with two constructors. As Just is parameterized over the the type it encasulates, Reader is parameterized over its environment type, e and value type a. Reader can use anything as an environment and return value, but we will use Env and FBAEValue.

Maybe captures the difference between a value and a failed computation. evalM uses bind and return to manage evaluation results that represent successful and failed computations. Reader values represent computations. The value encapsulated by Reader is a function that takes an environment and produces a value. It does not input an expression, but instead represents a computation that evaluates a specific expression. This is important. Reader values represent computations that have not yet been performed.

runR is an auxiliary function that performs the computation a Reader represents. runR is a rather trivial function that pulls the function representing a computation out of the Reader encapsulation and executes it on a specific environment. For this reason runR is frequently read as “run reader”. The definition of runR is really quite simple:

runR :: Reader e a -> e -> a
runR (Reader f) e = f e


Given some R of type Reader, runR e extracts the function representing its computation and applies it to a specific environment resulting in a value. We use the Reader by constructing aReader instance then extracting and executing its computation using runR and an environment.

So far this looks nothing like Maybe in either form or function. If it is a monad, then we know return and bind must be defined. To understand Reader, let’s look at the Monad instance for Reader and see how return andbind are implemented. Following is the Monad instance for Reader:

instance Monad (Reader e) where
return x = Reader $\e -> x g >>= f = Reader$ \e -> runR (f (runR g e)) e


There is a bit of Haskell-foo in this definition that needs explanation. The shorthand f $v is the same as (f v). It gets used quite often to reduce the number of parenthesis to parse mentally. The following two expressions result in the same value: Reader$ \e -> x


The notation \n -> n + 1 is an example of Haskell’s anonymous function definition mechanism. Evaluating \n -> n + 1 results in the increment function. The following two definitions result in the same definition of Inc:

inc x = x + 1
inc = \x -> x + 1


Now we’re set to go.

The return function creates trivial comutations that simply return values. Looking at its definition, return takes a value, x, creates a function from an environment, e to that value, and wraps it up in the Reader constructor. This function returns the argument to return regardless of the input environment. return creates a constant function from an environment to a specific value. Let’s see it at work with runR:

runR (return 5) 0
== runR (Reader \e -> 5) 0
== (\e -> 5) 0
== 5


return 5 creates the Reader value (Reader \e -> 5). runR extracts \e -> 5 and applies it to the environment value 0 resulting in 5. A second example shows the same result with the alternative environment [6,7,8]:

runR (return 5) [6,7,8]
== runR (Reader \e -> 5) [6,7,8]
== (\e -> 5) [6,7,8]
== 5


In fact, running (return 5) will always result in 5 regardless of its environment value. return encapsulates the most trivial computation - returning a constant value. Foreshadowing, the return will frequently be used at the end of strings of computations to return values.

In Maybe, bind passed through a Nothing value and performed a specified operation on a Just value. In Reader, bind will always perform a computation and pass the result to a subsequent computation. It is, in effect, a sequence operator. For reference, the type from the Monad class and the bind instance for Reader are:

  g >>= f :: M a -> (a -> M b) -> M b
g >>= f = Reader $\e -> runR (f (runR g e)) e  It’s quite simple to think that g >>= f runs f and uses its output as an input for running g. Looking at the inside of the definition we even see a term: runR (f (runR g e)) e  that looks exactly like what we want. Unfortunately, this is wrong. Remember, g >>= f returns a monad not a value. In the case of Reader the monad encapsulates a function that will be run later with an environment. g >>= f needs to create a function and encapsulate that function in Readerfor runR to use later. Looking more carefully, the actual argument to the Reader constructor is the function: \e -> runR (f (runR g e)) e  runR is not executed when the Reader value is created, but deferred until the function is evaluated with some e as input. Note also that both f and g are evaluated with e as their environment argument. The same argument for both that is input when runR evaluates the monad. In fact, we can bind as many functions together as we want and the same e will always be the environment argument and will never vary. Thus the name Reader. The environment allows passing in data to runR, but is constant over the entire Reader evaluation. Let’s look at at some simple examples. First, let’s start with a computation than returns 5 and bind it to a computation that adds 1. The computation that returns 5 is a simple application of return. Remember, return simply creates a Reader value that returns a constant value for any environment input: (return 5)  Now let’s create a computation that adds 1 to the result of the previous computation. Remember the signature for f is f::a -> M b. f will take a value and return a monad that uses that value in its computation. At first glance, a function that returns its argument plus 1 is simply \x -> x+1. But we need the function to return a monad, not a value. Our friend return will help us out here: \x -> (return (x + 1))  There is an important concept working here. The computation resulting in x sets up the computation that follows. Evaluating \x -> (return (x + 1)) will replace x with a value in the computation created by return. Instead of calculating, this function creates a computation using the result of the previous computation. The Reader builds up a computation then runR performs that computation. Our monad now becomes: ((return 5) >>= (\x -> (return (x + 1))))  To evaluate our monad we simply call runR and specify an environment: Example under development runR ((return 5) >>= (\x -> (return (x + 1)))) 0 == runR (Reader$ \e -> runR ((\x -> (return (x + 1))) (runR (return 5) e)) e) 0
== \e -> runR ((\x -> (return (x + 1))) (runR (return 5) e)) e) 0
== runR ((\x -> (return (x + 1))) (runR (return 5) 0)) 0)
== runR ((\x -> (return (x + 1))) 5) 0
== runR (return (5 + 1)) 0
== 6


g is return 5, the computation that simple returns 5. f is the function that takes a value and produces a Reader that returns the result of adding 1 to the input. Looking at executing >>=, runR runs (return 5) resulting in 5. It then applies f to 5 resulting in a Reader that simply returns 6. Finally, runR evaluates (return 6) resulting in 6.

Because the result of runR is a number, we can add other operations to the sequence:

runR ((return 5)
>>= (\x -> (return (x + 1)))
>>= (\x -> (return (x - 3)))
>>= (\x -> (return (x mod 2)))) 0
== 1


Now we can sequence operations as we did with Maybe, but none of our operations reference the environment. In fact, the value of the environment makes no difference at all in our examples thus far. How do we include the environment in a computation?

Let’s assume that instead of adding 1 to a constant 5, we want to add 1 to the value stored in the environment. In the definition of bind, the variable bound to the environment is e, so let’s just use it:

((return e) >>= (\x -> (return (x+1))))


Nice idea, but e is not in scope when the new monad is defined. The answer follows quickly by lookin at return again:

return x = (Reader \e -> x)


There is our missing environment! What happens if instead of returning x, we return e:

((Reader \e->e) >>= (\x -> return (x+1))))


This new monad represents the computation that returns the environment:

runR (Reader \e->e) 5
== 5


and can be used to include the environment in subsequent computations like the one above:

runR ((Reader \e->e) >>= (\x -> return (x+1)))) 5
== 6


This monad is commonly called ask. ask simply returns its environment:

ask :: Reader a a


It’s important to realize that ask is not a function, but a Reader instance. It simply names the Reader we used above and operates the same way:

runR ask 5
== 5


ask returns the environment. If the result of ask is the environment value, then ask >>= f should use the environment as the input to f:

runR (ask >>= (\x -> (return (x+1)))) 5
== 6


That is what happens here. The environment is used as a value in subsequent calculations.

Similarly, asks will apply a function to the environment and return the result. asks does exactly what ask does, but calls a function on the environment before returning it:

asks :: (e -> a) -> Reader e a


asks is not a Reader, but instead a function from environment to value to Reader that builds a Reader using ask and bind. For example, asks (\x -> head x) is an operation that takes the first element of the environment and returns it. Let’s try it out:

runR ((asks (\e -> head e)) >>= (\x -> (return (x+1))) [4,5,6]
== 4


asks runs and returns the head element from the environment list. The result is passed to a computation that returns its input plus 1. asks works just like ask, but applies a function to the environment instead of returning it.

Let’s look at one more example of asks that gets us closer to the environment that we want for our languages. In this example our environment will be a list of string, integer pairs:

type Env = [(String,Int)]


and we will define a lookup function that either returns an integer or throws an exception:

lookupName :: String -> Env -> Int
lookupName s e = case (lookup s e) of
Just x -> x


Nothin special here, we’ve wrapped lookup in a function that makes its return value non-Monadic. Now let’s write a Reader instance that uses lookupName to find an element in an environment of thpe Env:

runM (asks (lookupName "b")) >>= (\x -> (return (x+1))) [("a",1)("b",2),("c",3)]


The computation constructed by asks performs a lookup on "b" whose associated value in the environment is 2. That 2 is then input to (\x -> (return (x+1))) resulting in a computation that will add 2 to 1 and return 3. In this example, we are using the environment as a lookup table just as we would for identifier references in one of our languages. We simply need to add the ability to define new identifiers in the table and we’ll be done.

local creates a local environment for running a Reader. local will perform an operation on the environment and then evaluate a Reader with that new environment. The effects are local in that when the local evaluation ends, the environment will be unchanged. Like asks, local is a function that creates a Reader that can be used in a bind sequence:

local :: (e -> t) -> Reader t a -> Reader e a
local f r = ask >>= \e -> return (runR r (f e))


local’s two arguments are a function on the environment and a Reader to execute in the new environment. local’s function is applied to the environment in a similar manner as asks. The Reader argument is a monad that will be run with the environment resulting from the function application. Looking at the above definition, r is a Reader that will be run in the environment created by f e. How does local do this?

local first executes ask to get the environment that is then passed as input to the second bind argument as e. Remember, the second argument to bind is a function from an environment to a Reader. Look carefully at the returned Reader. return computes a value by running the Reader passed to local using f e as the environment. The function runR r (f e) accomplishes this task. The Reader passed to local as r is run inside the Reader created by local with the new environment (f e). Thus the name local. After the nested Reader runs, the local environment is lost and the original environment restored.

Let’s have a look at one quick example before diving into a monadic interpreter that uses Reader. In this example, the pair ("b",5) is added to the environment by local creating a new list. The local computation is the same as our previous computation where b is dereferenced and its value added to 1 and returned:

runR (local (\e -> ("b",5):e) ((asks (lookupName "b")) >>= \x -> return (x+1)) []
== 6


We will use local and lookup functions extensively in the definition of a new interpreter for FBAE.

To summarize, we can now create computations that: (i) ignore the environment; (ii) return the environment using ask; (iii) apply a function to the environment using asks; and (iv) create a new, local environment using local. These tools for manipulating the environment give us what we need to write more concise interpreters for languages that bind and use identifiers.

Before moving on, take a step back and think about what we’ve done in a different way. All Reader instances are encapsulated computations wrapped up in a datatype. runR executes those computations. return encapsulates single, atomic computations. bind sequences computations allowing results from prior computations to flow to later computations. Reader adds and environment, but all instances of Monad do roughly the same thing - encapsulate and sequence computations.

We now have all the pieces in place to us Reader to implement an interpreter. In earlier versions of interpreters with an environment, we passed the environment as an argument to eval. In this new version we will use the Reader, ask, asks, and local to manage the environment without passing it around as a parameter.

Let’s start through the definition of evalM, a monadic evaluator for FBAE based on Reader. The signature for evalM is:

evalM :: FBAE -> Reader Env FBAE


where:

type Env = [(String,FBAEVal)]


Not much changes other than the return type and removal of the environment parameter. The return result is a Reader that we must evaluate with runR. We’ll define another function later that does just this, but for now lets focus on evalM. The environment type remains the same, but as noted there is no Env parameter to evalM.

When discussing the Reader we established that we can ignore, query, and make local changes to the environment. Thinking about expressions in FBAE, we can divide them into three groups based on how they use the environment:

1. No direct reference - mathematical and boolean expressions, constants, if
2. Lookup identifiers in the environment - identifiers
3. Locally modify entries - add identifiers to the environment

The first set includes returning constants and evaluating mathematical expressions. None require accessing the environment directly. As has been our practice, subterms are evaluated the the results used to calculate expression values:

evalM (Num n) = return (Num n)
evalM (Lambda i b) = return (Lambda i b)
evalM (Plus l r) = do { (Num l') <- (evalM l) ;
(Num r') <- (evalM r) ;
return (Num l'+r') }
evalM (Minus l r) = do { (Num l') <- (evalM l) ;
(Num r') <- (evalM r) ;
return (Num l'-r') }


The only change here is removal of the environment parameter. The Reader takes are of threading the environment through to subterm evaluation.

Evaluating Id requires accessing the environment to find the value of an identifier. This is easily done using ask to get the environment and using a lookup function to find the needed environment record.

evalM (Id id) = do { env <- ask ;
return (case (lookup id env) of
Just x -> x


First, ask returns the environment from the Reader and binds it to env. lookup is used to find id just as it was in earlier implementations. lookup returns a Maybe that we will not use as a Monad. Instead case distinguishes between Just and Nothing returning a value while throwing an exception for Nothing. At this point it is easier to use error than manage errors using the monad, but we’ll come back to that later.

The last two expressions require adding information to the environment. local does exactly what we need. Evaluating both Bind and App requires adding a variable binding to the environment:

evalM (Bind i v b) = do { v' <- evalM v ;
local (addVar i v') (evalM b) }
evalM (App f v) = do { (Lambda i b) <- evalM f ;
v' <- evalM v ;
local (addVar i v') (evalM b) }


Both Bind and App use in local exactly the same manner. The value associated with the added identifier is calculated first and use with the identifier to partially instantiate addVar. When supplied with an environment, addVar will result in a new environment with the addition. evalM b evaluates b in the context of the environment created by addVar.

Again for completeness, the definition of addVar is:

addVar :: String -> FBAE -> Env -> Env
addVar s i e = (s,i):e


Does this monadic interpreter implement static or dynamic scoping? How can you tell? I’ll give you a hint and say we’ll look at a statically scoped interpreter next and forever leave dynamically scoped languages.

To implement static scoping we add closures that record the environment where a function is defined. We’ve done this once already and will simply repeat the process hear using the Reader. First, the abstract syntax:

data FBAE where
Num :: Int -> FBAE
Plus :: FBAE -> FBAE -> FBAE
Minus :: FBAE -> FBAE -> FBAE
Bind :: String -> FBAE -> FBAE -> FBAE
Lambda :: String -> FBAETy -> FBAE -> FBAE
App :: FBAE -> FBAE -> FBAE
Id :: String -> FBAE
If :: FBAE -> FBAE -> FBAE -> FBAE
deriving (Show,Eq)


Nothing changed other than adding the argument type to Lambda. We’ll see if we need this for evaluation, but we will certainly need it for type checking. Still the same language, just with that small addition.

With types references from the abstract syntax, we need to include the datatype for FBAETy:

data FBAETy where
TNum :: FBAETy
TFun :: FBAETy -> FBAETy -> FBAETy
deriving (Show,Eq)


For completeness again we include the type for values introduced earlier for static scoping. The important bit is the inclusion of closures for recording the static environment:

data FBAEVal where
NumV :: Int -> FBAEVal
ClosureV :: String -> FBAE -> Env -> FBAEVal
deriving (Show,Eq)


Finally, the environment type:

type Env = [(String,FBAEVal)]


Now we can define evalM, the monadic evaluator using the Reader monad. evalM accepts an abstract syntax value and returns a Reader that we’ll evaluate with runR. The only change here is the Reader encapsulates a function of type Env -> FBAEVal rather than Env -> FBAE used previously:

evalM :: FBAE -> Reader Env FBAEVal


Now the interpreter. Same song, second verse. Everything is identical until we get to the Lambda:

evalM (Num x) = return (NumV x)
evalM (Plus l r) = do { (NumV l') <- (evalM l) ;
(NumV r') <- (evalM r) ;
return (NumV (l'+r')) }
evalM (Minus l r) = do { (NumV l') <- (evalM l) ;
(NumV r') <- (evalM r) ;
return (NumV (l'-r')) }
evalM (Bind i v b) = do { v' <- evalM v ;
local (addVar i v') (evalM b) }


When evaluating Lambda we need to grave the environment when it is defined. When env was a parameter, this was easy. Using ask it still is. We simply execute ask to return a copy of the environment and bind the result to env. Then creating the closure is exactly as it was in the non-monadic statically scoped interpreter.

evalM (Lambda i b) = do { env <- ask ;
return (ClosureV i b env) }


The returned closure contains the environment obtained with ask when the lambda is evaluated. Note that we are evaluating a lambda, not an application. That comes next.

Evaluating App is where the environment from the closure is actually used. In effect, when we evaluate the app we need to start with the environment from the closure rather than the enviornment maintained by the Reader to that point. We don’t want to add to the enviroment. Instead we want to replace it.

When evaluating the App we first evaluate f and a to get the closure and argument value. For dynamic scoping we added a pair to the result of ask and replaced the environment with local. We need to do the same thing again, but using the closure environment rather than the Reader environment. We’ll accomplish this by passing a new function to local. Specifically:

useClosure :: String -> FBAEVal -> Env -> Env -> Env
useClosure i v e _ = (i,v):e


Look below how useClosure is used. The first three arguments are instantiated with the identifier name, value and the environment from the closure. The result is a function of type Env -> Env, exactly what local needs. This particular function ignores that argument and produces a new environment using e from the closure:

evalM (App f a) = do { (ClosureV i b e) <- (evalM f) ;
a' <- (evalM a) ;
local (useClosure i a' e) (evalM b) }


Bingo. useClosure creates a new environment by adding the new binding needed for evaluating App to the environment from the closure. local plugs that in and we’re now good go go.

Now that we know how to build an environment from application and bind, it’s time to evaluate identifiers by looking them up. ask returns the environment that is in turn bound to env. A lookup is performed to find id in env. If it’s there, return it. If it’s not, throw an error.

evalM (Id id) = do { env <- ask ;
case (lookup id env) of
Just x -> return x


It’s useful to redefine eval using evalM so the interpreter operates in the same way as previous interpreters:

eval x = runR (evalM x) []


Now we have a statically scoped interpreter for FBAE using a Reader.

As you might have guessed, the Reader is also quite effective at type checking. What is particularly interesting is the similarly between the type checker and evaluator.

For completeness, the context type is defined as a list of string/type pairs:

type Cont = [(String,FBAETy)]

lookupVarTy = lookup
addVarTy :: String -> FBAETy -> Cont -> Cont
addVarTy s i e = (s,i):e


The signature for our new type inference function is roughly the same as the evaluator, except that we return a Reader that encapsulates types. We will still need to use runR to evaluate the result of called typeofM:

typeofM :: FBAE -> Reader Cont FBAETy


The type of number constants is simply TNum. Just return it:

typeofM (Num n) = return TNum


The binary operations on numbers are identical modulo error messages. Both find the types of their arguments and make sure both are numbers. If they are, return TNum as the type of the operation. If not, throw an error:

typeofM (Plus l r) = do {
l' <- (typeofM l) ;
r' <- (typeofM r) ;
return (if (l'==TNum && r'==TNum) then TNum else error "Type error in +") }
typeofM (Minus l r) = do {
l' <- (typeofM l) ;
r' <- (typeofM r) ;
return (if (l'==TNum && r'==TNum) then TNum else error "Type error in -") }


bind adds bindings to the context when type checking. typeofM uses the Reader to pass along the context rather than the environment, but the operations are almost identical. typeofM for bind first uses ask to get the current context. It calculates the type of the identifier being added, and then uses local in the same way as evalM to add the binding to the local context:

typeofM (Bind i v b) = do {
v' <- typeofM v ;
local (addVarTy i v') (typeofM b) }


To perform static type checking, we need to use the lambda variant that carries a type for its argument. (i,t) is added to the context and typeofM b used to get r', the range type, that is the typeof the function body. The type of the Lambda becomes (TFun t r'):

typeofM (Lambda i t b) = do {
r' <- local (addVarTy i t) (typeofM b) ;
return (TFun t r') }


The App case uses typeofM to get the type of the function and its argument. The function type provides the domain and range of the associated function. If the type of the argument is the domain type, then the application is the range type. If they do not match, then typeofM throws an error. The if expression is where all the work for this function is performed:

typeofM (App f v) = do }
(TFun i b) <- typeofM f ;
v' <- typeofM v ;
return (if i==v' then b else error "Type Error in app") }


Finally finding the type of an identifier is simply looking it up on the context. ask returns the context, a lookup is performed, and either a type is returned or an error message is thrown:

typeofM (Id id) = do
ask >>= \env -> return (case (lookupVarTy id env) of
Just x -> x


Like other functions, we can made the monadic version look like a traditional version with a quick definition:

typeof x = runR (typeofM x) []


Discussion

The Reader is an exceptionally powerful and useful programming pattern. Utility functions like ask, asks, and local are just few samples of what kinds of operations can be defined on the environment. Even the function useClosure could be rewritten as a custom operation rather than using local:

explicit :: e -> Reader t a -> Reader e a
explicit e r = return (runR r e)


In our work thus far we have used our own Reader. The standard Haskell libraries contain a Reader implementation. However, when learning how to use the Reader it is far better to have visibility into the implementation than simply try to use the Reader interface. Monad type signatures are not enough to understand their utility.

It is worth spending time with a good Haskell tutorial and learning the Reader well.

Exercises

1. Rewrite the identifier lookup to use asks rather than ask.
2. Extend the interpreter and type inference function of this chapter to include typed recrursion by implementing the fix operator. Note specifically whether this has any impact on already implemented language features.